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.
We discovered two transient events in the Kepler eld with light curves that strongly suggest they
are type II-P supernovae. Using the fast cadence of the Kepler observations we precisely estimate
the rise time to maximum for KSN2011a and KSN2011d as 10.50:4 and 13.30:4 rest-frame days
respectively. Based on ts to idealized analytic models, we nd the progenitor radius of KSN2011a
(28020 R) to be signicantly smaller than that for KSN2011d (49020 R) but both have similar
explosion energies of 2.00:3 1051 erg.
The rising light curve of KSN2011d is an excellent match to that predicted by simple models of
exploding red supergiants (RSG). However, the early rise of KSN2011a is faster than the models
predict possibly due to the supernova shockwave moving into pre-existing wind or mass-loss from the
RSG. A mass loss rate of 10 4 M yr 1 from the RSG can explain the fast rise without impacting
the optical
ux at maximum light or the shape of the post-maximum light curve.
No shock breakout emission is seen in KSN2011a, but this is likely due to the circumstellar inter-
action suspected in the fast rising light curve. The early light curve of KSN2011d does show excess
emission consistent with model predictions of a shock breakout. This is the rst optical detection of
a shock breakout from a type II-P supernova.
The physical conditions_in_a_pre_super_star_cluster_molecular_cloud_in_the_an...Sérgio Sacani
Artigo descreve estudo feitos pelos astrônomos utilizando o ALMA para descobrir um proto-aglomerado globular de estrelas gigantes se formando no interior das galáxias Antenas, o famoso par de galáxias em interação. É a primeira vez que os astrônomos conseguem observar um objeto desse tipo nos seus estágios iniciais de vida e com o ambiente ao redor inalterado.
Inverse Compton cooling limits the brightness temperature of the radiating plasma to a maximum of
1011.5 K. Relativistic boosting can increase its observed value, but apparent brightness temperatures
much in excess of 1013 K are inaccessible using ground-based very long baseline interferometry (VLBI)
at any wavelength. We present observations of the quasar 3C 273, made with the space VLBI mission
RadioAstron on baselines up to 171,000 km, which directly reveal the presence of angular structure as
small as 26 µas (2.7 light months) and brightness temperature in excess of 1013 K. These measurements
challenge our understanding of the non-thermal continuum emission in the vicinity of supermassive
black holes and require a much higher Doppler factor than what is determined from jet apparent
kinematics.
Keywords: galaxies: active — galaxies: jets — radio continuum: galaxies — techniques: interferometric
— quasars: individual (3C 273)
The characterization of_the_gamma_ray_signal_from_the_central_milk_way_a_comp...Sérgio Sacani
Past studies have identified a spatially extended excess of ∼1-3 GeV gamma rays from the region
surrounding the Galactic Center, consistent with the emission expected from annihilating dark
matter. We revisit and scrutinize this signal with the intention of further constraining its characteristics
and origin. By applying cuts to the Fermi event parameter CTBCORE, we suppress the tails
of the point spread function and generate high resolution gamma-ray maps, enabling us to more
easily separate the various gamma-ray components. Within these maps, we find the GeV excess
to be robust and highly statistically significant, with a spectrum, angular distribution, and overall
normalization that is in good agreement with that predicted by simple annihilating dark matter
models. For example, the signal is very well fit by a 36-51 GeV dark matter particle annihilating to
b
¯b with an annihilation cross section of σv = (1−3)×10−26 cm3
/s (normalized to a local dark matter
density of 0.4 GeV/cm3
). Furthermore, we confirm that the angular distribution of the excess is
approximately spherically symmetric and centered around the dynamical center of the Milky Way
(within ∼0.05◦
of Sgr A∗
), showing no sign of elongation along the Galactic Plane. The signal is
observed to extend to at least ' 10◦
from the Galactic Center, disfavoring the possibility that this
emission originates from millisecond pulsars.
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
Young remmants of_type_ia_supernovae_and_their_progenitors_a_study_of_snr_g19_03Sérgio Sacani
Type Ia supernovae, with their remarkably homogeneous light curves and spectra, have been used as
standardizable candles to measure the accelerating expansion of the Universe. Yet, their progenitors
remain elusive. Common explanations invoke a degenerate star (white dwarf) which explodes upon
reaching close to the Chandrasekhar limit, by either steadily accreting mass from a companion star
or violently merging with another degenerate star. We show that circumstellar interaction in young
Galactic supernova remnants can be used to distinguish between these single and double degenerate
progenitor scenarios. Here we propose a new diagnostic, the Surface Brightness Index, which can
be computed from theory and compared with Chandra and VLA observations. We use this method
to demonstrate that a double degenerate progenitor can explain the decades-long
ux rise and size
increase of the youngest known Galactic SNR G1.9+0.3. We disfavor a single degenerate scenario.
