This document describes a study investigating the dynamical evolution of the Orion Nebula Cluster through N-body simulations. The simulations assume the cluster was initially much more compact than currently observed, based on evidence that a large fraction of gas was expelled during formation. This likely led to strong few-body relaxation effects early in the cluster's evolution. The simulations find that three-body interactions among massive stars could have ejected them from the cluster while also forming a very massive object through runaway stellar mergers. This object may have collapsed directly into a massive (~100 solar mass) black hole, which could explain the velocity dispersion of stars in the Trapezium system at the cluster core. The simulations further suggest this putative black hole has a ~70
This document summarizes a study on the formation of polar ring galaxies through two proposed scenarios: the merging scenario and the accretion scenario. The merging scenario involves a head-on collision between two galaxies, where one becomes the host galaxy and the other's gas forms the polar ring. The accretion scenario involves tidal interactions that allow gas from one galaxy to be accreted by another galaxy to form a polar ring. Through simulations, the study finds that both scenarios can produce stable polar rings, but that the accretion scenario better matches observed properties of polar ring galaxies, such as the frequency and morphology of the rings. The study concludes the accretion scenario is both more likely and more supported by observations to explain how most polar ring galaxies form.
A possible carbonrich_interior_in_superearth_55_cancrieSérgio Sacani
1) The document analyzes the possibility that the interior of the super-Earth exoplanet 55 Cancri e could be carbon-rich rather than oxygen-rich.
2) Models that assume an oxygen-rich interior with iron, silicates, and a water envelope cannot fully explain 55 Cancri e's mass and radius measurements. However, a carbon-rich interior containing iron, silicon carbide, and/or carbon could explain the observations without needing a volatile envelope.
3) A carbon-rich interior for 55 Cancri e is plausible given the reported carbon-rich composition of its host star, though more data is needed on the star's elemental abundances and the planet's atmosphere.
1. Chandra observations of the galaxy NGC 3351 revealed X-ray emission from its circumnuclear star-forming ring that is composed of numerous point-like sources embedded in diffuse hot gas.
2. The morphology of the X-ray emission is similar to but not identical with UV and H-alpha hot spots in the ring, which can be understood if star formation occurs through intermittent starbursts around the ring with different emissions tracing later evolutionary stages.
3. X-ray emission also extends beyond the ring, which is interpreted as outflowing gas from the ring into the disk and halo of NGC 3351, providing evidence for confined outflow near the plane but less restricted outflow perpendicular to
This document summarizes observations of the debris disk around the subgiant star κ CrB using Herschel and Keck. Herschel spatially resolved images of the debris disk, the first such images of a disk around a subgiant star. Keck radial velocity monitoring provided evidence for a second planetary companion around κ CrB. Keck adaptive optics imaging placed an upper limit on the mass of this companion. Modeling of the Herschel images showed the dust is broadly distributed but could not distinguish between a single wide belt or two narrow belts. The observations are consistent with dynamical depletion or collisional erosion clearing the inner regions of the disk.
Multi wavelenth observations and surveys of galaxy clustersJoana Santos
This document provides an overview of galaxy clusters, focusing on their baryonic components (intracluster medium and galaxies) and how they relate to the cluster's physical properties like mass. It discusses that galaxy clusters form hierarchically through gravitational collapse. The intracluster medium, which makes up most of the baryonic mass, emits X-rays and has been heated to temperatures of millions of degrees. The document reviews properties of the intracluster medium like density, temperature, metallicity, and how they can be measured from X-ray spectra. It also discusses upcoming surveys that will advance the study of galaxy clusters.
This document summarizes a study that uses cosmological simulations to model the formation of stellar halos around galaxies via the tidal disruption of accreted dwarf galaxies. The simulations follow the dynamical evolution and disruption of satellites from high redshift in a fully cosmological setting. The simulations produce stellar halos with masses and density profiles consistent with observations of the Milky Way and M31. The stellar halos show complex structures composed of well-mixed components, tidal streams, shells, and other substructures rather than smooth distributions.
This document summarizes the results of a 180 ks Chandra-LETGS observation of Mrk 509 as part of a larger multi-wavelength campaign. The observation detected several absorption features in the X-ray spectrum originating from an ionized absorber, including ions with three distinct ionization degrees. The lowest ionized component is slightly redshifted and not in pressure equilibrium with the others, likely belonging to the host galaxy's interstellar medium. The other two components are outflowing at velocities of around -200 and -455 km/s. Simultaneous HST-COS observations detected 13 UV kinematic components, and at least three can be associated with the X-ray components, providing evidence that the UV and X-
The document summarizes the period of the universe known as the Dark Age, which occurred between the emission of the cosmic microwave background radiation and the formation of the first stars. During this time, the universe was nearly homogeneous and dark as the first structures like galaxies and stars had yet to form. The Cold Dark Matter model and observations of the cosmic microwave background and distant galaxies provide evidence that the first stars likely formed around redshift 20, ending the Dark Age and beginning the process of reionization. The formation of these first stars required the presence of dark matter for structure to grow and cooling mechanisms like molecular hydrogen to allow gas to collapse into the progenitors of stars.
This document summarizes a study on the formation of polar ring galaxies through two proposed scenarios: the merging scenario and the accretion scenario. The merging scenario involves a head-on collision between two galaxies, where one becomes the host galaxy and the other's gas forms the polar ring. The accretion scenario involves tidal interactions that allow gas from one galaxy to be accreted by another galaxy to form a polar ring. Through simulations, the study finds that both scenarios can produce stable polar rings, but that the accretion scenario better matches observed properties of polar ring galaxies, such as the frequency and morphology of the rings. The study concludes the accretion scenario is both more likely and more supported by observations to explain how most polar ring galaxies form.
A possible carbonrich_interior_in_superearth_55_cancrieSérgio Sacani
1) The document analyzes the possibility that the interior of the super-Earth exoplanet 55 Cancri e could be carbon-rich rather than oxygen-rich.
2) Models that assume an oxygen-rich interior with iron, silicates, and a water envelope cannot fully explain 55 Cancri e's mass and radius measurements. However, a carbon-rich interior containing iron, silicon carbide, and/or carbon could explain the observations without needing a volatile envelope.
3) A carbon-rich interior for 55 Cancri e is plausible given the reported carbon-rich composition of its host star, though more data is needed on the star's elemental abundances and the planet's atmosphere.
1. Chandra observations of the galaxy NGC 3351 revealed X-ray emission from its circumnuclear star-forming ring that is composed of numerous point-like sources embedded in diffuse hot gas.
2. The morphology of the X-ray emission is similar to but not identical with UV and H-alpha hot spots in the ring, which can be understood if star formation occurs through intermittent starbursts around the ring with different emissions tracing later evolutionary stages.
3. X-ray emission also extends beyond the ring, which is interpreted as outflowing gas from the ring into the disk and halo of NGC 3351, providing evidence for confined outflow near the plane but less restricted outflow perpendicular to
This document summarizes observations of the debris disk around the subgiant star κ CrB using Herschel and Keck. Herschel spatially resolved images of the debris disk, the first such images of a disk around a subgiant star. Keck radial velocity monitoring provided evidence for a second planetary companion around κ CrB. Keck adaptive optics imaging placed an upper limit on the mass of this companion. Modeling of the Herschel images showed the dust is broadly distributed but could not distinguish between a single wide belt or two narrow belts. The observations are consistent with dynamical depletion or collisional erosion clearing the inner regions of the disk.
Multi wavelenth observations and surveys of galaxy clustersJoana Santos
This document provides an overview of galaxy clusters, focusing on their baryonic components (intracluster medium and galaxies) and how they relate to the cluster's physical properties like mass. It discusses that galaxy clusters form hierarchically through gravitational collapse. The intracluster medium, which makes up most of the baryonic mass, emits X-rays and has been heated to temperatures of millions of degrees. The document reviews properties of the intracluster medium like density, temperature, metallicity, and how they can be measured from X-ray spectra. It also discusses upcoming surveys that will advance the study of galaxy clusters.
This document summarizes a study that uses cosmological simulations to model the formation of stellar halos around galaxies via the tidal disruption of accreted dwarf galaxies. The simulations follow the dynamical evolution and disruption of satellites from high redshift in a fully cosmological setting. The simulations produce stellar halos with masses and density profiles consistent with observations of the Milky Way and M31. The stellar halos show complex structures composed of well-mixed components, tidal streams, shells, and other substructures rather than smooth distributions.
This document summarizes the results of a 180 ks Chandra-LETGS observation of Mrk 509 as part of a larger multi-wavelength campaign. The observation detected several absorption features in the X-ray spectrum originating from an ionized absorber, including ions with three distinct ionization degrees. The lowest ionized component is slightly redshifted and not in pressure equilibrium with the others, likely belonging to the host galaxy's interstellar medium. The other two components are outflowing at velocities of around -200 and -455 km/s. Simultaneous HST-COS observations detected 13 UV kinematic components, and at least three can be associated with the X-ray components, providing evidence that the UV and X-
The document summarizes the period of the universe known as the Dark Age, which occurred between the emission of the cosmic microwave background radiation and the formation of the first stars. During this time, the universe was nearly homogeneous and dark as the first structures like galaxies and stars had yet to form. The Cold Dark Matter model and observations of the cosmic microwave background and distant galaxies provide evidence that the first stars likely formed around redshift 20, ending the Dark Age and beginning the process of reionization. The formation of these first stars required the presence of dark matter for structure to grow and cooling mechanisms like molecular hydrogen to allow gas to collapse into the progenitors of stars.
The document discusses evidence for dark matter from astrophysical observations. It is established that dark matter is massive, cold, collisionless, and does not interact electromagnetically. However, its fundamental nature and interactions are unknown. The evidence includes missing mass observations from galaxy rotation curves, cluster dynamics, gravitational lensing as well as cosmological measurements of the cosmic microwave background and matter power spectrum. Future experiments aim to directly detect dark matter particle signatures through anomalies in cosmic ray spectra or indirect signals from early structure formation.
EMU M.Sc. Thesis Presentation
Thesis Title: "Dark Matter; Modification of f(R) or WIMPS Miracle"
Student: Ali Övgün
Supervisor: Prof. Dr. Mustafa Halilsoy
First detection of_molecular_gas_in_shells_in_cena_galaxySérgio Sacani
This document reports the first detection of molecular gas (CO) in the shells of the galaxy Centaurus A (Cen A), located 15 kpc from the galaxy's center. The ratio of CO to HI emission in the shells matches what is found in the galaxy's central regions, which is unexpected given typical metallicity gradients in galaxies. The detection of molecular gas in the shells provides evidence that molecular gas in galaxy mergers may be spread further from nuclear regions than previously thought. The dynamics of the gas can be understood if the interstellar medium is considered clumpy and less dissipative than assumed, allowing dense gas to orbit with stars and form shells.
This document discusses a proposed explanation for dark energy and the accelerated expansion of the universe involving antimatter hidden in cosmic voids. It suggests that general relativity extended to antimatter predicts a gravitational repulsion between matter and antimatter. Observational evidence of faster than expected void evacuation and structure formation in our local region could be explained by the antigravity field produced by antimatter masses equivalent to typical superclusters hidden in voids. This "dark repulsor" model could account for dark energy and various anomalies without needing an initial explosion or a cosmological constant.
1) The document provides a summary of a course on high-energy astrophysics that the author took. It discusses various topics covered in the course including accretion disks, pulsars, black holes, supernovae, and more.
2) The author argues that high-energy astrophysics is important for understanding the universe and requests that the provost offer a similar course at their university.
3) Key concepts in high-energy astrophysics discussed include accretion and its relation to luminosity, binary star systems, properties of neutron stars and black holes, and x-ray emissions from astrophysical phenomena like supernovae.
Observational evidence against long lived spiral arms in galaxies1Sérgio Sacani
This document presents evidence against the theory of long-lived spiral arms in galaxies. The authors analyze simulations and observations to test whether spiral patterns are quasi-stationary features with a constant pattern speed, as implied by density wave theory. They find that in simulations with imposed spiral potentials, gas and young stars are spatially ordered as expected. However, in more realistic simulations where spirals form naturally, the gas and stars are coincident with the spiral potential minors and not separated. Analyzing observations of 12 galaxies, the authors calculate angular cross-correlations between different star formation tracers but find no systematic offsets, contrary to the predictions of stationary spiral density waves. This suggests spiral density waves are not important for explaining spiral structure in
Relativistic collapse and explosion of rotating supermassive stars with therm...Sérgio Sacani
1) The document describes general relativistic simulations of collapsing supermassive stars with and without rotation using a numerical code called Nada.
2) The simulations include effects of gas pressure, radiation, electron-positron pairs, and thermonuclear energy from hydrogen and helium burning.
3) Objects with a mass of around 5×105 solar masses explode if non-rotating with a metallicity over 0.007, while rotation lowers the threshold to 0.001. More massive objects have a higher critical metallicity for explosion.
A rapidly spinning_supermassive_black_hole_at_the_centre_of_ngc1365Sérgio Sacani
1) XMM-Newton and NuSTAR observed the galaxy NGC 1365 simultaneously over 130 ks, detecting broadband X-ray emission from 3-79 keV.
2) The observations revealed variable absorption below 10 keV and prominent broad emission features between 5-7 keV and 10-30 keV, indicative of relativistic reflection from an accretion disk near a spinning supermassive black hole.
3) Time-resolved spectral analysis was able to disentangle the variable absorption component from the relativistic reflection component. Absorption-dominated models without reflection could be ruled out statistically and on physical grounds.
Mass and motion_of_globulettes_in_the_rosette_nebulaSérgio Sacani
This document summarizes observations of globulettes (small molecular clumps) in the Rosette Nebula made using radio telescopes and infrared imaging. Radio observations of 16 globulettes detected molecular line emission from 12CO and 13CO, allowing masses to be estimated. Masses ranged from about 50 to 500 Jupiter masses. Infrared imaging found that several globulettes are very opaque and contain dense cores. Analysis of velocities found that globulettes and other nebula structures are expanding at about 22 km/s, with globulettes having only small velocity differences, suggesting they are moving with the general expansion. Some globulettes appear to be detaching from larger structures as they expand outwards.
The tumbling rotational state of 1I/‘OumuamuaSérgio Sacani
The discovery1
of 1I/2017 U1 (1I/‘Oumuamua) has provided
the first glimpse of a planetesimal born in another planetary
system. This interloper exhibits a variable colour within a
range that is broadly consistent with local small bodies, such
as the P- and D-type asteroids, Jupiter Trojans and dynamically
excited Kuiper belt objects2–7
. 1I/‘Oumuamua appears
unusually elongated in shape, with an axial ratio exceeding
5:1 (refs 1,4,5,8). Rotation period estimates are inconsistent
and varied, with reported values between 6.9 and 8.3 h
(refs 4–6,9). Here, we analyse all the available optical photometry
data reported to date. No single rotation period can explain
the exhibited brightness variations. Rather, 1I/‘Oumuamua
appears to be in an excited rotational state undergoing nonprincipal
axis rotation, or tumbling. A satisfactory solution
has apparent lightcurve frequencies of 0.135 and 0.126 h−1
and
implies a longest-to-shortest axis ratio of ≳5:1, although the
available data are insufficient to uniquely constrain the true
frequencies and shape. Assuming a body that responds to
non-principal axis rotation in a similar manner to Solar System
asteroids and comets, the timescale to damp 1I/‘Oumuamua’s
tumbling is at least one billion years. 1I/‘Oumuamua was
probably set tumbling within its parent planetary system and
will remain tumbling well after it has left ours.