We attribute the observed properties to the interaction between a steep ejecta prole and a constant
density environment. We suggest using the upgraded VLA to detect circumstellar interaction in
the remnants of historical Type Ia supernovae in the Local Group of galaxies. This may settle the
long-standing debate over their progenitors.
Subject headings: ISM: supernova remnants | radio continuum: general | X-rays: general | bi-
naries: general | circumstellar matter | supernovae: general | ISM: individual
objects(SNR G1.9+0.3)
We discovered two transient events in the Kepler eld with light curves that strongly suggest they
are type II-P supernovae. Using the fast cadence of the Kepler observations we precisely estimate
the rise time to maximum for KSN2011a and KSN2011d as 10.50:4 and 13.30:4 rest-frame days
respectively. Based on ts to idealized analytic models, we nd the progenitor radius of KSN2011a
(28020 R) to be signicantly smaller than that for KSN2011d (49020 R) but both have similar
explosion energies of 2.00:3 1051 erg.
The rising light curve of KSN2011d is an excellent match to that predicted by simple models of
exploding red supergiants (RSG). However, the early rise of KSN2011a is faster than the models
predict possibly due to the supernova shockwave moving into pre-existing wind or mass-loss from the
RSG. A mass loss rate of 10 4 M yr 1 from the RSG can explain the fast rise without impacting
the optical
ux at maximum light or the shape of the post-maximum light curve.
No shock breakout emission is seen in KSN2011a, but this is likely due to the circumstellar inter-
action suspected in the fast rising light curve. The early light curve of KSN2011d does show excess
emission consistent with model predictions of a shock breakout. This is the rst optical detection of
a shock breakout from a type II-P supernova.
The physical conditions_in_a_pre_super_star_cluster_molecular_cloud_in_the_an...Sérgio Sacani
Artigo descreve estudo feitos pelos astrônomos utilizando o ALMA para descobrir um proto-aglomerado globular de estrelas gigantes se formando no interior das galáxias Antenas, o famoso par de galáxias em interação. É a primeira vez que os astrônomos conseguem observar um objeto desse tipo nos seus estágios iniciais de vida e com o ambiente ao redor inalterado.
Inverse Compton cooling limits the brightness temperature of the radiating plasma to a maximum of
1011.5 K. Relativistic boosting can increase its observed value, but apparent brightness temperatures
much in excess of 1013 K are inaccessible using ground-based very long baseline interferometry (VLBI)
at any wavelength. We present observations of the quasar 3C 273, made with the space VLBI mission
RadioAstron on baselines up to 171,000 km, which directly reveal the presence of angular structure as
small as 26 µas (2.7 light months) and brightness temperature in excess of 1013 K. These measurements
challenge our understanding of the non-thermal continuum emission in the vicinity of supermassive
black holes and require a much higher Doppler factor than what is determined from jet apparent
kinematics.
Keywords: galaxies: active — galaxies: jets — radio continuum: galaxies — techniques: interferometric
— quasars: individual (3C 273)
The characterization of_the_gamma_ray_signal_from_the_central_milk_way_a_comp...Sérgio Sacani
Past studies have identified a spatially extended excess of ∼1-3 GeV gamma rays from the region
surrounding the Galactic Center, consistent with the emission expected from annihilating dark
matter. We revisit and scrutinize this signal with the intention of further constraining its characteristics
and origin. By applying cuts to the Fermi event parameter CTBCORE, we suppress the tails
of the point spread function and generate high resolution gamma-ray maps, enabling us to more
easily separate the various gamma-ray components. Within these maps, we find the GeV excess
to be robust and highly statistically significant, with a spectrum, angular distribution, and overall
normalization that is in good agreement with that predicted by simple annihilating dark matter
models. For example, the signal is very well fit by a 36-51 GeV dark matter particle annihilating to
b
¯b with an annihilation cross section of σv = (1−3)×10−26 cm3
/s (normalized to a local dark matter
density of 0.4 GeV/cm3
). Furthermore, we confirm that the angular distribution of the excess is
approximately spherically symmetric and centered around the dynamical center of the Milky Way
(within ∼0.05◦
of Sgr A∗
), showing no sign of elongation along the Galactic Plane. The signal is
observed to extend to at least ' 10◦
from the Galactic Center, disfavoring the possibility that this
emission originates from millisecond pulsars.
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
Young remmants of_type_ia_supernovae_and_their_progenitors_a_study_of_snr_g19_03Sérgio Sacani
Type Ia supernovae, with their remarkably homogeneous light curves and spectra, have been used as
standardizable candles to measure the accelerating expansion of the Universe. Yet, their progenitors
remain elusive. Common explanations invoke a degenerate star (white dwarf) which explodes upon
reaching close to the Chandrasekhar limit, by either steadily accreting mass from a companion star
or violently merging with another degenerate star. We show that circumstellar interaction in young
Galactic supernova remnants can be used to distinguish between these single and double degenerate
progenitor scenarios. Here we propose a new diagnostic, the Surface Brightness Index, which can
be computed from theory and compared with Chandra and VLA observations. We use this method
to demonstrate that a double degenerate progenitor can explain the decades-long
ux rise and size
increase of the youngest known Galactic SNR G1.9+0.3. We disfavor a single degenerate scenario.
We attribute the observed properties to the interaction between a steep ejecta prole and a constant
density environment. We suggest using the upgraded VLA to detect circumstellar interaction in
the remnants of historical Type Ia supernovae in the Local Group of galaxies. This may settle the
long-standing debate over their progenitors.
Subject headings: ISM: supernova remnants | radio continuum: general | X-rays: general | bi-
naries: general | circumstellar matter | supernovae: general | ISM: individual
objects(SNR G1.9+0.3)
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.
Constraints on Neutrino Natal Kicks from Black-Hole Binary VFTS 243Sérgio Sacani
The recently reported observation of VFTS 243 is the first example of a massive black-hole binary
system with negligible binary interaction following black-hole formation. The black-hole mass (≈10M⊙)
and near-circular orbit (e ≈ 0.02) of VFTS 243 suggest that the progenitor star experienced complete
collapse, with energy-momentum being lost predominantly through neutrinos. VFTS 243 enables us to
constrain the natal kick and neutrino-emission asymmetry during black-hole formation. At 68% confidence
level, the natal kick velocity (mass decrement) is ≲10 km=s (≲1.0M⊙), with a full probability distribution
that peaks when ≈0.3M⊙ were ejected, presumably in neutrinos, and the black hole experienced a natal
kick of 4 km=s. The neutrino-emission asymmetry is ≲4%, with best fit values of ∼0–0.2%. Such a small
neutrino natal kick accompanying black-hole formation is in agreement with theoretical predictions.
Detectability of Solar Panels as a TechnosignatureSérgio Sacani
In this work, we assess the potential detectability of solar panels made of silicon on an Earth-like
exoplanet as a potential technosignature. Silicon-based photovoltaic cells have high reflectance in the
UV-VIS and in the near-IR, within the wavelength range of a space-based flagship mission concept
like the Habitable Worlds Observatory (HWO). Assuming that only solar energy is used to provide
the 2022 human energy needs with a land cover of ∼ 2.4%, and projecting the future energy demand
assuming various growth-rate scenarios, we assess the detectability with an 8 m HWO-like telescope.
Assuming the most favorable viewing orientation, and focusing on the strong absorption edge in the
ultraviolet-to-visible (0.34 − 0.52 µm), we find that several 100s of hours of observation time is needed
to reach a SNR of 5 for an Earth-like planet around a Sun-like star at 10pc, even with a solar panel
coverage of ∼ 23% land coverage of a future Earth. We discuss the necessity of concepts like Kardeshev
Type I/II civilizations and Dyson spheres, which would aim to harness vast amounts of energy. Even
with much larger populations than today, the total energy use of human civilization would be orders of
magnitude below the threshold for causing direct thermal heating or reaching the scale of a Kardashev
Type I civilization. Any extraterrrestrial civilization that likewise achieves sustainable population
levels may also find a limit on its need to expand, which suggests that a galaxy-spanning civilization
as imagined in the Fermi paradox may not exist.