Small dust particles escaping from Pluto's moons Nix and Hydra could temporarily form a ring around Pluto due to impacts, but the solar radiation pressure would remove about half of 1 micrometer particles within a year by causing collisions with Pluto and its moons. Numerical simulations found that a tenuous ring with an optical depth of 6×10^-11 could be maintained by dust released from Nix and Hydra.
Periodic mass extinctions_and_the_planet_x_model_reconsideredSérgio Sacani
The 27 Myr periodicity in the fossil extinction record has been con-
firmed in modern data bases dating back 500 Myr, which is twice the time
interval of the original analysis from thirty years ago. The surprising regularity
of this period has been used to reject the Nemesis model. A second
model based on the sun’s vertical galactic oscillations has been challenged
on the basis of an inconsistency in period and phasing. The third astronomical
model originally proposed to explain the periodicity is the Planet
X model in which the period is associated with the perihelion precession
of the inclined orbit of a trans-Neptunian planet. Recently, and unrelated
to mass extinctions, a trans-Neptunian super-Earth planet has been proposed
to explain the observation that the inner Oort cloud objects Sedna
and 2012VP113 have perihelia that lie near the ecliptic plane. In this
Letter we reconsider the Planet X model in light of the confluence of the
modern palaeontological and outer solar system dynamical evidence.
Key Words: astrobiology - planets and satellites - Kuiper belt:
general - comets: general
The characterization of_the_gamma_ray_signal_from_the_central_milk_way_a_comp...Sérgio Sacani
This document analyzes the gamma-ray signal from the central Milky Way that is consistent with emission from annihilating dark matter particles. The authors re-examine Fermi data using cuts on an event parameter to improve gamma-ray maps and more easily separate components. They find the GeV excess is robust and well-fit by a 36-51 GeV dark matter particle annihilating to bottom quarks with a cross section of 1-3×10−26 cm3/s. The signal extends over 10 degrees from the Galactic Center and is spherically symmetric, disfavoring explanations from millisecond pulsars or gas interactions.
Evidence for reflected_lightfrom_the_most_eccentric_exoplanet_knownSérgio Sacani
Planets in highly eccentric orbits form a class of objects not seen within our Solar System. The most extreme case known amongst these objects is the planet orbiting HD 20782, with an orbital period of 597 days and an eccentricity of 0.96. Here we present new data and analysis for this system as part of the Transit Ephemeris Refinement and Monitoring Survey (TERMS). We obtained CHIRON spectra to perform an independent estimation of the fundamental stellar parameters. New radial velocities from AAT and PARAS observations during periastron passage greatly improve our knowledge of the eccentric nature of the orbit. The combined analysis of our Keplerian orbital and Hipparcos astrometry show that the inclination of the planetary orbit is > 1.22◦, ruling out stellar masses for the companion. Our long-term robotic photometry show that the star is extremely stable over long timescales. Photometric monitoring of the star during predicted transit and periastron times using MOST rule out a transit of the planet and reveal evidence of phase variations during periastron. These possible photometric phase variations may be caused by reflected light from the planet’s atmosphere and the dramatic change in star–planet separation surrounding the periastron passage.
An interacting binary_system_powers_precessing_outflows_of_an_evolved_starSérgio Sacani
1) The authors discovered that the central star system in the planetary nebula Fleming 1 is a binary system with an orbital period of 1.1953 days.
2) They determine that the binary likely consists of a white dwarf primary star with a mass of 0.56-0.7 solar masses and a hotter white dwarf secondary star with a temperature over 120,000 K that provides the ionizing photons to power the nebula.
3) The discovery confirms that binary interactions can explain the precessing outflows and point-symmetric structures seen in Fleming 1 and other planetary nebulae with similar features.
Evidence for the_thermal_sunyaev-zeldovich_effect_associated_with_quasar_feed...Sérgio Sacani
Using a radio-quiet subsample of the Sloan Digital Sky Survey spectroscopic quasar
catalogue, spanning redshifts 0.5–3.5, we derive the mean millimetre and far-infrared
quasar spectral energy distributions (SEDs) via a stacking analysis of Atacama Cosmology
Telescope and Herschel-Spectral and Photometric Imaging REceiver data. We
constrain the form of the far-infrared emission and find 3σ–4σ evidence for the thermal
Sunyaev-Zel’dovich (SZ) effect, characteristic of a hot ionized gas component with
thermal energy (6.2 ± 1.7) × 1060 erg. This amount of thermal energy is greater than
expected assuming only hot gas in virial equilibrium with the dark matter haloes of
(1 − 5) × 1012h
−1M that these systems are expected to occupy, though the highest
quasar mass estimates found in the literature could explain a large fraction of this
energy. Our measurements are consistent with quasars depositing up to (14.5±3.3) τ
−1
8
per cent of their radiative energy into their circumgalactic environment if their typical
period of quasar activity is τ8 × 108 yr. For high quasar host masses, ∼ 1013h
−1M,
this percentage will be reduced. Furthermore, the uncertainty on this percentage is
only statistical and additional systematic uncertainties enter at the 40 per cent level.
The SEDs are dust dominated in all bands and we consider various models for dust
emission. While sufficiently complex dust models can obviate the SZ effect, the SZ
interpretation remains favoured at the 3σ–4σ level for most models.
The document discusses high energy astrophysical research conducted at the Maidanak Observatory in Uzbekistan. It provides historical context about the transfer of Islamic science to the region in the 11th century. It then describes the observatory's facilities, including its six telescopes and five CCD cameras. The main fields of scientific research at the observatory are also listed, such as gravitational lensing systems, active galactic nuclei, gamma-ray bursts, and blazars. Specific research on gravitational lensing systems and blazars using data from Maidanak is summarized.
1) The galaxy merger rate over cosmic time is a fundamental measure of galaxy evolution. Mergers are thought to drive galaxy assembly, rapid star formation, and the formation of spheroidal galaxies and supermassive black holes.
2) While simulations predict that the dark matter halo merger rate increases strongly with redshift, the theoretical predictions for the galaxy merger rate remain uncertain due to the complex relationship between galaxies and dark matter halos.
3) The paper aims to address this problem by calibrating three observational galaxy merger rate estimators - close galaxy pairs, G-M20 morphology, and asymmetry - using hydrodynamic merger simulations. This allows them to determine merger timescales and consistently apply them to observed merger fractions up to
The effects of_the_target_material_properties_and layering_on_the_crater_chro...Sérgio Sacani
This document summarizes a study analyzing the effects of target material properties and layering on determining crater ages for the Raditladi and Rachmaninoff basins on Mercury. The authors derive model ages of about 3.6 billion years for Rachmaninoff basin and 1.1 billion years for Raditladi basin. They also constrain the age of smooth plains within the basin floors, suggesting volcanic activity on Mercury may have continued until around 1 billion years ago or less. The study addresses the importance of considering terrain parameters such as mechanical properties and layering in the age determination process.
Nickel and helium_evidence_for_melt_above_the_core_mantle_boundarySérgio Sacani
1) Olivine phenocrysts from basalts in several locations have higher nickel contents than expected from normal mantle peridotite sources, indicating a nickel-rich source.
2) Modeling of crystallization processes shows these olivines require a source with around 20% more nickel (2,360 ppm) than estimated for normal fertile peridotite (1,960 ppm).
3) The nickel-rich source is proposed to have formed from interaction of the core and mantle during crystallization of a basal magma ocean or melt-rich layer, and continues to be sampled by mantle plumes, such as those producing the basalts studied.
The document discusses evidence for dark matter from astrophysical observations. It is established that dark matter is massive, cold, collisionless, and does not interact electromagnetically. However, its fundamental nature and interactions are unknown. The evidence includes missing mass observations from galaxy rotation curves, cluster dynamics, gravitational lensing as well as cosmological measurements of the cosmic microwave background and matter power spectrum. Future experiments aim to directly detect dark matter particle signatures through anomalies in cosmic ray spectra or indirect signals from early structure formation.
EMU M.Sc. Thesis Presentation
Thesis Title: "Dark Matter; Modification of f(R) or WIMPS Miracle"
Student: Ali Övgün
Supervisor: Prof. Dr. Mustafa Halilsoy
First detection of_molecular_gas_in_shells_in_cena_galaxySérgio Sacani
This document reports the first detection of molecular gas (CO) in the shells of the galaxy Centaurus A (Cen A), located 15 kpc from the galaxy's center. The ratio of CO to HI emission in the shells matches what is found in the galaxy's central regions, which is unexpected given typical metallicity gradients in galaxies. The detection of molecular gas in the shells provides evidence that molecular gas in galaxy mergers may be spread further from nuclear regions than previously thought. The dynamics of the gas can be understood if the interstellar medium is considered clumpy and less dissipative than assumed, allowing dense gas to orbit with stars and form shells.
This document discusses a proposed explanation for dark energy and the accelerated expansion of the universe involving antimatter hidden in cosmic voids. It suggests that general relativity extended to antimatter predicts a gravitational repulsion between matter and antimatter. Observational evidence of faster than expected void evacuation and structure formation in our local region could be explained by the antigravity field produced by antimatter masses equivalent to typical superclusters hidden in voids. This "dark repulsor" model could account for dark energy and various anomalies without needing an initial explosion or a cosmological constant.
1) The document provides a summary of a course on high-energy astrophysics that the author took. It discusses various topics covered in the course including accretion disks, pulsars, black holes, supernovae, and more.
2) The author argues that high-energy astrophysics is important for understanding the universe and requests that the provost offer a similar course at their university.
3) Key concepts in high-energy astrophysics discussed include accretion and its relation to luminosity, binary star systems, properties of neutron stars and black holes, and x-ray emissions from astrophysical phenomena like supernovae.
Observational evidence against long lived spiral arms in galaxies1Sérgio Sacani
This document presents evidence against the theory of long-lived spiral arms in galaxies. The authors analyze simulations and observations to test whether spiral patterns are quasi-stationary features with a constant pattern speed, as implied by density wave theory. They find that in simulations with imposed spiral potentials, gas and young stars are spatially ordered as expected. However, in more realistic simulations where spirals form naturally, the gas and stars are coincident with the spiral potential minors and not separated. Analyzing observations of 12 galaxies, the authors calculate angular cross-correlations between different star formation tracers but find no systematic offsets, contrary to the predictions of stationary spiral density waves. This suggests spiral density waves are not important for explaining spiral structure in
Relativistic collapse and explosion of rotating supermassive stars with therm...Sérgio Sacani
1) The document describes general relativistic simulations of collapsing supermassive stars with and without rotation using a numerical code called Nada.
2) The simulations include effects of gas pressure, radiation, electron-positron pairs, and thermonuclear energy from hydrogen and helium burning.
3) Objects with a mass of around 5×105 solar masses explode if non-rotating with a metallicity over 0.007, while rotation lowers the threshold to 0.001. More massive objects have a higher critical metallicity for explosion.
A rapidly spinning_supermassive_black_hole_at_the_centre_of_ngc1365Sérgio Sacani
1) XMM-Newton and NuSTAR observed the galaxy NGC 1365 simultaneously over 130 ks, detecting broadband X-ray emission from 3-79 keV.
2) The observations revealed variable absorption below 10 keV and prominent broad emission features between 5-7 keV and 10-30 keV, indicative of relativistic reflection from an accretion disk near a spinning supermassive black hole.
3) Time-resolved spectral analysis was able to disentangle the variable absorption component from the relativistic reflection component. Absorption-dominated models without reflection could be ruled out statistically and on physical grounds.
Mass and motion_of_globulettes_in_the_rosette_nebulaSérgio Sacani
This document summarizes observations of globulettes (small molecular clumps) in the Rosette Nebula made using radio telescopes and infrared imaging. Radio observations of 16 globulettes detected molecular line emission from 12CO and 13CO, allowing masses to be estimated. Masses ranged from about 50 to 500 Jupiter masses. Infrared imaging found that several globulettes are very opaque and contain dense cores. Analysis of velocities found that globulettes and other nebula structures are expanding at about 22 km/s, with globulettes having only small velocity differences, suggesting they are moving with the general expansion. Some globulettes appear to be detaching from larger structures as they expand outwards.
The tumbling rotational state of 1I/‘OumuamuaSérgio Sacani
The discovery1
of 1I/2017 U1 (1I/‘Oumuamua) has provided
the first glimpse of a planetesimal born in another planetary
system. This interloper exhibits a variable colour within a
range that is broadly consistent with local small bodies, such
as the P- and D-type asteroids, Jupiter Trojans and dynamically
excited Kuiper belt objects2–7
. 1I/‘Oumuamua appears
unusually elongated in shape, with an axial ratio exceeding
5:1 (refs 1,4,5,8). Rotation period estimates are inconsistent
and varied, with reported values between 6.9 and 8.3 h
(refs 4–6,9). Here, we analyse all the available optical photometry
data reported to date. No single rotation period can explain
the exhibited brightness variations. Rather, 1I/‘Oumuamua
appears to be in an excited rotational state undergoing nonprincipal
axis rotation, or tumbling. A satisfactory solution
has apparent lightcurve frequencies of 0.135 and 0.126 h−1
and
implies a longest-to-shortest axis ratio of ≳5:1, although the
available data are insufficient to uniquely constrain the true
frequencies and shape. Assuming a body that responds to
non-principal axis rotation in a similar manner to Solar System
asteroids and comets, the timescale to damp 1I/‘Oumuamua’s
tumbling is at least one billion years. 1I/‘Oumuamua was
probably set tumbling within its parent planetary system and
will remain tumbling well after it has left ours.
Small dust particles escaping from Pluto's moons Nix and Hydra could temporarily form a ring around Pluto due to impacts, but the solar radiation pressure would remove about half of 1 micrometer particles within a year by causing collisions with Pluto and its moons. Numerical simulations found that a tenuous ring with an optical depth of 6×10^-11 could be maintained by dust released from Nix and Hydra.
Periodic mass extinctions_and_the_planet_x_model_reconsideredSérgio Sacani
The 27 Myr periodicity in the fossil extinction record has been con-
firmed in modern data bases dating back 500 Myr, which is twice the time
interval of the original analysis from thirty years ago. The surprising regularity
of this period has been used to reject the Nemesis model. A second
model based on the sun’s vertical galactic oscillations has been challenged
on the basis of an inconsistency in period and phasing. The third astronomical
model originally proposed to explain the periodicity is the Planet
X model in which the period is associated with the perihelion precession
of the inclined orbit of a trans-Neptunian planet. Recently, and unrelated
to mass extinctions, a trans-Neptunian super-Earth planet has been proposed
to explain the observation that the inner Oort cloud objects Sedna
and 2012VP113 have perihelia that lie near the ecliptic plane. In this
Letter we reconsider the Planet X model in light of the confluence of the
modern palaeontological and outer solar system dynamical evidence.
Key Words: astrobiology - planets and satellites - Kuiper belt:
general - comets: general
The characterization of_the_gamma_ray_signal_from_the_central_milk_way_a_comp...Sérgio Sacani
This document analyzes the gamma-ray signal from the central Milky Way that is consistent with emission from annihilating dark matter particles. The authors re-examine Fermi data using cuts on an event parameter to improve gamma-ray maps and more easily separate components. They find the GeV excess is robust and well-fit by a 36-51 GeV dark matter particle annihilating to bottom quarks with a cross section of 1-3×10−26 cm3/s. The signal extends over 10 degrees from the Galactic Center and is spherically symmetric, disfavoring explanations from millisecond pulsars or gas interactions.
Evidence for reflected_lightfrom_the_most_eccentric_exoplanet_knownSérgio Sacani
Planets in highly eccentric orbits form a class of objects not seen within our Solar System. The most extreme case known amongst these objects is the planet orbiting HD 20782, with an orbital period of 597 days and an eccentricity of 0.96. Here we present new data and analysis for this system as part of the Transit Ephemeris Refinement and Monitoring Survey (TERMS). We obtained CHIRON spectra to perform an independent estimation of the fundamental stellar parameters. New radial velocities from AAT and PARAS observations during periastron passage greatly improve our knowledge of the eccentric nature of the orbit. The combined analysis of our Keplerian orbital and Hipparcos astrometry show that the inclination of the planetary orbit is > 1.22◦, ruling out stellar masses for the companion. Our long-term robotic photometry show that the star is extremely stable over long timescales. Photometric monitoring of the star during predicted transit and periastron times using MOST rule out a transit of the planet and reveal evidence of phase variations during periastron. These possible photometric phase variations may be caused by reflected light from the planet’s atmosphere and the dramatic change in star–planet separation surrounding the periastron passage.
An interacting binary_system_powers_precessing_outflows_of_an_evolved_starSérgio Sacani
1) The authors discovered that the central star system in the planetary nebula Fleming 1 is a binary system with an orbital period of 1.1953 days.
2) They determine that the binary likely consists of a white dwarf primary star with a mass of 0.56-0.7 solar masses and a hotter white dwarf secondary star with a temperature over 120,000 K that provides the ionizing photons to power the nebula.
3) The discovery confirms that binary interactions can explain the precessing outflows and point-symmetric structures seen in Fleming 1 and other planetary nebulae with similar features.
Evidence for the_thermal_sunyaev-zeldovich_effect_associated_with_quasar_feed...Sérgio Sacani
Using a radio-quiet subsample of the Sloan Digital Sky Survey spectroscopic quasar
catalogue, spanning redshifts 0.5–3.5, we derive the mean millimetre and far-infrared
quasar spectral energy distributions (SEDs) via a stacking analysis of Atacama Cosmology
Telescope and Herschel-Spectral and Photometric Imaging REceiver data. We
constrain the form of the far-infrared emission and find 3σ–4σ evidence for the thermal
Sunyaev-Zel’dovich (SZ) effect, characteristic of a hot ionized gas component with
thermal energy (6.2 ± 1.7) × 1060 erg. This amount of thermal energy is greater than
expected assuming only hot gas in virial equilibrium with the dark matter haloes of
(1 − 5) × 1012h
−1M that these systems are expected to occupy, though the highest
quasar mass estimates found in the literature could explain a large fraction of this
energy. Our measurements are consistent with quasars depositing up to (14.5±3.3) τ
−1
8
per cent of their radiative energy into their circumgalactic environment if their typical
period of quasar activity is τ8 × 108 yr. For high quasar host masses, ∼ 1013h
−1M,
this percentage will be reduced. Furthermore, the uncertainty on this percentage is
only statistical and additional systematic uncertainties enter at the 40 per cent level.
The SEDs are dust dominated in all bands and we consider various models for dust
emission. While sufficiently complex dust models can obviate the SZ effect, the SZ
interpretation remains favoured at the 3σ–4σ level for most models.
The document discusses high energy astrophysical research conducted at the Maidanak Observatory in Uzbekistan. It provides historical context about the transfer of Islamic science to the region in the 11th century. It then describes the observatory's facilities, including its six telescopes and five CCD cameras. The main fields of scientific research at the observatory are also listed, such as gravitational lensing systems, active galactic nuclei, gamma-ray bursts, and blazars. Specific research on gravitational lensing systems and blazars using data from Maidanak is summarized.
1) The galaxy merger rate over cosmic time is a fundamental measure of galaxy evolution. Mergers are thought to drive galaxy assembly, rapid star formation, and the formation of spheroidal galaxies and supermassive black holes.
2) While simulations predict that the dark matter halo merger rate increases strongly with redshift, the theoretical predictions for the galaxy merger rate remain uncertain due to the complex relationship between galaxies and dark matter halos.
3) The paper aims to address this problem by calibrating three observational galaxy merger rate estimators - close galaxy pairs, G-M20 morphology, and asymmetry - using hydrodynamic merger simulations. This allows them to determine merger timescales and consistently apply them to observed merger fractions up to
The effects of_the_target_material_properties_and layering_on_the_crater_chro...Sérgio Sacani
This document summarizes a study analyzing the effects of target material properties and layering on determining crater ages for the Raditladi and Rachmaninoff basins on Mercury. The authors derive model ages of about 3.6 billion years for Rachmaninoff basin and 1.1 billion years for Raditladi basin. They also constrain the age of smooth plains within the basin floors, suggesting volcanic activity on Mercury may have continued until around 1 billion years ago or less. The study addresses the importance of considering terrain parameters such as mechanical properties and layering in the age determination process.
Nickel and helium_evidence_for_melt_above_the_core_mantle_boundarySérgio Sacani
1) Olivine phenocrysts from basalts in several locations have higher nickel contents than expected from normal mantle peridotite sources, indicating a nickel-rich source.
2) Modeling of crystallization processes shows these olivines require a source with around 20% more nickel (2,360 ppm) than estimated for normal fertile peridotite (1,960 ppm).
3) The nickel-rich source is proposed to have formed from interaction of the core and mantle during crystallization of a basal magma ocean or melt-rich layer, and continues to be sampled by mantle plumes, such as those producing the basalts studied.
The GRAIL spacecraft measured the gravity field of the Moon at high resolution, allowing determination of the bulk density and porosity of the lunar crust. The analysis found:
1) The bulk density of the lunar highlands crust is 2550 kg/m3, substantially lower than previous estimates due to impact-induced porosity.
2) The average porosity of the crust is 12%, varying regionally from 4-21% and correlated with impact basins.
3) A new global crustal thickness model was constructed satisfying seismic constraints with an average thickness of 34-43 km, indicating the Moon's composition is not highly enriched compared to Earth.
1) The document presents a new model for star formation in simulations of interacting galaxies that is governed by the local rate of energy dissipation in shocks, rather than the typical density-dependent model.
2) It compares the new shock-induced star formation model to the traditional density-dependent model using simulations of the interacting galaxy NGC 4676.
3) The shock-induced star formation model provides a better match to observations of extended star formation in interacting galaxies than the density-dependent model.
This document discusses observations of the Carina region made by the AGILE gamma-ray satellite over 130 days between 2007 and 2009. It detected a gamma-ray source, 1AGL J1043-5931, that is consistent with the position of Eta Carinae. Notably, 1AGL J1043-5931 exhibited a two-day gamma-ray flaring episode in October 2008. If associated with the Eta Car system, this would provide the first detection above 100 MeV of gamma rays from a colliding wind binary star system. The Carina region contains many massive stars, clusters, nebulae and a giant molecular cloud, making it complex to study.
Star formation history_in_the_smc_the_case_of_ngc602Sérgio Sacani
This document summarizes a study of the star formation history in the NGC 602 region of the Small Magellanic Cloud based on Hubble Space Telescope observations. Deep photometry reveals numerous pre-main sequence stars as well as young stars on the main sequence, allowing the study of star formation into recent times. New pre-main sequence stellar evolutionary tracks are presented. A stellar population synthesis code is used to derive the best estimate of the star formation history by comparing observed and synthetic color-magnitude diagrams. The star formation rate in the region has increased over the past tens of Myr, reaching 0.3-0.7×10−3 M☉ yr−1 in the last 2.5 Myr, comparable to
An earth mass_planet_orbiting_alpha_centauri_bSérgio Sacani
1) Researchers have detected the smallest minimum mass planet found to date orbiting a solar-type star, Alpha Centauri B.
2) The planet has a minimum mass similar to Earth and orbits Alpha Centauri B with a period of 3.236 days, making it the closest exoplanet to our solar system discovered so far.
3) The detection demonstrates the ability to reach precision needed to detect potentially habitable super-Earth planets around Sun-like stars using radial velocity techniques, though this planet is too close to its star to be habitable.
This document appears to be a citation to a conference paper from 1981 that does not provide any other contextual information. As such, it is not possible to generate a meaningful summary with only this citation provided. Additional context and details would be needed to understand the topic and main points discussed in the referenced conference paper in order to write an informative summary.
A massive pulsar_in_a_compact_relativistic_binarySérgio Sacani
1) Researchers observed the pulsar PSR J0348+0432 in a compact orbit with a white dwarf companion. Timing observations yielded component masses - the pulsar has a precisely determined mass of 2.01 solar masses, among the highest yet observed.
2) Optical spectroscopy of the white dwarf companion found it has a mass of 0.172 solar masses. Analysis of the orbital parameters and component masses showed the system's orbital decay matches predictions from general relativity.
3) The high pulsar mass and compact orbit make this a sensitive laboratory for testing strong-field gravity. The observed orbital decay is consistent with general relativity, supporting its validity even in extreme gravity regimes not previously tested. This has implications
1. The spectrum of the bright Kuiper Belt object 2005 FY9 is dominated by methane absorption features in the near-infrared region. However, the methane absorption lines are significantly broader than seen on any other solar system body, indicating unusually long optical path lengths through methane grains on 2005 FY9, estimated to be around 1 cm in size.
2. In addition to methane, the spectrum also shows clear evidence for ethane, which is expected to form from UV photolysis of methane. No evidence is found for nitrogen or carbon monoxide, both known to be present on Pluto.
3. The differences between 2005 FY9's spectrum and those of Pluto and 2003 UB313 are suggested
Investigating the nuclear_activity_of_ngc1672Sérgio Sacani
1) The document investigates the nuclear activity of the barred spiral galaxy NGC 1672 using X-ray data from Chandra and XMM-Newton, as well as optical data from Hubble and infrared data from Spitzer.
2) It detects 28 X-ray sources within NGC 1672, with nine being ultraluminous X-ray sources located at the ends of the galaxy's bar.
3) For the first time, it shows NGC 1672 possesses a hard nuclear X-ray source surrounded by an X-ray bright circumnuclear star-forming ring, which dominates the central X-ray emission of the galaxy. The data indicates the nuclear source is a low-luminosity active gal
The physical conditions_in_a_pre_super_star_cluster_molecular_cloud_in_the_an...Sérgio Sacani
The document summarizes a study of an extreme molecular cloud in the Antennae galaxies that has properties consistent with forming a globular cluster. ALMA observations reveal a cloud with a radius of 24 pc and mass greater than 5 million solar masses. While capable of forming a globular cluster, a lack of associated thermal radio emission indicates star formation has not yet begun to alter the environment, suggesting the cloud is in an early stage of evolution. For the cloud to be confined as observed, an external pressure over 10,000 times greater than typical interstellar pressure is required, supporting the theory that high pressures are needed to form globular clusters in extreme environments like mergers.
This pilot survey used modest aperture telescopes to image 8 nearby spiral galaxies in order to search for stellar tidal streams. Ultra-deep imaging revealed 6 previously undetected extensive (up to 30 kpc) stellar structures likely from tidally disrupted satellites. A diversity of tidal feature morphologies was found, including great circle-like streams, remote shells, and jets emerging from disks. Simulations predict tidal debris should be common and match the observed variety, providing evidence minor mergers have shaped disk galaxies since z=1.
Too much pasta_for_pulsars_to_spin_downSérgio Sacani
This document summarizes a study investigating why no isolated X-ray pulsars have been observed with spin periods longer than 12 seconds. The researchers suggest this is due to a highly resistive layer in the inner crust of neutron stars, which is expected to be in a state called "nuclear pasta". Nuclear pasta has an irregular structure that increases electrical resistivity, limiting the spin-down of pulsars. Modeling the long-term magnetic field evolution incorporating a resistive nuclear pasta layer successfully reproduced the observed 12 second period limit. The results provide the first potential observational evidence for the existence of nuclear pasta in neutron star crusts.
X ray emission-from_strongly_asymmetric_circumstellar_material_in_the_remnant...Sérgio Sacani
This document summarizes a study of X-ray emission from circumstellar material (CSM) in the remnant of Kepler's supernova. The researchers used a statistical technique to isolate X-ray emission from CSM versus ejecta based on spectral characteristics. They found that most CSM is distributed along the bright north rim, but substantial amounts are also projected against the center, indicating a disk-like distribution of CSM from the progenitor system before the supernova. Hydrodynamic simulations support an AGB star companion as the origin of the asymmetric CSM. Quantitative analysis of magnesium emission identifies CSM and requires Kepler to have originated from a close binary system.
This document summarizes the discovery of a stellar tidal stream around the nearby dwarf galaxy NGC 4449 using deep imaging. The stream implies a massive dwarf spheroidal progenitor that is disrupting and depositing stars into NGC 4449's halo. The luminosity ratio between the two galaxies is around 1:50, but their dynamical mass ratio including dark matter may be 1:10-1:5. This "stealth merger" suggests that satellite accretion can significantly build up stellar halos of low-mass galaxies and possibly trigger their starbursts.
A study of_the_dark_core_in_a520_with_hubble_space_telescope_the_mystery_deepensSérgio Sacani
This document summarizes a study using Hubble Space Telescope images to further examine the dark core detected in the galaxy cluster A520 in a previous study using ground-based data. The HST data provides over 3 times as many galaxies that can be used for weak lensing analysis, significantly enhancing the mass reconstruction. The analysis confirms the detection of the dark core coincident with the X-ray peak but lacking luminous galaxies at over 10 sigma significance. Several substructures are also clearly detected, including a new one labeled P5. However, the comparison of dark matter, hot gas, and galaxy distributions remains peculiar compared to other merging clusters.
1) Newly-born pulsars offer favorable conditions for accelerating heavy nuclei like iron to ultrahigh energies via unipolar induction. However, these nuclei must escape the surrounding dense supernova envelope.
2) The paper analytically and numerically examines the escape of ultrahigh energy cosmic rays (UHECRs) from supernova envelopes. It finds that at early times when protons could reach energies above 10^20 eV, the envelope prevents their escape. However, heavier iron nuclei can still reach the highest observed energies at later times when the envelope has thinned.
3) The authors conclude that a small fraction (0.01%) of extragalactic rapidly rotating young pulsars embedded in supernov
Asymmetrical tidal tails of open star clusters: stars crossing their cluster’...Sérgio Sacani
The document discusses asymmetrical tidal tails observed around five open star clusters, which challenges Newtonian gravity. It summarizes how tidal tails form as stars escape clusters due to energy equipartition. Observations of the Hyades, Praesepe, Coma Berenices, COIN-Gaia 13, and NGC 752 clusters found more stars in the leading tidal tails within 50 pc of the clusters. Simulations show that in Newtonian gravity, tidal tails should be symmetrical, but asymmetries can arise in Milgromian dynamics. Future work is needed to better map tidal tails and develop Milgromian simulations.
The document summarizes a study that presents the first panoramic view of the outskirts of the Milky Way analogue galaxy NGC 891 based on ground-based imaging. The key findings are:
1) Numerous stellar overdensities and coherent substructures were detected out to distances of ~90 kpc from the center of NGC 891, including a giant stream looping around the galaxy up to ~50 kpc.
2) A previously undetected large, flat, and thick cocoon-like stellar structure was found surrounding the bulge and disk of NGC 891 out to radial distances of ~40 kpc and vertical distances of ~15 kpc.