Jet reorientation in central galaxies of clusters and groups: insights from V...Sérgio Sacani
Recent observations of galaxy clusters and groups with misalignments between their central AGN jets
and X-ray cavities, or with multiple misaligned cavities, have raised concerns about the jet – bubble
connection in cooling cores, and the processes responsible for jet realignment. To investigate the
frequency and causes of such misalignments, we construct a sample of 16 cool core galaxy clusters and
groups. Using VLBA radio data we measure the parsec-scale position angle of the jets, and compare
it with the position angle of the X-ray cavities detected in Chandra data. Using the overall sample
and selected subsets, we consistently find that there is a 30% – 38% chance to find a misalignment
larger than ∆Ψ = 45◦ when observing a cluster/group with a detected jet and at least one cavity. We
determine that projection may account for an apparently large ∆Ψ only in a fraction of objects (∼35%),
and given that gas dynamical disturbances (as sloshing) are found in both aligned and misaligned
systems, we exclude environmental perturbation as the main driver of cavity – jet misalignment.
Moreover, we find that large misalignments (up to ∼ 90◦
) are favored over smaller ones (45◦ ≤ ∆Ψ ≤
70◦
), and that the change in jet direction can occur on timescales between one and a few tens of Myr.
We conclude that misalignments are more likely related to actual reorientation of the jet axis, and we
discuss several engine-based mechanisms that may cause these dramatic changes.
The solar dynamo begins near the surfaceSérgio Sacani
The magnetic dynamo cycle of the Sun features a distinct pattern: a propagating
region of sunspot emergence appears around 30° latitude and vanishes near the
equator every 11 years (ref. 1). Moreover, longitudinal flows called torsional oscillations
closely shadow sunspot migration, undoubtedly sharing a common cause2. Contrary
to theories suggesting deep origins of these phenomena, helioseismology pinpoints
low-latitude torsional oscillations to the outer 5–10% of the Sun, the near-surface
shear layer3,4. Within this zone, inwardly increasing differential rotation coupled with
a poloidal magnetic field strongly implicates the magneto-rotational instability5,6,
prominent in accretion-disk theory and observed in laboratory experiments7.
Together, these two facts prompt the general question: whether the solar dynamo is
possibly a near-surface instability. Here we report strong affirmative evidence in stark
contrast to traditional models8 focusing on the deeper tachocline. Simple analytic
estimates show that the near-surface magneto-rotational instability better explains
the spatiotemporal scales of the torsional oscillations and inferred subsurface
magnetic field amplitudes9. State-of-the-art numerical simulations corroborate these
estimates and reproduce hemispherical magnetic current helicity laws10. The dynamo
resulting from a well-understood near-surface phenomenon improves prospects
for accurate predictions of full magnetic cycles and space weather, affecting the
electromagnetic infrastructure of Earth.
Extensive Pollution of Uranus and Neptune’s Atmospheres by Upsweep of Icy Mat...Sérgio Sacani
In the Nice model of solar system formation, Uranus and Neptune undergo an orbital upheaval,
sweeping through a planetesimal disk. The region of the disk from which material is accreted by
the ice giants during this phase of their evolution has not previously been identified. We perform
direct N-body orbital simulations of the four giant planets to determine the amount and origin of solid
accretion during this orbital upheaval. We find that the ice giants undergo an extreme bombardment
event, with collision rates as much as ∼3 per hour assuming km-sized planetesimals, increasing the
total planet mass by up to ∼0.35%. In all cases, the initially outermost ice giant experiences the
largest total enhancement. We determine that for some plausible planetesimal properties, the resulting
atmospheric enrichment could potentially produce sufficient latent heat to alter the planetary cooling
timescale according to existing models. Our findings suggest that substantial accretion during this
phase of planetary evolution may have been sufficient to impact the atmospheric composition and
thermal evolution of the ice giants, motivating future work on the fate of deposited solid material.