3) The properties of the substructures suggest NGC
Kinematics and simulations_of_the_stellar_stream_in_the_halo_of_the_umbrella_...Sérgio Sacani
This document summarizes a study of the stellar stream and substructures around the Umbrella Galaxy (NGC 4651). Deep imaging and spectroscopy were used to characterize the properties and kinematics of the stream. Tracer objects like globular clusters and planetary nebulae were identified and found to delineate a kinematically cold feature in position-velocity space. Dynamical modeling suggests the stream originated from the tidal disruption of a dwarf galaxy on a highly eccentric orbit about 6-10 billion years ago. This work demonstrates the feasibility of using discrete tracers to recover the kinematics and model the dynamics of low surface brightness stellar streams around distant galaxies.
This document summarizes observations of the W49 giant molecular cloud (GMC) using the PMO 14m telescope and the Submillimeter Array (SMA). The PMO observations mapped the entire GMC in various molecular lines at scales up to 113 pc, while the SMA mosaic mapped the central star-forming region W49N at scales down to 0.5 pc. The observations are used to derive the mass structure of the GMC across all scales. The main findings are that the W49 GMC has a total gas mass of 1.1 million solar masses within 60 pc and 2x10^5 solar masses within 6 pc. The mass is distributed in a hierarchical network of filaments converging toward the central
1) The growth of the first stars was halted by ultraviolet radiation feedback from the stars themselves.
2) Radiation from the protostar evaporated the circumstellar accretion disk when the star's mass reached 43 times the mass of the Sun.
3) These massive primordial stars may help explain the lack of pair-instability supernovae signatures in ancient metal-poor stars.
1) The Fermi bubbles are giant gamma-ray emitting structures extending above and below the galactic center.
2) The bubbles may have been formed by periodic capture of stars by the supermassive black hole at the galactic center, releasing energy of around 3x10^52 ergs per capture.
3) This energy injection could produce very hot plasma, accelerating electrons that produce radio and gamma-ray emission through synchrotron radiation and inverse Compton scattering.
Three newly discovered_globular_clusters_in_ngc6822Sérgio Sacani
This document describes the discovery of three new globular clusters in the Local Group dwarf irregular galaxy NGC 6822. The authors present optical photometry of the clusters from archival CFHT/Megacam data. Two clusters are luminous and compact, while the third is a lower luminosity diffuse cluster. The positions of the new clusters are consistent with the linear alignment of previously known clusters in NGC 6822. Studying globular cluster systems in dwarf galaxies provides insights into the formation of larger galaxies through accretion and mergers.
A structure in_the_early_universe_at_z_1_3_that_exceeds_the_homogeneity_scale...Sérgio Sacani
This document summarizes the discovery of an exceptionally large quasar group (LQG) in the Sloan Digital Sky Survey (SDSS) DR7 quasar catalog, designated U1.27. The LQG, termed the "Huge-LQG", contains 73 quasars within a redshift range of 1.1742 to 1.3713, and has a characteristic size of approximately 500 megaparsecs (Mpc), making it the largest LQG found to date. Its size suggests incompatibility with the expected homogeneity scale in the standard cosmological model and challenges the cosmological principle.
An exceptional xray_view_of_the_young_open_cluster_ngc6231Sérgio Sacani
The XMM-Newton satellite observed the young open star cluster NGC 6231 for a total of 180 kiloseconds, providing an unprecedented X-ray view. Around 600 point-like X-ray sources were detected, including many early-type O stars. The data helped refine the relationship between X-ray and bolometric luminosities for O stars, finding less scatter than previously thought, with the main exception being X-ray emission from wind interactions in binary systems. The data also provided insight into the low-mass pre-main sequence star population and star formation history of the cluster.
A 100 parsec elliptical and twisted ring of cold and dense molecular clouds r...Sérgio Sacani
The document summarizes observations from the Herschel satellite that reveal a 100-parsec elliptical and twisted ring of cold, dense molecular clouds orbiting the Galactic Center. The ring has a mass of about 3 million solar masses and semi-major axes of 100 and 60 parsecs. Its major axis is inclined 40 degrees to the plane of the sky and perpendicular to the Galactic Bar. The ring appears to trace stable x2 orbits predicted for the barred Galactic potential.
This document summarizes the results of a deep near-infrared survey of the Carina Nebula complex using the HAWK-I instrument on the VLT. The survey imaged an area of 0.36 square degrees down to magnitudes of J=23, H=22, and Ks=21, detecting over 600,000 infrared sources. Color-magnitude diagrams of the sources were analyzed to determine properties of the low-mass stellar population such as ages and masses. The survey found that about 3200 sources have masses above 1 solar mass, consistent with expectations from the initial mass function. It also found that about half of the young stars in Carina are in a widely distributed, non-clustered configuration. Six
This document summarizes a study of the galaxy NGC 3801 using panchromatic data from radio, ultraviolet, optical and infrared wavelengths. The study finds that NGC 3801 is a post-merger early-type galaxy that shows evidence of recent star formation in spiral wisps as well as feedback from active galactic nucleus jets. Specifically, the galaxy has a young radio jet that has not yet impacted the outer gaseous regions, but will likely quench star formation within the next 10 million years and further transform the galaxy into a red elliptical. The multi-wavelength data reveal complex gas and dust structures that may be due to a kinematically decoupled core or warped disk.
Similar to Catch me if_you_can_is_there_a_runaway_mass_black_hole_in_the_orion_nebula_cluster (20)
Compositions of iron-meteorite parent bodies constrainthe structure of the pr...Sérgio Sacani
Magmatic iron-meteorite parent bodies are the earliest planetesimals in the Solar System,and they preserve information about conditions and planet-forming processes in thesolar nebula. In this study, we include comprehensive elemental compositions andfractional-crystallization modeling for iron meteorites from the cores of five differenti-ated asteroids from the inner Solar System. Together with previous results of metalliccores from the outer Solar System, we conclude that asteroidal cores from the outerSolar System have smaller sizes, elevated siderophile-element abundances, and simplercrystallization processes than those from the inner Solar System. These differences arerelated to the formation locations of the parent asteroids because the solar protoplane-tary disk varied in redox conditions, elemental distributions, and dynamics at differentheliocentric distances. Using highly siderophile-element data from iron meteorites, wereconstruct the distribution of calcium-aluminum-rich inclusions (CAIs) across theprotoplanetary disk within the first million years of Solar-System history. CAIs, the firstsolids to condense in the Solar System, formed close to the Sun. They were, however,concentrated within the outer disk and depleted within the inner disk. Future modelsof the structure and evolution of the protoplanetary disk should account for this dis-tribution pattern of CAIs.
Signatures of wave erosion in Titan’s coastsSérgio Sacani
The shorelines of Titan’s hydrocarbon seas trace flooded erosional landforms such as river valleys; however, it isunclear whether coastal erosion has subsequently altered these shorelines. Spacecraft observations and theo-retical models suggest that wind may cause waves to form on Titan’s seas, potentially driving coastal erosion,but the observational evidence of waves is indirect, and the processes affecting shoreline evolution on Titanremain unknown. No widely accepted framework exists for using shoreline morphology to quantitatively dis-cern coastal erosion mechanisms, even on Earth, where the dominant mechanisms are known. We combinelandscape evolution models with measurements of shoreline shape on Earth to characterize how differentcoastal erosion mechanisms affect shoreline morphology. Applying this framework to Titan, we find that theshorelines of Titan’s seas are most consistent with flooded landscapes that subsequently have been eroded bywaves, rather than a uniform erosional process or no coastal erosion, particularly if wave growth saturates atfetch lengths of tens of kilometers.
SDSS1335+0728: The awakening of a ∼ 106M⊙ black hole⋆Sérgio Sacani
Context. The early-type galaxy SDSS J133519.91+072807.4 (hereafter SDSS1335+0728), which had exhibited no prior optical variations during the preceding two decades, began showing significant nuclear variability in the Zwicky Transient Facility (ZTF) alert stream from December 2019 (as ZTF19acnskyy). This variability behaviour, coupled with the host-galaxy properties, suggests that SDSS1335+0728 hosts a ∼ 106M⊙ black hole (BH) that is currently in the process of ‘turning on’. Aims. We present a multi-wavelength photometric analysis and spectroscopic follow-up performed with the aim of better understanding the origin of the nuclear variations detected in SDSS1335+0728. Methods. We used archival photometry (from WISE, 2MASS, SDSS, GALEX, eROSITA) and spectroscopic data (from SDSS and LAMOST) to study the state of SDSS1335+0728 prior to December 2019, and new observations from Swift, SOAR/Goodman, VLT/X-shooter, and Keck/LRIS taken after its turn-on to characterise its current state. We analysed the variability of SDSS1335+0728 in the X-ray/UV/optical/mid-infrared range, modelled its spectral energy distribution prior to and after December 2019, and studied the evolution of its UV/optical spectra. Results. From our multi-wavelength photometric analysis, we find that: (a) since 2021, the UV flux (from Swift/UVOT observations) is four times brighter than the flux reported by GALEX in 2004; (b) since June 2022, the mid-infrared flux has risen more than two times, and the W1−W2 WISE colour has become redder; and (c) since February 2024, the source has begun showing X-ray emission. From our spectroscopic follow-up, we see that (i) the narrow emission line ratios are now consistent with a more energetic ionising continuum; (ii) broad emission lines are not detected; and (iii) the [OIII] line increased its flux ∼ 3.6 years after the first ZTF alert, which implies a relatively compact narrow-line-emitting region. Conclusions. We conclude that the variations observed in SDSS1335+0728 could be either explained by a ∼ 106M⊙ AGN that is just turning on or by an exotic tidal disruption event (TDE). If the former is true, SDSS1335+0728 is one of the strongest cases of an AGNobserved in the process of activating. If the latter were found to be the case, it would correspond to the longest and faintest TDE ever observed (or another class of still unknown nuclear transient). Future observations of SDSS1335+0728 are crucial to further understand its behaviour. Key words. galaxies: active– accretion, accretion discs– galaxies: individual: SDSS J133519.91+072807.4
Discovery of An Apparent Red, High-Velocity Type Ia Supernova at 𝐳 = 2.9 wi...Sérgio Sacani
We present the JWST discovery of SN 2023adsy, a transient object located in a host galaxy JADES-GS
+
53.13485
−
27.82088
with a host spectroscopic redshift of
2.903
±
0.007
. The transient was identified in deep James Webb Space Telescope (JWST)/NIRCam imaging from the JWST Advanced Deep Extragalactic Survey (JADES) program. Photometric and spectroscopic followup with NIRCam and NIRSpec, respectively, confirm the redshift and yield UV-NIR light-curve, NIR color, and spectroscopic information all consistent with a Type Ia classification. Despite its classification as a likely SN Ia, SN 2023adsy is both fairly red (
�
(
�
−
�
)
∼
0.9
) despite a host galaxy with low-extinction and has a high Ca II velocity (
19
,
000
±
2
,
000
km/s) compared to the general population of SNe Ia. While these characteristics are consistent with some Ca-rich SNe Ia, particularly SN 2016hnk, SN 2023adsy is intrinsically brighter than the low-
�
Ca-rich population. Although such an object is too red for any low-
�
cosmological sample, we apply a fiducial standardization approach to SN 2023adsy and find that the SN 2023adsy luminosity distance measurement is in excellent agreement (
≲
1
�
) with
Λ
CDM. Therefore unlike low-
�
Ca-rich SNe Ia, SN 2023adsy is standardizable and gives no indication that SN Ia standardized luminosities change significantly with redshift. A larger sample of distant SNe Ia is required to determine if SN Ia population characteristics at high-
�
truly diverge from their low-
�
counterparts, and to confirm that standardized luminosities nevertheless remain constant with redshift.
Evidence of Jet Activity from the Secondary Black Hole in the OJ 287 Binary S...Sérgio Sacani
Wereport the study of a huge optical intraday flare on 2021 November 12 at 2 a.m. UT in the blazar OJ287. In the binary black hole model, it is associated with an impact of the secondary black hole on the accretion disk of the primary. Our multifrequency observing campaign was set up to search for such a signature of the impact based on a prediction made 8 yr earlier. The first I-band results of the flare have already been reported by Kishore et al. (2024). Here we combine these data with our monitoring in the R-band. There is a big change in the R–I spectral index by 1.0 ±0.1 between the normal background and the flare, suggesting a new component of radiation. The polarization variation during the rise of the flare suggests the same. The limits on the source size place it most reasonably in the jet of the secondary BH. We then ask why we have not seen this phenomenon before. We show that OJ287 was never before observed with sufficient sensitivity on the night when the flare should have happened according to the binary model. We also study the probability that this flare is just an oversized example of intraday variability using the Krakow data set of intense monitoring between 2015 and 2023. We find that the occurrence of a flare of this size and rapidity is unlikely. In machine-readable Tables 1 and 2, we give the full orbit-linked historical light curve of OJ287 as well as the dense monitoring sample of Krakow.
Candidate young stellar objects in the S-cluster: Kinematic analysis of a sub...Sérgio Sacani
Context. The observation of several L-band emission sources in the S cluster has led to a rich discussion of their nature. However, a definitive answer to the classification of the dusty objects requires an explanation for the detection of compact Doppler-shifted Brγ emission. The ionized hydrogen in combination with the observation of mid-infrared L-band continuum emission suggests that most of these sources are embedded in a dusty envelope. These embedded sources are part of the S-cluster, and their relationship to the S-stars is still under debate. To date, the question of the origin of these two populations has been vague, although all explanations favor migration processes for the individual cluster members. Aims. This work revisits the S-cluster and its dusty members orbiting the supermassive black hole SgrA* on bound Keplerian orbits from a kinematic perspective. The aim is to explore the Keplerian parameters for patterns that might imply a nonrandom distribution of the sample. Additionally, various analytical aspects are considered to address the nature of the dusty sources. Methods. Based on the photometric analysis, we estimated the individual H−K and K−L colors for the source sample and compared the results to known cluster members. The classification revealed a noticeable contrast between the S-stars and the dusty sources. To fit the flux-density distribution, we utilized the radiative transfer code HYPERION and implemented a young stellar object Class I model. We obtained the position angle from the Keplerian fit results; additionally, we analyzed the distribution of the inclinations and the longitudes of the ascending node. Results. The colors of the dusty sources suggest a stellar nature consistent with the spectral energy distribution in the near and midinfrared domains. Furthermore, the evaporation timescales of dusty and gaseous clumps in the vicinity of SgrA* are much shorter ( 2yr) than the epochs covered by the observations (≈15yr). In addition to the strong evidence for the stellar classification of the D-sources, we also find a clear disk-like pattern following the arrangements of S-stars proposed in the literature. Furthermore, we find a global intrinsic inclination for all dusty sources of 60 ± 20◦, implying a common formation process. Conclusions. The pattern of the dusty sources manifested in the distribution of the position angles, inclinations, and longitudes of the ascending node strongly suggests two different scenarios: the main-sequence stars and the dusty stellar S-cluster sources share a common formation history or migrated with a similar formation channel in the vicinity of SgrA*. Alternatively, the gravitational influence of SgrA* in combination with a massive perturber, such as a putative intermediate mass black hole in the IRS 13 cluster, forces the dusty objects and S-stars to follow a particular orbital arrangement. Key words. stars: black holes– stars: formation– Galaxy: center– galaxies: star formation
JAMES WEBB STUDY THE MASSIVE BLACK HOLE SEEDSSérgio Sacani
The pathway(s) to seeding the massive black holes (MBHs) that exist at the heart of galaxies in the present and distant Universe remains an unsolved problem. Here we categorise, describe and quantitatively discuss the formation pathways of both light and heavy seeds. We emphasise that the most recent computational models suggest that rather than a bimodal-like mass spectrum between light and heavy seeds with light at one end and heavy at the other that instead a continuum exists. Light seeds being more ubiquitous and the heavier seeds becoming less and less abundant due the rarer environmental conditions required for their formation. We therefore examine the different mechanisms that give rise to different seed mass spectrums. We show how and why the mechanisms that produce the heaviest seeds are also among the rarest events in the Universe and are hence extremely unlikely to be the seeds for the vast majority of the MBH population. We quantify, within the limits of the current large uncertainties in the seeding processes, the expected number densities of the seed mass spectrum. We argue that light seeds must be at least 103 to 105 times more numerous than heavy seeds to explain the MBH population as a whole. Based on our current understanding of the seed population this makes heavy seeds (Mseed > 103 M⊙) a significantly more likely pathway given that heavy seeds have an abundance pattern than is close to and likely in excess of 10−4 compared to light seeds. Finally, we examine the current state-of-the-art in numerical calculations and recent observations and plot a path forward for near-future advances in both domains.