Exomoons & Exorings with the Habitable Worlds Observatory I: On the Detection...Sérgio Sacani
The highest priority recommendation of the Astro2020 Decadal Survey for space-based astronomy
was the construction of an observatory capable of characterizing habitable worlds. In this paper series
we explore the detectability of and interference from exomoons and exorings serendipitously observed
with the proposed Habitable Worlds Observatory (HWO) as it seeks to characterize exoplanets, starting
in this manuscript with Earth-Moon analog mutual events. Unlike transits, which only occur in systems
viewed near edge-on, shadow (i.e., solar eclipse) and lunar eclipse mutual events occur in almost every
star-planet-moon system. The cadence of these events can vary widely from ∼yearly to multiple events
per day, as was the case in our younger Earth-Moon system. Leveraging previous space-based (EPOXI)
lightcurves of a Moon transit and performance predictions from the LUVOIR-B concept, we derive
the detectability of Moon analogs with HWO. We determine that Earth-Moon analogs are detectable
with observation of ∼2-20 mutual events for systems within 10 pc, and larger moons should remain
detectable out to 20 pc. We explore the extent to which exomoon mutual events can mimic planet
features and weather. We find that HWO wavelength coverage in the near-IR, specifically in the 1.4 µm
water band where large moons can outshine their host planet, will aid in differentiating exomoon signals
from exoplanet variability. Finally, we predict that exomoons formed through collision processes akin
to our Moon are more likely to be detected in younger systems, where shorter orbital periods and
favorable geometry enhance the probability and frequency of mutual events.
Emergent ribozyme behaviors in oxychlorine brines indicate a unique niche for...Sérgio Sacani
Mars is a particularly attractive candidate among known astronomical objects
to potentially host life. Results from space exploration missions have provided
insights into Martian geochemistry that indicate oxychlorine species, particularly perchlorate, are ubiquitous features of the Martian geochemical landscape. Perchlorate presents potential obstacles for known forms of life due to
its toxicity. However, it can also provide potential benefits, such as producing
brines by deliquescence, like those thought to exist on present-day Mars. Here
we show perchlorate brines support folding and catalysis of functional RNAs,
while inactivating representative protein enzymes. Additionally, we show
perchlorate and other oxychlorine species enable ribozyme functions,
including homeostasis-like regulatory behavior and ribozyme-catalyzed
chlorination of organic molecules. We suggest nucleic acids are uniquely wellsuited to hypersaline Martian environments. Furthermore, Martian near- or
subsurface oxychlorine brines, and brines found in potential lifeforms, could
provide a unique niche for biomolecular evolution.
Continuum emission from within the plunging region of black hole discsSérgio Sacani
The thermal continuum emission observed from accreting black holes across X-ray bands has the potential to be leveraged as a
powerful probe of the mass and spin of the central black hole. The vast majority of existing ‘continuum fitting’ models neglect
emission sourced at and within the innermost stable circular orbit (ISCO) of the black hole. Numerical simulations, however,
find non-zero emission sourced from these regions. In this work, we extend existing techniques by including the emission
sourced from within the plunging region, utilizing new analytical models that reproduce the properties of numerical accretion
simulations. We show that in general the neglected intra-ISCO emission produces a hot-and-small quasi-blackbody component,
but can also produce a weak power-law tail for more extreme parameter regions. A similar hot-and-small blackbody component
has been added in by hand in an ad hoc manner to previous analyses of X-ray binary spectra. We show that the X-ray spectrum
of MAXI J1820+070 in a soft-state outburst is extremely well described by a full Kerr black hole disc, while conventional
models that neglect intra-ISCO emission are unable to reproduce the data. We believe this represents the first robust detection of
intra-ISCO emission in the literature, and allows additional constraints to be placed on the MAXI J1820 + 070 black hole spin
which must be low a• < 0.5 to allow a detectable intra-ISCO region. Emission from within the ISCO is the dominant emission
component in the MAXI J1820 + 070 spectrum between 6 and 10 keV, highlighting the necessity of including this region. Our
continuum fitting model is made publicly available.
WASP-69b’s Escaping Envelope Is Confined to a Tail Extending at Least 7 RpSérgio Sacani
Studying the escaping atmospheres of highly irradiated exoplanets is critical for understanding the physical
mechanisms that shape the demographics of close-in planets. A number of planetary outflows have been observed
as excess H/He absorption during/after transit. Such an outflow has been observed for WASP-69b by multiple
groups that disagree on the geometry and velocity structure of the outflow. Here, we report the detection of this
planet’s outflow using Keck/NIRSPEC for the first time. We observed the outflow 1.28 hr after egress until the
target set, demonstrating the outflow extends at least 5.8 × 105 km or 7.5 Rp This detection is significantly longer
than previous observations, which report an outflow extending ∼2.2 planet radii just 1 yr prior. The outflow is
blueshifted by −23 km s−1 in the planetary rest frame. We estimate a current mass-loss rate of 1 M⊕ Gyr−1
. Our
observations are most consistent with an outflow that is strongly sculpted by ram pressure from the stellar wind.