Anti-Universe And Emergent Gravity and the Dark UniverseSérgio Sacani
Recent theoretical progress indicates that spacetime and gravity emerge together from the entanglement structure of an underlying microscopic theory. These ideas are best understood in Anti-de Sitter space, where they rely on the area law for entanglement entropy. The extension to de Sitter space requires taking into account the entropy and temperature associated with the cosmological horizon. Using insights from string theory, black hole physics and quantum information theory we argue that the positive dark energy leads to a thermal volume law contribution to the entropy that overtakes the area law precisely at the cosmological horizon. Due to the competition between area and volume law entanglement the microscopic de Sitter states do not thermalise at sub-Hubble scales: they exhibit memory effects in the form of an entropy displacement caused by matter. The emergent laws of gravity contain an additional ‘dark’ gravitational force describing the ‘elastic’ response due to the entropy displacement. We derive an estimate of the strength of this extra force in terms of the baryonic mass, Newton’s constant and the Hubble acceleration scale a0 = cH0, and provide evidence for the fact that this additional ‘dark gravity force’ explains the observed phenomena in galaxies and clusters currently attributed to dark matter.
The binding of cosmological structures by massless topological defectsSérgio Sacani
Assuming spherical symmetry and weak field, it is shown that if one solves the Poisson equation or the Einstein field
equations sourced by a topological defect, i.e. a singularity of a very specific form, the result is a localized gravitational
field capable of driving flat rotation (i.e. Keplerian circular orbits at a constant speed for all radii) of test masses on a thin
spherical shell without any underlying mass. Moreover, a large-scale structure which exploits this solution by assembling
concentrically a number of such topological defects can establish a flat stellar or galactic rotation curve, and can also deflect
light in the same manner as an equipotential (isothermal) sphere. Thus, the need for dark matter or modified gravity theory is
mitigated, at least in part.
EWOCS-I: The catalog of X-ray sources in Westerlund 1 from the Extended Weste...Sérgio Sacani
Context. With a mass exceeding several 104 M⊙ and a rich and dense population of massive stars, supermassive young star clusters
represent the most massive star-forming environment that is dominated by the feedback from massive stars and gravitational interactions
among stars.
Aims. In this paper we present the Extended Westerlund 1 and 2 Open Clusters Survey (EWOCS) project, which aims to investigate
the influence of the starburst environment on the formation of stars and planets, and on the evolution of both low and high mass stars.
The primary targets of this project are Westerlund 1 and 2, the closest supermassive star clusters to the Sun.
Methods. The project is based primarily on recent observations conducted with the Chandra and JWST observatories. Specifically,
the Chandra survey of Westerlund 1 consists of 36 new ACIS-I observations, nearly co-pointed, for a total exposure time of 1 Msec.
Additionally, we included 8 archival Chandra/ACIS-S observations. This paper presents the resulting catalog of X-ray sources within
and around Westerlund 1. Sources were detected by combining various existing methods, and photon extraction and source validation
were carried out using the ACIS-Extract software.
Results. The EWOCS X-ray catalog comprises 5963 validated sources out of the 9420 initially provided to ACIS-Extract, reaching a
photon flux threshold of approximately 2 × 10−8 photons cm−2
s
−1
. The X-ray sources exhibit a highly concentrated spatial distribution,
with 1075 sources located within the central 1 arcmin. We have successfully detected X-ray emissions from 126 out of the 166 known
massive stars of the cluster, and we have collected over 71 000 photons from the magnetar CXO J164710.20-455217.
The debris of the ‘last major merger’ is dynamically youngSérgio Sacani
The Milky Way’s (MW) inner stellar halo contains an [Fe/H]-rich component with highly eccentric orbits, often referred to as the
‘last major merger.’ Hypotheses for the origin of this component include Gaia-Sausage/Enceladus (GSE), where the progenitor
collided with the MW proto-disc 8–11 Gyr ago, and the Virgo Radial Merger (VRM), where the progenitor collided with the
MW disc within the last 3 Gyr. These two scenarios make different predictions about observable structure in local phase space,
because the morphology of debris depends on how long it has had to phase mix. The recently identified phase-space folds in Gaia
DR3 have positive caustic velocities, making them fundamentally different than the phase-mixed chevrons found in simulations
at late times. Roughly 20 per cent of the stars in the prograde local stellar halo are associated with the observed caustics. Based
on a simple phase-mixing model, the observed number of caustics are consistent with a merger that occurred 1–2 Gyr ago.
We also compare the observed phase-space distribution to FIRE-2 Latte simulations of GSE-like mergers, using a quantitative
measurement of phase mixing (2D causticality). The observed local phase-space distribution best matches the simulated data
1–2 Gyr after collision, and certainly not later than 3 Gyr. This is further evidence that the progenitor of the ‘last major merger’
did not collide with the MW proto-disc at early times, as is thought for the GSE, but instead collided with the MW disc within
the last few Gyr, consistent with the body of work surrounding the VRM.
Observation of Io’s Resurfacing via Plume Deposition Using Ground-based Adapt...Sérgio Sacani
Since volcanic activity was first discovered on Io from Voyager images in 1979, changes
on Io’s surface have been monitored from both spacecraft and ground-based telescopes.
Here, we present the highest spatial resolution images of Io ever obtained from a groundbased telescope. These images, acquired by the SHARK-VIS instrument on the Large
Binocular Telescope, show evidence of a major resurfacing event on Io’s trailing hemisphere. When compared to the most recent spacecraft images, the SHARK-VIS images
show that a plume deposit from a powerful eruption at Pillan Patera has covered part
of the long-lived Pele plume deposit. Although this type of resurfacing event may be common on Io, few have been detected due to the rarity of spacecraft visits and the previously low spatial resolution available from Earth-based telescopes. The SHARK-VIS instrument ushers in a new era of high resolution imaging of Io’s surface using adaptive
optics at visible wavelengths.
Earliest Galaxies in the JADES Origins Field: Luminosity Function and Cosmic ...Sérgio Sacani
We characterize the earliest galaxy population in the JADES Origins Field (JOF), the deepest
imaging field observed with JWST. We make use of the ancillary Hubble optical images (5 filters
spanning 0.4−0.9µm) and novel JWST images with 14 filters spanning 0.8−5µm, including 7 mediumband filters, and reaching total exposure times of up to 46 hours per filter. We combine all our data
at > 2.3µm to construct an ultradeep image, reaching as deep as ≈ 31.4 AB mag in the stack and
30.3-31.0 AB mag (5σ, r = 0.1” circular aperture) in individual filters. We measure photometric
redshifts and use robust selection criteria to identify a sample of eight galaxy candidates at redshifts
z = 11.5 − 15. These objects show compact half-light radii of R1/2 ∼ 50 − 200pc, stellar masses of
M⋆ ∼ 107−108M⊙, and star-formation rates of SFR ∼ 0.1−1 M⊙ yr−1
. Our search finds no candidates
at 15 < z < 20, placing upper limits at these redshifts. We develop a forward modeling approach to
infer the properties of the evolving luminosity function without binning in redshift or luminosity that
marginalizes over the photometric redshift uncertainty of our candidate galaxies and incorporates the
impact of non-detections. We find a z = 12 luminosity function in good agreement with prior results,
and that the luminosity function normalization and UV luminosity density decline by a factor of ∼ 2.5
from z = 12 to z = 14. We discuss the possible implications of our results in the context of theoretical
models for evolution of the dark matter halo mass function.
THE IMPORTANCE OF MARTIAN ATMOSPHERE SAMPLE RETURN.Sérgio Sacani
The return of a sample of near-surface atmosphere from Mars would facilitate answers to several first-order science questions surrounding the formation and evolution of the planet. One of the important aspects of terrestrial planet formation in general is the role that primary atmospheres played in influencing the chemistry and structure of the planets and their antecedents. Studies of the martian atmosphere can be used to investigate the role of a primary atmosphere in its history. Atmosphere samples would also inform our understanding of the near-surface chemistry of the planet, and ultimately the prospects for life. High-precision isotopic analyses of constituent gases are needed to address these questions, requiring that the analyses are made on returned samples rather than in situ.
Multi-source connectivity as the driver of solar wind variability in the heli...Sérgio Sacani
The ambient solar wind that flls the heliosphere originates from multiple
sources in the solar corona and is highly structured. It is often described
as high-speed, relatively homogeneous, plasma streams from coronal
holes and slow-speed, highly variable, streams whose source regions are
under debate. A key goal of ESA/NASA’s Solar Orbiter mission is to identify
solar wind sources and understand what drives the complexity seen in the
heliosphere. By combining magnetic feld modelling and spectroscopic
techniques with high-resolution observations and measurements, we show
that the solar wind variability detected in situ by Solar Orbiter in March
2022 is driven by spatio-temporal changes in the magnetic connectivity to
multiple sources in the solar atmosphere. The magnetic feld footpoints
connected to the spacecraft moved from the boundaries of a coronal hole
to one active region (12961) and then across to another region (12957). This
is refected in the in situ measurements, which show the transition from fast
to highly Alfvénic then to slow solar wind that is disrupted by the arrival of
a coronal mass ejection. Our results describe solar wind variability at 0.5 au
but are applicable to near-Earth observatories.
Gliese 12 b: A Temperate Earth-sized Planet at 12 pc Ideal for Atmospheric Tr...Sérgio Sacani
Recent discoveries of Earth-sized planets transiting nearby M dwarfs have made it possible to characterize the
atmospheres of terrestrial planets via follow-up spectroscopic observations. However, the number of such planets
receiving low insolation is still small, limiting our ability to understand the diversity of the atmospheric
composition and climates of temperate terrestrial planets. We report the discovery of an Earth-sized planet
transiting the nearby (12 pc) inactive M3.0 dwarf Gliese 12 (TOI-6251) with an orbital period (Porb) of 12.76 days.
The planet, Gliese 12 b, was initially identified as a candidate with an ambiguous Porb from TESS data. We
confirmed the transit signal and Porb using ground-based photometry with MuSCAT2 and MuSCAT3, and
validated the planetary nature of the signal using high-resolution images from Gemini/NIRI and Keck/NIRC2 as
well as radial velocity (RV) measurements from the InfraRed Doppler instrument on the Subaru 8.2 m telescope
and from CARMENES on the CAHA 3.5 m telescope. X-ray observations with XMM-Newton showed the host
star is inactive, with an X-ray-to-bolometric luminosity ratio of log 5.7 L L X bol » - . Joint analysis of the light
curves and RV measurements revealed that Gliese 12 b has a radius of 0.96 ± 0.05 R⊕,a3σ mass upper limit of
3.9 M⊕, and an equilibrium temperature of 315 ± 6 K assuming zero albedo. The transmission spectroscopy metric
(TSM) value of Gliese 12 b is close to the TSM values of the TRAPPIST-1 planets, adding Gliese 12 b to the small
list of potentially terrestrial, temperate planets amenable to atmospheric characterization with JWST.
Gliese 12 b, a temperate Earth-sized planet at 12 parsecs discovered with TES...Sérgio Sacani
We report on the discovery of Gliese 12 b, the nearest transiting temperate, Earth-sized planet found to date. Gliese 12 is a
bright (V = 12.6 mag, K = 7.8 mag) metal-poor M4V star only 12.162 ± 0.005 pc away from the Solar system with one of the
lowest stellar activity levels known for M-dwarfs. A planet candidate was detected by TESS based on only 3 transits in sectors
42, 43, and 57, with an ambiguity in the orbital period due to observational gaps. We performed follow-up transit observations
with CHEOPS and ground-based photometry with MINERVA-Australis, SPECULOOS, and Purple Mountain Observatory,
as well as further TESS observations in sector 70. We statistically validate Gliese 12 b as a planet with an orbital period of
12.76144 ± 0.00006 d and a radius of 1.0 ± 0.1 R⊕, resulting in an equilibrium temperature of ∼315 K. Gliese 12 b has excellent
future prospects for precise mass measurement, which may inform how planetary internal structure is affected by the stellar
compositional environment. Gliese 12 b also represents one of the best targets to study whether Earth-like planets orbiting cool
stars can retain their atmospheres, a crucial step to advance our understanding of habitability on Earth and across the galaxy.
The importance of continents, oceans and plate tectonics for the evolution of...Sérgio Sacani
Within the uncertainties of involved astronomical and biological parameters, the Drake Equation
typically predicts that there should be many exoplanets in our galaxy hosting active, communicative
civilizations (ACCs). These optimistic calculations are however not supported by evidence, which is
often referred to as the Fermi Paradox. Here, we elaborate on this long-standing enigma by showing
the importance of planetary tectonic style for biological evolution. We summarize growing evidence
that a prolonged transition from Mesoproterozoic active single lid tectonics (1.6 to 1.0 Ga) to modern
plate tectonics occurred in the Neoproterozoic Era (1.0 to 0.541 Ga), which dramatically accelerated
emergence and evolution of complex species. We further suggest that both continents and oceans
are required for ACCs because early evolution of simple life must happen in water but late evolution
of advanced life capable of creating technology must happen on land. We resolve the Fermi Paradox
(1) by adding two additional terms to the Drake Equation: foc
(the fraction of habitable exoplanets
with significant continents and oceans) and fpt
(the fraction of habitable exoplanets with significant
continents and oceans that have had plate tectonics operating for at least 0.5 Ga); and (2) by
demonstrating that the product of foc
and fpt
is very small (< 0.00003–0.002). We propose that the lack
of evidence for ACCs reflects the scarcity of long-lived plate tectonics and/or continents and oceans on
exoplanets with primitive life.
A Giant Impact Origin for the First Subduction on EarthSérgio Sacani
Hadean zircons provide a potential record of Earth's earliest subduction 4.3 billion years ago. Itremains enigmatic how subduction could be initiated so soon after the presumably Moon‐forming giant impact(MGI). Earlier studies found an increase in Earth's core‐mantle boundary (CMB) temperature due to theaccumulation of the impactor's core, and our recent work shows Earth's lower mantle remains largely solid, withsome of the impactor's mantle potentially surviving as the large low‐shear velocity provinces (LLSVPs). Here,we show that a hot post‐impact CMB drives the initiation of strong mantle plumes that can induce subductioninitiation ∼200 Myr after the MGI. 2D and 3D thermomechanical computations show that a high CMBtemperature is the primary factor triggering early subduction, with enrichment of heat‐producing elements inLLSVPs as another potential factor. The models link the earliest subduction to the MGI with implications forunderstanding the diverse tectonic regimes of rocky planets.
Climate extremes likely to drive land mammal extinction during next supercont...Sérgio Sacani
Mammals have dominated Earth for approximately 55 Myr thanks to their
adaptations and resilience to warming and cooling during the Cenozoic. All
life will eventually perish in a runaway greenhouse once absorbed solar
radiation exceeds the emission of thermal radiation in several billions of
years. However, conditions rendering the Earth naturally inhospitable to
mammals may develop sooner because of long-term processes linked to
plate tectonics (short-term perturbations are not considered here). In
~250 Myr, all continents will converge to form Earth’s next supercontinent,
Pangea Ultima. A natural consequence of the creation and decay of Pangea
Ultima will be extremes in pCO2 due to changes in volcanic rifting and
outgassing. Here we show that increased pCO2, solar energy (F⨀;
approximately +2.5% W m−2 greater than today) and continentality (larger
range in temperatures away from the ocean) lead to increasing warming
hostile to mammalian life. We assess their impact on mammalian
physiological limits (dry bulb, wet bulb and Humidex heat stress indicators)
as well as a planetary habitability index. Given mammals’ continued survival,
predicted background pCO2 levels of 410–816 ppm combined with increased
F⨀ will probably lead to a climate tipping point and their mass extinction.
The results also highlight how global landmass configuration, pCO2 and F⨀
play a critical role in planetary habitability.
Climate extremes likely to drive land mammal extinction during next supercont...
Catch me if_you_can_is_there_a_runaway_mass_black_hole_in_the_orion_nebula_cluster
1. ApJ accepted, September 12, 2012
Preprint typeset using L TEX style emulateapj v. 12/16/11
A
CATCH ME IF YOU CAN: IS THERE A ‘RUNAWAY-MASS’ BLACK HOLE
IN THE ORION NEBULA CLUSTER?
Ladislav Šubr1 , Pavel Kroupa2 and Holger Baumgardt3
ApJ accepted, September 12, 2012
Abstract
We investigate the dynamical evolution of the Orion Nebula Cluster (ONC) by means of direct
arXiv:1209.2114v1 [astro-ph.GA] 10 Sep 2012
N -body integrations. A large fraction of residual gas was probably expelled when the ONC formed,
so we assume that the ONC was much more compact when it formed compared to its current size,
in agreement with the embedded cluster radius-mass relation from Marks & Kroupa (2012). Hence,
we assume that few-body relaxation played an important role during the initial phase of evolution
of the ONC. In particular, three body interactions among OB stars likely led to their ejection from
the cluster and, at the same time, to the formation of a massive object via ‘runaway’ physical stellar
collisions. The resulting depletion of the high mass end of the stellar mass function in the cluster
is one of the important points where our models fit the observational data. We speculate that the
runaway-mass star may have collapsed directly into a massive black hole (M• 100M⊙). Such a
dark object could explain the large velocity dispersion of the four Trapezium stars observed in the
ONC core. We further show that the putative massive black hole is likely to be a member of a binary
system with ≈ 70% probability. In such a case, it could be detected either due to short periods of
enhanced accretion of stellar winds from the secondary star during pericentre passages, or through a
measurement of the motion of the secondary whose velocity would exceed 10 km s−1 along the whole
orbit.
Subject headings: black hole physics — stars: kinematics and dynamics — stars: massive
1. INTRODUCTION ficiency 50% (e.g. Baumgardt & Kroupa 2007). The
The Orion Nebula Cluster (ONC, M42) is a dense star assumption that the ONC has been more compact in
cluster which is part of a complex star forming region at a the past is in accord with the fact that this star clus-
distance of about 400pc (Jeffries 2007; Sandstrom et al. ter is missing wide binaries with separations > 103 AU
2007; Menten et al. 2007). Due to its relative proxim- which may have been disrupted through close three-body
ity the ONC is one of the best observationally studied interactions during the initial compact stage (Kroupa
star clusters. Its age is estimated to be 3Myr, the 2000; Parker et al. 2009; Marks & Kroupa 2012). The
resolved stellar mass is Mc ≈ 1800M⊙ and it has a com- process of dynamical ejections has been suggested by
pact core of radius 0.5pc (Hillenbrand & Hartmann Pflamm-Altenburg & Kroupa (2006) as a reason for the
1998). Based on the data presented by Huff & Stahler depletion of the cluster mass function at the high-mass
(2006), we estimate a half-mass radius of the cluster of end. This hypothesis aims to explain the observed deficit
rh ≈ 0.8pc. Hence, the ONC is considered to be a proto- of massive stars in the ONC (Hillenbrand 1997) with re-
type of a dense young star cluster and it naturally serves spect to the standard Kroupa (2001) initial mass func-
as a test bed for theoretical models of various astrophys- tion. Finally, observations (Zapata et. al. 2009) have
ical processes (e.g. Kroupa et al. 2001 considered the brought evidence that stellar disruptions occurred in the
dynamical evolution of ONC-type star clusters with gas ONC recently. These may be a consequence of physical
expulsion; Olczak et al. 2008 studied the destruction of stellar collisions in the dense cluster core.
protoplanetary discs due to close stellar encounters). In this paper we address the history of the ONC over
Nevertheless, the morphological and dynamical state its lifetime. We concentrate on initialy very compact
of the ONC is still not fully understood. Some of it’s star clusters and show that they can evolve within a few
characteristics indicate that it has undergone a period millions years into a state compatible with the current
of violent evolution and that the current state does not observations. In particular, we address the fact that the
represent a true picture of a newly born star cluster. central system of OB stars, the so-called Trapezium, is
There is a lack of gas in the cluster (Wilson et al. 1997) supervirial and, at the same time, that the ONC hosts
which has been expelled due to the radiation pressure unexpectedly few OB stars.
of the OB stars that reside in the cluster core. The 2. MODEL
effect of gas removal is likely to have led to the clus-
ter expanding by a factor 3 for a star formation ef- We restricted ourselves to the stage of cluster evolu-
tion when the stars have already formed as individual
subr@mbox.troja.mff.cuni.cz entities which can be characterised by a constant mass
1 Astronomical Institute, Charles University, V Holešovičkách
and radius. Stellar dynamics is then driven mainly by
2, CZ-180 00 Praha, Czech Republic
2 Argelander Institute for Astronomy (AIfA), Auf dem Hügel gravitational interactions. We used the numerical code
71, D-53121 Bonn, Germany NBODY6 (Aarseth 2003) which is a suitable tool for
3 University of Queensland, School of Mathematics and
modeling self-gravitating stellar systems with a consid-
Physics, Brisbane, QLD 4072, Australia erable amount of binaries.
2. 2 Šubr et al.
At the initial stage of its evolution, a substantial con- 0.8
tribution to the gravitational field of the cluster is due 0.7
to gas. We incorporated external gas into our model
0.6
by means of a special type of low mass particles (mg ≤
0.4 M⊙) whose gravitational interaction with the rest of 0.5
rh [pc]
the cluster was treated directly via the N-body scheme. 0.4
Beside that, we modified the original NBODY6 code, in-
cluding an option for a repulsive force (with the direction 0.3
radial from the cluster centre) acting upon the gas par- 0.2
ticles in order to mimic the radiation pressure from the
stars starting at a given time Tex (see Sec. 3.2). Alterna- 0.1
tively, we also modelled the gas expulsion by an instan- 0
taneous removal of the gas particles at a given time. 2800
Stellar masses were calculated according to the Kroupa
2700
(2001) mass function with an upper mass limit of 80 M⊙
2600
(however, the mass of the most massive star is typically
2500
≈ 63 M⊙ ). For several numerical reasons (e.g. quadratic
growth of CPU time with the number of particles, higher 2400
Mc [M ]
probability of numerical errors for extreme mass ratios) 2300
we replaced low-mass stars (M⋆ ≤ mmin ) by stars with 2200
mass mmin , keeping the total mass unchanged. We have 2100
verified (cf. models #4 and #6 introduced below which 2000
have mmin = 0.5 M⊙ and 0.2 M⊙, respectively) that our 1900
results are not affected considerably by this approxima- 1800
tion. 1700
0 0.5 1 1.5 2 2.5
The stars were considered to have finite radii R⋆ =
T [Myr]
R⊙ (M⋆ /M⊙ )0.8 (e.g. Lang 1980; this simple formula
also fits well the radii of zero age main sequence stars for
M⋆ M⊙ according to Eggleton et al. 1989; our overes- Fig. 1.— Half-mass radius of the star cluster of our canonical
model of the ONC (upper panel). Expansion is accelerated at
timation of stellar radii for M⋆ M⊙ does not affect our T ≈ 0.5Myr due to gas expulsion. The total mass of the stars
results, as the low-mass stars contribute only marginally within a 3pc radius (lower panel) monotonically decreases as stars
to the merging tree). If the separation of two stars got escape out of the observer’s field of view. This curve reflects gas
below the sum of their radii, they were merged into a expulsion with a time delay of ≈ 0.5Myr which is the time the
stars need to reach the 3pc boundary. Shaded area represents the
single star. We also switched off the option of stellar evo- 1σ variance of the individual realisations of the model.
lution in the numerical code. Loss of realism is assumed
to be negligible as we followed the cluster evolution for a dial binaries with a secondary mass Ms ≥ 1M⊙ with the
period of only 2.5Myr. The stars in our models are not pairing algorithm being biased towards assigning a mas-
assigned a spectral type. For the sake of brevity, we refer sive secondary to a massive primary. More specifically,
to all stars with M⋆ ≥ 5 M⊙ as ‘OB stars’. the algorithm first sorts the stars from the most massive
to the lighest one. The most massive star from the set
2.1. The ‘canonical’ model is taken as the primary. The secondary star index, id, in
The canonical (best-fit) model of the ONC has an ini- the ordered set is generated as a random number with the
tial mass of 5400M⊙, half of which is in the form of probability density ∝ id−β and max(id) corresponding to
stars, while the other half accounts for gas. We consid- a certain mass limit, Ms,min . The two stars are removed
ered a Kroupa (2001) initial mass function which, for the from the set and the whole procedure is repeated until
given cluster star mass, predicts ≈ 50 OB stars to have stars with M⋆ ≥ Mp,min remain. Typically, we used as
been formed in the ONC (Pflamm-Altenburg & Kroupa the minimal mass of the primary Mp,min = 5M⊙ , as the
2006). In order to avoid additional statistical noise in the minimal mass of the secondary Ms,min = 1M⊙ and the
evolution of the number of OB stars, we used identical pairing algorithm index β = 40. For the assigned bina-
samples of stellar masses for all numerical realisations of ries we used a semi-major axis distribution according to
the model. The initial half-mass radius of the cluster is Öpik’s law n(a) ∝ a−1 (Kobulnicky & Fryer 2007) and
rh ≈ 0.11pc, which is compatible with the initial or birth a thermal distribution of eccentricities, n(e) ∝ e. Ex-
radius-mass relation inferred for star clusters ranging in cept for the possibility of being a secondary member of
mass up to globular clusters (Marks & Kroupa 2012). a binary containing an OB star, we did not generate bi-
Positions and velocities are generated in a mass segre- naries of low mass stars. This limitation comes from the
gated state according to the prescription of Šubr et al. requirement of numerical effectivity and it is not likely
(2008) with mass segregation index S = 0.4. The al- to affect our results substantially.
gorithm by definition places massive stars in the cluster Let us note that the canonical model presented above
core and, therefore, the initial half-mass radius of the OB fits into the range of the expanding class of models of
stars is ≈ 0.05pc. In order not to place the light gas parti- the ONC discussed by Kroupa (2000). Other possible
cles at the cluster outskirts, we generated initial positions initial conditions would be fractal models that collapse
for stellar and gas particles separately, i.e. we had a con- (e.g. Allison et al. 2009). These require star formation
stant gas to star mass ratio throughout the whole cluster to be synchronised across the pre-cluster cloud core to
at T = 0. All OB stars were set to be members of primor- much shorter than the dynamical time though.
3. Catch me if you can 3
150
2.2. Dynamical evolution
140
Most of the features of the evolution of a star cluster
with strong few-body relaxation and gas expulsion can 130
be demonstrated with the canonical model. In order to 120
distinguish systematic effects from rather large fluctua-
M [M ]
110
tions of cluster parameters, we present quantities aver-
aged over 100 realisations of the model. The upper panel 100
of Fig. 1 shows the temporal evolution of the half-mass 90
radius of the stellar component, rh . During the initial 80
phase, the cluster expands due to two-body relaxation. 70
Gas expulsion that starts at Tex = 0.5Myr removes all gas
from the cluster within a few hundred thousand years. 60
From the point of view of the stars, this event leads to an 55
abrupt decrease of their potential energy. Consequently,
50
they can reach larger distances from the cluster centre,
which manifests itself as an accelerated growth of rh at 45
0.5Myr T 1Myr. Initially weakly bound stars be- 40
came unbound due to the gas expulsion and they escape
NOB
from the cluster. Hence, reduction of the cluster mass 35
also accelerates – see the lower panel of Fig. 1 where we 30
plot the mass of the star cluster. We define Mc as the
sum of the mass of the stars within a sphere of radius 25
rlim from the cluster centre. We set rlim = 3 pc in order 20
to match typical observations of the ONC, which usually 15
count stars within a projected distance of ≈ 3pc from the 0 0.5 1 1.5 2 2.5
Trapezium. Note that the decrease of Mc is accelerated T [Myr]
with ≈ 0.5Myr delay after the time of gas expulsion.
This is due to the unbound stars taking some time to
Fig. 2.— Top: Mean mass of the most massive (collisional) star
reach rlim . Besides the escape of stars due to the gas in the model regardless of its location (solid line) and mean mass of
expulsion, there is also continuous stellar mass loss due the most massive star within 3pc from the cluster centre (dashed
to the few-body relaxation which is capable to accelerate line). Bottom: number of OB stars (solid line) and number of OB
stars above the escape velocity. From T ≈ 1Myr onwards systems (dashed line) within 3pc from the cluster centre.
the cluster expansion is again dominated by relaxational
The same is true also for our model of the ONC: The
processes driving a revirialisation (Kroupa et al. 2001).
upper panel of Fig. 2 shows the growth of the mass of
Our model indicates that the initial mass of the ONC
the most massive object, M• , due to stellar collisions; the
must have been at least 20% larger than it is now, af-
number of OB stars left in the cluster is plotted in the
ter a few million years of dynamical evolution. We also
lower panel. Approximately one third of the missing OB
see that, mainly due to the effect of gas expulsion, the
stars have disappeared due to merging, while the other
ONC must have been more compact by a factor 5 at
two thirds have been ejected from the cluster with ve-
the time of its birth than it is now. Hence, the initial
relaxation time was much shorter than what we would locities > 10km s−1 . The merging tree varies significanly
infer from present-day observations. Consequently, close among individual realisations of the particular model. In
few-body interactions must have been more frequent in general, more than one merging star is formed during the
the past. In particular, we suggest that scattering of initial ≈ 0.5Myr. Among other processes, this is due to
single stars on binaries is a process that has played an the merging of several primordial OB binaries. At later
important role in the evolution of the ONC. This kind of stages, usually one runaway-mass object dominates the
interaction has been studied intensively in the literature merging process.
(e.g. Heggie & Hut 2003 and references therein). A rich In the Appendix we provide an approximate descrip-
variety of outcomes can be obtained, depending on the tion of the rate of stellar ejections due to the scattering
initial conditions. Nevertheless, some general results can of single stars on massive binaries. It gives an estimate of
be formulated. In particular, if the binary is hard, i.e. its the decay time of the number of the OB stars of τ 5Myr
orbital velocity is larger than the impact velocity of the for the canonical model. If we assume that the decay rate
third star, it typically shrinks, losing its energy which is linearly proportional to the number of OB stars, we ex-
is transferred to the accelerated single star. Therefore, pect roughly an exponential decay of NOB , i.e. it should
we expect two processes to happen in correlation: (i) reach half of its initial value at ∼ 3.5Myr, which is in
ejections of high velocity stars and (ii) physical collisions good agreement with the numerical results.
of massive binaries which lead to the formation of more A mean number of OB stars of ≈ 28 remain at T =
massive objects. 2.5Myr. This considerably exceeds the number of OB
The probability of collisions increases with the mass of systems observed in the ONC — the study of Hillenbrand
the interacting stars. Therefore, numerical models by dif- (1997) reports only 10 stars heavier than 5M⊙ . How-
ferent authors often show a ‘runaway’ process when most ever, the real number of massive stars may be somewhat
of the collisions involve the most massive object which larger due to their ‘hiding’ in OB binary systems. Our
grows continuously (e.g. Portegies-Zwart et al. 2004). models always end up with several OB stars having an
OB companion and, therefore, a better agreement with
4. 4 Šubr et al.
250
TABLE 1
Initial parameters and final states of several models.
200
id S Tex /Myr ǫ rh /pc Mc /M⊙ NOB M• /M⊙
1 0.00 0.5 0.50c 0.56 2183 35.1 102.9
M [M ]
150 2 0.00 0.7 0.50c 0.48 2061 28.1 138.1
3 0.25 0.7 0.50c 0.51 1980 26.2 140.9
4 0.40 0.5 0.50c 0.59 1895 28.0 140.1
100
5 0.40 0.5 0.50c 0.60 1938 27.3 138.5
6 0.40 0.5 0.50c 0.63 1887 28.3 144.2
50 7 0.40 0.5 0.50c 0.66 1660 26.0 142.7
8 0.40 0.5 0.50v 0.47 2197 29.7 122.1
9 0.40 0.7 0.50c 0.62 1895 26.5 152.1
0
10 0.40 0.7 0.50c 0.58 1882 27.0 144.2
250 11 0.40 0.7 0.50c 0.67 1636 26.1 132.6
12 0.00 0.7 0.33c 0.52 1717 24.2 173.7
13 0.25 0.5 0.33v 0.51 2109 32.1 124.2
200
14 0.40 0.5 0.33v 0.50 2196 33.1 112.1
15 0.40 0.5 0.33c 0.77 1300 23.2 171.0
M [M ]
150
Common initial parameters of all models are the initial mass of
the stellar component, Mc = 2700M⊙ which implies NOB (T =
100 0) = 50 and M• (T = 0) ≈ 63M⊙ . The initial half-mass radius
varies slightly throughout the models, but is generally ≈ 0.1pc.
The gas particles are of mass 0.2M⊙ for models 6 and 10; in all
50
other cases mgas = 0.4M⊙ is considered. In models 7 and 11, gas
particles were removed instantaneously at T = Tex ; in all other
0
cases, Tex is the time when the repulsive force was switched on.
15 20 25 30 35 40 S is the mass segregation index as defined in Šubr et al. (2008).
NOB ǫ ≡ Mc /(Mc + Mgas ) is the star formation efficiency; suffix ‘c’
stands for constant ǫ throughout the whole cluster, while ‘v’ means
variable ǫ (decreasing outwards). Öpik’s distribution of semi-major
Fig. 3.— Mass of the most massive object in the cluster vs. axes of the primordial binaries is considered in all models except
the number of OB stars within a radius of 3pc. Individual points for #5, where we set n(a) = const. The canonical model has id 6.
represent states of different realisations of the canonical model at The final state of the canonical model presented in the
T = 2.5Myr. Mass of the most massive object in the whole set
is plotted in the upper panel, while in the lower one, only objects previous section matches basic observables of the ONC
found within 3pc are considered. quite well, in particular its mass and half-mass radius.
The mean number of OB stars is somewhat larger than
what is observed in the ONC, nevertheless, some of the
the observations is achieved if we count the OB systems realisations reach NOB < 20. Hence, we consider this
as indicated with the dashed line in Fig. 2. Moreover, model to be a realistic representation of the ONC. We
due to the stochastic nature of dynamical cluster evo- have investigated several tens of different models with
lution, different realisations of the model end up with different values of the parameters including the index
quite different values of NOB and M• . The final state of the initial mass segregation, initial half-mass radius,
(T = 2.5Myr) of all realisations of the canonical model mass of the cluster, star to gas mass fraction and the
is shown in Fig. 3. Although the distribution of points time of the gas expulsion. Models whose final state can
in the NOB –M• space is rather noisy, we can deduce an be considered compatible with the current state of the
anti-corelation between these two quantities (their linear ONC, at least in some of their characteristics, are listed
correlation coefficient is −0.42). In particular, all reali- in Table 1.
sations that end up with NOB ≤ 20 lead to the formation
of a runaway-mass star of mass M• > 100M⊙. In other 3.1. NOB − M• anticorelation
words, the underabundance of the high-mass stars in the As mentioned above, the canonical model indicates an
ONC not only indicates a period of prominent two-body anticorelation between the final number of the OB stars
relaxation in the past, but also the merging formation and the final mass of the runaway-mass star. In Fig. 4 we
of a massive object which may represent an important plot the mean mass of the most massive object vs. the
footprint of the cluster’s history. mean number of remaining OB stars for all models listed
Interestingly, the term ‘runaway’ has a secondary in Table 1. Now, each point represents an average over
meaning in some cases: Despite of its high mass, in sev- several tens of realisations of the particular model and a
eral realisations the merging object has been ejected out NOB − M• anticorelation becomes evident. We attribute
of the cluster with velocity exceeding 10 km s−1 . These this relation to the fact that both mechanisms of OB star
cases can be identified due to differences of the upper and removal, i.e. physical collisions and ejections, are driven
lower panels of Fig. 3. The escape of the most massive by a common underlyig process of close three-body in-
body beyond the 3pc boundary is also responsible for the teractions (see Appendix). Naturally, their importance
drops of the dashed line in the plot of the temporal evo- grows with increasing stellar density. Hence, the models
lution of M• in Fig. 2 which shows the mean mass of the that stay more compact during the course of their evolu-
most massive object within 3pc from the cluster centre. tion are located in the top-left corner of the graph. They
better fit the observations from the point of view of the
3. DISCUSSION number of OB stars, however, either their half-mass ra-
5. Catch me if you can 5
180
TABLE 2
Comparison of models with different masses of gas
160
particles
140 mg NOB M• Nrun PKS,NOB PKS,M• comment
M [M ]
120 0.40 39.7 125.1 40 95% 42% model #4
0.20 39.8 124.9 100 — — canonical model
100 0.05 38.2 128.5 14 82% 87%
0.03 39.9 126.7 14 32% 75%
80
Mean values of the number of OB stars within the radius of 3 pc
from the cluster core, NOB , and mass of the runaway-mass star,
60 M• , for models with different values of mg . All other parameters
20 25 30 35 40 45 50
of the models, including the total mass of gas, are identical to the
NOB canonical model. PKS,∗ is the Kolmogorov-Smirnov probability
that the null hypothesis (assuming the particular data set comes
from the same distribution as the canonical one) is valid. Note
Fig. 4.— Mean mass of the most massive object in the cluster vs. that the value of PKS,∗ ≈ 5% is usually considered to be a limit
the mean number of OB stars (both within 3 pc) at T = 2.5Myr for
below which the null hypothesis should be rejected.
several different models of compact star clusters listed in Table 1.
The canonical model is represented with the × sign; full circle servations (Huff & Stahler 2006), the star formation in
stands for the initial state which is common for all models.
the ONC was continuous with the most massive stars
dius or total stellar mass at T = 2.5Myr is too small, i.e. starting to be formed no more than 2 Myr ago. Setting
not consistent with the observations. Tex in our models to either 0.5 or 0.7 Myr appears to have
only a marginal impact on the final state of the cluster.
3.2. Gas expulsion Hence, we assume that a more complicated temporal pre-
In most of our models, the gas particles were blown out scription of the gas removal would not affect our results
of the cluster due to a repulsive external force ∝ r/r3 considerably.
centered on the cluster core. The strength of the force
was set such that the gas particles were accelerated to ve- 3.3. The runaway-mass star or black hole
locities ≈ 8km s−1 on the time-scale of ≈ 0.1 Myr. This Under the assumption of an invariant canonical IMF
approach is the most realistic one, keeping the continu- and a small initial cluster radius, the formation of the
ity equation fulfilled, but, at the same time, it is nu- runaway-mass object via stellar collisions appears to
merically the most expensive. The alternative method be an inevitable process that, together with the high-
with instantaneous removal of the gas particles at Tex velocity ejections, decreases the number of OB stars in
leads to similar results. The clusters integrated with this a dense star cluster. It’s possible detection would defi-
method typically result in somewhat lower total mass and nitely strongly support the scenario of the ONC history
larger half-mass radius at the final time. Let us note, for presented in this paper.
completness, that in the literature, another method that The current state of the runaway-mass star is, how-
mimics gas removal in numerical models of star clusters ever, not clear. The massive star would definitely have
is also used (e.g. Geyer & Burkert 2001; Kroupa et al. undergone a fast internal evolution. While successive
2001; Baumgardt & Klessen 2011). It is based on the merging events may lead to very fast mass growth, stellar
time-variable smooth external potential that represents winds act in the opposite way. The state of the runaway-
the weakening gravitational potential of the gas. It has mass object after ≈ 2 Myr of evolution depends on the
been found that the two approaches (i.e. modelling gas (dis)balance of these two processes. In the literature,
via particles vs. external potential) lead to nearly indis- quite different predictions on this subject can be found.
tinguishable results (Geyer & Burkert 2001). In order to Glebbeek et al. (2009) suggest that winds should work
check for a possible bias of our gas particles approach in a self-regulatory manner, limiting the maximum mass
which may stem from two-body relaxational processes, and, consequently the star’s lifetime. Other authors (e.g.
we have integrated two additional models with very low Suzuki et al. 2007; Pauldrach et al. 2011) state that stel-
masses for the gas particles. Due to the large num- lar winds of the runaway-mass star will not be able to
ber of particles in these integrations, we have followed terminate its growth and it may then collapse to a mas-
their evolution only up to the time of the gas expulsion, sive black hole. There is no observational evidence for a
Tex = 0.5 Myr. Outcomes of these models in terms of the star with a mass above 50M⊙ in the ONC which could be
mean number of remaining OB stars, NOB , and the mass interpreted as a runaway-mass object. The most massive
of the runaway-mass star, M• , are presented in Table 2. stellar member of the ONC, Θ1 C, is a binary with a total
Besides just giving the mean values of NOB and M• , we mass of about 45M⊙ and mass ratio ≈ 0.25 (Kraus et al.
have also performed a Kolmogorv-Smirnov test compar- 2009), i.e. the primary has mass 35 M⊙. It does not
ing the sets of NOB and M• coming from these models exhibit a stellar wind strong enough to reduce its mass
with the canonical one. As can be seen, there is no ap- by several tens of M⊙ . Hence, our scenario assumes a
parent dependence of the results on mgas ranging from low efficiency of stellar winds, i.e. continuous growth of
0.03 to 0.4 M⊙. Hence, we conclude that the two-body the runaway-mass star which forms a massive black hole
relaxation due to the gas partiles does notsignificantly at the end of its lifetime. There is no general consensus
affect our results. whether the collapse of a star more massive than 150M⊙
The evolution of the radiation pressure in the real ONC will be followed by an ejection of most of its mass. As
has been definitely more complicated. According to ob- there is no evidence for a supernova remnant in the ONC,
6. 6 Šubr et al.
9 6.0
8
5.5
7
5.0
number count
6
σ [km/s]
5 4.5
4 4.0
3
3.5
2
3.0
1
0 2.5
0.1 1 10 100 1000 0.1 1
a, rp [AU] r [pc]
Fig. 5.— Histogram of the semi-major axes (a, empty boxes) Fig. 6.— Velocity dispersion σ within a specified radius r at the
and pericentre distances (rp , full boxes) of the binaries involving final state of the canonical model. For distinguished bound multi-
the runaway-mass star in the canonical model. A total of 100 runs ple systems, the centre-of-mass velocity is counted instead of the
were examined, out of which 70 harbour the runaway-mass star proper velocities of the individual components. Solid line repre-
with a stellar companion (three of them with a > 1000AU). sents the average over all realisations; thin dashed line corresponds
to half of the computations yielding more massive (M• 120 M⊙ )
we assume most of the mass of the runaway-mass star, if runaway-mass object, while the dotted line stands for the com-
putaions with lower M• .
it is present, to have collapsed directly into a black hole.
The putative black hole in the ONC would be de- The accretion events should be recurrent with a period
tectable only through an interaction with its environ- ranging from years to hundreds of years. A shorter pe-
ment. One possibility is an accretion of surrounding gas riod generally implies a smaller rp , i.e. larger values of
which could be detectable through X-ray radiation. The the peak luminosity.
main source of the gas in the Trapezium region are stel-
lar winds from the remaining OB stars. We estimate 3.4. Velocity dispersion
that they produce at most 10−5 M⊙ yr−1 within the cen-
Another way of an indirect detection of the massive
tral 0.2pc. However, the black hole is likely to capture
only the gas that falls within its Bondi radius ≈ 100 AU, black hole in the ONC lies in velocity dispersion measure-
ments. It has been stated by several authors that the core
i.e. the accretion rate could be 10−15 M⊙ yr−1 . Such
of the ONC (r 0.25 pc) is dynamically hot with a ve-
a highly sub-Eddington accretion will not be detectable
in the region confused with several X-ray young stellar locity dispersion 4 km s−1 (e.g. Jones & Walker 1988;
sources. van Altena et al. 1988; Tobin et al. 2009). The missing
A better chance for detecting the black hole would be stellar mass required for virial equilibrum in the inner-
given if it is a member of a binary. This indeed appears most region of the ONC was estimated to be 2000 M⊙
to be the case for about two thirds of the realisations of which exceeds the observed mass ≈ 200 M⊙ by an or-
the canonical model. However, the typical separation of der of magnitude. Kroupa et al. (2001) demonstrate that
the black hole and the secondary appears to be from sev- the globally super-virial velocity dispersion is readily ob-
eral tens to hundreds of AU (see Fig. 5). Even with the tained if the ONC is expanding now after expulsion of its
relatively large eccentricites achieved, in no case does the residual gas, but some of the missing mass in the inner
secondary star fill its Roche radius at the pericentre of its region could be attributed to the invisible remnant of the
orbit, i.e. we do not expect the black hole to be a mem- runaway-mass star.
ber of a mass transferring system. Still there would be a As we show in Fig. 6, our canonical model gives a ve-
chance for a detectable accretion provided the secondary locity dispersion 4km s−1 in the inner 0.25pc, which
star is massive (M⋆ 10 M⊙). In such a case, its stel- is in accord with the observational data. In order to
lar wind may produce short periods of enhanced activity inspect the influence of the mass of the runaway-mass
during pericentre passages, but the corresponding lumi- object on the velocity dispersion, we have divided the
nosity is rather uncertain. Let us consider the well kown 100 realisations of the canonical model into two groups
Cyg-X1 system as a kind of template. Having a black of 50 according to the mass of the most massive object
hole mass of ≈ 10 M⊙ and separation from the secondary that remained in the cluster. It appears that the clus-
star of ≈ 0.2 AU, it has a luminosity ≈ 2 × 1037 erg s−1 ters with M• 120M⊙ have a velocity dispersion within
(Wilms et al. 2006) which corresponds to ≈ 10−4 of the 0.25pc somewhat larger than those with M• 120M⊙,
rest-mass energy of the winds emitted by the secondary nevertheless, even within the latter group we have σ
star (≈ 10−6 M⊙ yr−1 ). Assuming the same effectivity in 3.5km s−1 , i.e. an apparently super-virial core region.
conversion of the stellar winds into radiation, we estimate Together with the fact that none of our numerical ex-
the luminosity to be similar to the Cyg-X1 system, i.e. periments led to M• 2000M⊙, virtually required by
≈ 104 L⊙ , for separations of the order of 1 AU. Consid- the condition of virial equilibrium, this indicates that
ering the density of the stellar wind to decrease with the the large velocity dispersion can be only partly due to
sqaure of the distance, we obtain a rough estimate of the the hidden mass of the runaway-mass object. Detailed
peak luminosity L ≈ 104 (rp /1 AU)−2 L⊙ . Acording to analysis of our models shows that there are two other
the mass of the putative black hole, the maximum of the reasons for having large velocity dispersion in the core.
emitted radiation is expected to lie in the X-ray band. First, according to our models, the ONC is a post-core-
7. Catch me if you can 7
TABLE 3 to be ≈ 7.5 km s−1 . The runaway-mass star was ex-
Trapezium stars data cluded from the search algorithm. Furthermore, we
name M/M⊙ vlos /km s−1
have distinguished systems according to their centre of
mass distance to the runaway-mass object. In approx-
Θ1 A 18.9 33.4 ± 2
Θ1 B 7.2 24.0 ± 2
imately two thirds of the Trapezium-like systems, the
Θ1 C 44 ± 7 23.6
runaway-mass object was found within a distance smaller
Θ1 D 16.6 32.4 ± 1 than max(∆rij ), with ∆rij denoting separations of indi-
vidual members. The mean velocity dispersion of the
Stellar masses of Θ1 A, Θ1 B and Θ1 D are taken from Hillenbrand Trapezium-like system of this subset was ≈ 9.0 km s−1 ,
(1997). Radial velocity data of Θ1 A and Θ1 B come from the C.D.S. while it was only ≈ 4.3 km s−1 for the remainig cases.
– SIMBAD database; data of Θ1 D follows Vitrichenko (2002). Finally, let us remark that kinematical evidence of a
Both mass and radial velocity of Θ1 C are according to Kraus et al.
(2009).
massive black hole in the ONC may also lie in the ≈ 70%
probability that it is a member of a binary (see Sec. 3.3).
The typical velocity of the secondary star should be
collapse cluster with centrally peaked density. This leads > 10 km s−1 , i.e. considerably exceeding the observed
to a deep gravitational potential well which allows the central velocity dispersion.
central velocity dispersion to be larger than what would
be expected e.g. for the Plummer profile. Second, in 4. CONCLUSIONS
our models and perhaps also in the real ONC, there are We have carried out extensive modelling of the dy-
likely to be present several not identified wide binaries namical evolution of compact young star clusters, aimig
with orbital velocities exceeding 10 km s−1 which affect to reconstruct the history of the ONC. Assuming that
the velocity dispersion. the ONC underwent primordial gas expulsion, we have
Fig. 6 indicates that the presence of the dark mas- shown that the ONC must have been several times more
sive body influences remarkably (i.e. by more than compact than it is now in agreement with Kroupa (2000)
1 km s−1 ) the velocity dispersion in a small region of a and Kroupa et al. (2001). This implies that its two-body
radius of ≈ 0.1 pc. This is in accord with an analyt- relaxation time was considerably shorter in the past and,
ical estimate of the influence radius of the black hole, consequently, that close few-body interactions between
rh = GM• /σ 2 , which for M• = 100 M⊙ and σ = 3 km s−1 massive stars were rather frequent. We have concen-
gives rh ≈ 0.05 pc. Due to the small number count of trated on two significant tracks of such interactions that
stars in such a small region, standard statistical meth- lead either to high-velocity ejections of massive stars
ods do not represent a robust tool for determining the from the cluster or to their physical collisions that are
presence of a dark massive body. Nevertheless, hav- likely to lead to the formation of a massive ‘runaway’
ing this caution in mind, let us discuss the velocity merging object. Both of these processes decrease the
dispersion of the compact core of the ONC, the four number of massive stars in the cluster. Hence, the ob-
Trapezium stars. According to the published observa- served significant lack of massive OB stars in the ONC
tional data (see Table 3), the line-of-sight velocity dis- further supports our assumption that this star cluster
persion of the Trapezium is σlos ≈ 4.6 km s−1 which im- has undergone a relaxation dominated period since its
plies a 3D velocity dispersion σ ≈ 7.9 km s−1 . Consid- birth.
ering a sphere of radius rT ≈ 0.025 pc covering the four We have shown that it is not unfeasible that the centre
Trapezium stars, we obtain an estimate of the enclosed of the ONC harbours a black hole of mass 100M⊙ as
mass required for the system to be in virial equilibrium: the remnant of the massive runaway-mass star. Its pres-
Mbind ≈ rT σ 2 /G ≈ 350 M⊙. Taking into account the ence could be revealed either by episodic accretion events
stellar mass of the Trapezium (≈ 90 M⊙ ), we find it to be or by a thorough kinematical study of the innermost
an apparently dynamically very hot system. Our models ≈ 0.05 pc region of the ONC, where the hidden mass
suggest that the virial equilibrium state can be achieved would increase the velocity dispersion above 5km s−1 . In
by the presence of the runaway-mass object. particular, we have shown that the observed velocity dis-
Yet another, and statistically better founded, argu- persion of the Trapezium system can be achieved by the
ment for a hidden massive body comes from compari- presence of an object of mass ≈ 150 M⊙ which is fully
son of the kinematical state of the Trapezium and sim- consistent with the hypothesis of the runaway-mass ob-
ilar subsystems found in our numerial models. We ject. Our model also shows that the apparent super-
have implemented an algorithm similar to that used by viriality of the central ≈ 0.25pc can be explained as a
Allison & Goodwin (2011) for detection of Trapezium- natural attribute of a post-core-collapse dynamical state
like systems. In particular, for each OB star we de- of a star cluster with a considerable binary fraction.
termined its three nearest OB neighbours (either sin- The possible detection of the remnant of the merger
gle OB stars, or bound systems containing at least one object could bring new light into several fields of con-
OB star). The set of stars was considered Trapezium- temporary astrophysics. First, it would confirm the hy-
like if all members had the same OB neighbours. In pothesis that star clusters similar to the ONC are being
order to obtain a statistically significant sample, we ex- formed very compact with a half-mass radius of the or-
amined the last 20 snapshots (covering the period from der of a few tenths of a parsec (Marks & Kroupa 2012).
≈ 1.5 Myr to 2.5 Myr) of 100 independet realisations Second, and probably more importantly, it would have
of the canonical model. We found Trapezium-like sys- important implications for the evolution of very massive
tems in a third of cluster snapshots. The mean value stars and merger products for which we have only a lim-
of the velocity dispersion of these systems was found ited understanding now.
8. 8 Šubr et al.
ACKNOWLEDGMENTS search Program MSM0021620860 of the Czech Ministry
We thank the anonymous referee for helpful comments. of Education. HB acknowledges support from the Ger-
This work was supported by the Czech Science Foun- man Science Foundation through a Heisenberg Fellow-
dation via grant GACR-205/07/0052 and from the Re- ship and from the Australian Research Council through
a Future Fellowship grant FT0991052.
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APPENDIX
THEORETICAL PREDICTIONS
Unlike two-body relaxation, interactions among three or more stars allow considerable energy transfer from one
component to another. Therefore, multiple-body scattering is essential for both processes (merging and ejections) that
lead to the reduction of the number of massive stars in the cluster. In spite of the chaotic nature of the dynamical
evolution of the star cluster core, it is possible to derive a raw estimate of the rate of OB stars ejections and collisions.
Our approximation of three-body scattering follows the work of Perets & Šubr (2012): Consider an interaction of a
binary of mass MB = M1 + M2 and a single star M⋆ . Large accelerations of the single star are assumed to occur when
it passes around one of the binary components within the semi-major axis, a, of the binary. The cross section of the
interaction is assumed to be determined by gravitational focusing:
2πGMB a
Σ(a) ≈ 2
, (A1)
vc
where vc is the characteristic stellar velocity in the cluster and G stands for the gravitational constant.
The energy transfer estimate is based on the approximation that the impacting star moves along a hyperbolic orbit
around M1 perturbed by M2 . The typical perturbing force is
GM2 M⋆
F ≈ (A2)
a2
and it acts along the star’s trajectory segment of length ≈ a. Hence, the energy transfer to the star is
GM2 M⋆ GMB M⋆
∆E⋆ ≈ ≈ . (A3)
a a
Here we for simplicity assume the mass of the binary to be of the same order as the mass of the secondary, MB ≈ M2 .
In the cases when ∆E⋆ is much larger or at least comparable to the star’s energy before the interaction, it will be
accelerated to
2GMB
vacc ≈ . (A4)
a
Finally, let us assume the distributions of the binary mass, semi-major axis and eccentricity in a simple power-law
form,
n(a, e, MB ) = 2A e a−1 MB−α
(A5)
9. Catch me if you can 9
in the interval a ∈ amin , amax , e ∈ 0, 1 and MB ∈ Mmin , Mmax , i.e. n(a, e, MB ) da de dMB is the number of
binaries with semi-major axis, eccentricity and mass in a, a + da , e, e + de and MB , MB + dMB respectively. The
normalisation constant then reads:
ln−1 amax
amin ln−1 Mmax
Mmin for α=1,
A= (A6)
ln−1 amax
amin 1−α
1−α
1−α
Mmax −Mmin
for α=1.
HIGH-VELOCITY EJECTIONS
The frequency of scattering events of a star with velocity vc on binaries with number density nB and semi-major
axis a is
2πGMB nB a
ν = Σ nB vc = , (A7)
vc
where we assumed the binary cross-section to be determined by the gravitational focusing, i.e. Σ ≈ 2πGMB a/vc . The
2
mean frequency of ejections can be obtained via integration of (A7) weighted by the distribution function (A5):
Mmax alim
νe =
¯ dMB da ν(a, MB ) n(a, e, MB ) . (A8)
Mmin amin
The upper limit of the semi-major axis, alim , has to be set such that only interactions that lead to acceleration above
the escape velocity from the cluster are considered, i.e.
2GMc
vacc (alim ) = vesc ≈ , (A9)
rc
where rc is a characteristic radius of the cluster. Combining (A9) and (A4) gives alim ≈ rc MB /Mc . For the canonical
model with initial Mc = 5400M⊙ and rc = 0.11pc and for 5M⊙ ≤ MB ≤ 100M⊙ we obtain alim 100AU which is the
upper limit of the semi-major axis distribution used in our numerical integrations. Hence, let us for simplicity assume
alim = amax = const. which yields
2πGnB amax − amin Mmax − Mmin
for α=1,
vc ln(amax /amin ) ln(Mmax /Mmin)
νe =
¯ 2−α 2−α (A10)
2πGnB amax − amin 1 − α Mmax − Mmin
1−α 1−α for α=1.
vc ln(amax /amin ) 2 − α Mmax − Mmin
Considering an initial OB binary density nB ≈ 105 pc−3 and velocity dispersion vc ≈ 10km s−1 , a ∈ 0.1AU, 100AU
and MB ∈ 5M⊙ , 100M⊙ with a Salpeter mass function (α = 2.35) we obtain νe ≈ 0.1Myr−1 . Hence, the mean time
¯
for an OB star to undergo scattering on a massive binary that leads to its ejection from the cluster is τe ≡ 1/¯e ≈ 10Myr
ν
for the canonical model.
STELLAR COLLISIONS
Unlike in the case of stellar ejections, we assume that stellar collisions are caused by binary shrinking due to successive
three body interactions. We further assume all the massive binaries to be hard, i.e. their interaction with the third
body leads to the acceleration of the impact star and growth of the binding energy of the binary. The frequency of
the events of scattering of a star of mass M⋆ on the massive binary is ν ′ = Σvc n(M⋆ )dM⋆ , where n(M⋆ )dM⋆ is the
number of stars of given mass per unit volume. The rate of the energy transfer from the binary to the stars of mass
in M⋆ , M⋆ + dM⋆ is
dE 2πG2 MB M⋆
2
≈ ∆E⋆ ν ′ = n(M⋆ )dM⋆ . (A11)
dt vc
The net rate of energy transfer to stars within the whole mass spectrum gives
dE 2πG2 MB M⋆
2
2πG2 MB ρ⋆
2
≈ n(M⋆ )dM⋆ = . (A12)
dt vc vc
The stars collide when the binary separation at pericentre becomes less than the sum of the stellar radii. In terms of
binding energy and with approximation R⋆ (M1 ) + R⋆ (M2 ) ≈ R⋆ (MB ), the condition for collision reads:
2
1 − e GMB
Ecoll ≈ . (A13)
8 R⋆ (MB )
The time required to grow the binding energy from the initial value E0 ≈ 1 GMB /a0 to Ecoll is
8
2
−1
dE vc 1−e 1
tcoll ≈ (E0 − Ecoll ) ≈ − . (A14)
dt 8πGρ⋆ R⋆ (MB ) a0
10. 10 Šubr et al.
Integration of tcoll over the whole parameter space of binaries with the distribution function (A5) gives the characteristic
time of stellar collisions:
amax Mmax 1−R⋆ (MB )/a
vc 1−e 1
τcoll ≈ da dMB de − n(a, e, MB )
16πGρ⋆ amin Mmin 0 R⋆ (MB ) a0
amax Mmax
vc 1 1 RB R3
≈ da −α
dMB A a−1 MB − + 2 − B , (A15)
16πGρ⋆ amin Mmin 3RB a a 3a3
where we denoted RB ≡ R⋆ (MB ); the upper limit on the eccentricity is set such that the binaries are not collisional
initially. Assuming RB ≫ a, we omit the two least significant terms in (A15) and perform the integration over a:
Mmax
Avc 1 amax 1 1
τcoll ≈ −α
dMB MB ln + − . (A16)
16πGρ⋆ Mmin 3RB amin amax amin
˜ ˜
Finally, using the notation M ≡ M/M⊙ we obtain for R⋆ (M ) = R⊙ M 0.8
vc 5 ˜ −0.8 ˜ −0.8 Mmax R⊙ R⊙ amax
Mmin − Mmax ln−1 + − ln−1 for α=1,
16πGρ⋆ R⊙ 12 Mmin amax amin amin
τcoll ≈ (A17)
vc ˜ 0.2−α ˜ 0.2−α
1 − α Mmax − Mmin R⊙ R⊙ amax
+ − −1
ln for α=1.
16πGρ⋆ R⊙ ˜ 1−α ˜ 1−α
0.6 − 3α Mmax − Mmin amax amin amin
The canonical model has an initial central density ρ⋆ ≈ 2 × 106 M⊙ pc−3 which gives τcoll ≈ 40 Myr, i.e. we estimate
stellar collisions to be roughly a factor of 4 less efficient process for OB star removal than three-body scattering. This
is somewhat less than what our numerical experiment shows. This discrepancy can be due to several reasons. Most
important is probably the fact that stellar collisions may occur sooner due to perturbations which lead to a strong
growth of orbital eccentricity. Indeed, the numerical experiments show that most of the collisions occur with e 0.99.
Hence, the above given derivation can only serve as an order of magnitude estimate.
Altogether, the three-body interactions are expected to decrease the number of OB stars in the canonical model of
the ONC by a factor of 2 on a time-scale of ≈ 5 Myr which is in accord with the results of the numerical experiment.