However, potential variability in the outflow could be due to time-varying interactions with the stellar wind or
differences in instrumental precision.
X-rays from a Central “Exhaust Vent” of the Galactic Center ChimneySérgio Sacani
Using deep archival observations from the Chandra X-ray Observatory, we present an analysis of
linear X-ray-emitting features located within the southern portion of the Galactic center chimney,
and oriented orthogonal to the Galactic plane, centered at coordinates l = 0.08◦
, b = −1.42◦
. The
surface brightness and hardness ratio patterns are suggestive of a cylindrical morphology which may
have been produced by a plasma outflow channel extending from the Galactic center. Our fits of the
feature’s spectra favor a complex two-component model consisting of thermal and recombining plasma
components, possibly a sign of shock compression or heating of the interstellar medium by outflowing
material. Assuming a recombining plasma scenario, we further estimate the cooling timescale of this
plasma to be on the order of a few hundred to thousands of years, leading us to speculate that a
sequence of accretion events onto the Galactic Black Hole may be a plausible quasi-continuous energy
source to sustain the observed morphology
X-rays from a Central “Exhaust Vent” of the Galactic Center Chimney
X ray emission-from_strongly_asymmetric_circumstellar_material_in_the_remnant_of_kepler_supernova
1. X-ray Emission from Strongly Asymmetric Circumstellar
Material in the Remnant of Kepler’s Supernova
Mary T. Burkey,1 Stephen P. Reynolds,1 Kazimierz J. Borkowski,1 and John M. Blondin1
arXiv:1212.4534v1 [astro-ph.GA] 18 Dec 2012
ABSTRACT
Kepler’s supernova remnant resulted from a thermonuclear explosion, but is
interacting with circumstellar material (CSM) lost from the progenitor system.
We describe a statistical technique for isolating X-ray emission due to CSM from
that due to shocked ejecta. Shocked CSM coincides well in position with 24 µm
emission seen by Spitzer. We find most CSM to be distributed along the bright
north rim, but substantial concentrations are also found projected against the
center of the remnant, roughly along a diameter with position angle ∼ 100◦ .
We interpret this as evidence for a disk distribution of CSM before the SN,
with the line of sight to the observer roughly in the disk plane. We present 2-
D hydrodynamic simulations of this scenario, in qualitative agreement with the
observed CSM morphology. Our observations require Kepler to have originated
in a close binary system with an AGB star companion.
Subject headings: ISM: individual objects (G4.5+6.8) — ISM: supernova rem-
nants — X-rays: ISM — supernovae: general
1. Introduction
The remnant of Kepler’s supernova of 1604 (“Kepler” henceforth) has defied classifica-
tion since its optical recovery in 1943 (Baade 1943), because of its awkward combination
of clear evidence for dense circumstellar material (originally identified through an optical
spectrum indicating enhanced N; Minkowski 1943), suggesting a core-collapse supernova,
and light curve and location (470 kpc above the Galactic plane, for a distance of 4 kpc;
Sankrit et al. 2005) indicating a Type Ia origin. X-ray studies with ASCA indicated a large
mass of Fe (Kinugasa & Tsunemi 1999), and finally detailed X-ray studies with Chandra
(Reynolds et al. 2007; Patnaude et al. 2012) showed conclusively that the event must have
1
Department of Physics, North Carolina State University, Raleigh NC 27695-8202; reynolds@ncsu.edu
2. –2–
been a thermonuclear explosion (though its spectrum at maximum light may or may not
have resembled a traditional Ia event).
But the evidence for circumstellar material (CSM) has not gone away. Since the ques-
tion of the progenitor systems of SNe Ia is unsettled at this time, characterization of the
CSM is of great importance. Observations of Type Ia supernovae consistently fail to show
evidence for CSM (except in rare cases such as 2002ic, Hamuy et al. 2003, and 2005gj,
Aldering et al. 2006), a finding used as evidence in favor of binary white-dwarf progeni-
tor systems (double-degenerate, DD). Single-degenerate (SD) scenarios may involve either
a main-sequence or an evolved companion (see, for example, Branch 1995; Hillebrandt &
Niemeyer 2000), but searches for a surviving companion have for the most part been neg-
ative (e.g., Schaefer & Pagnotta 2012). Just as finding a companion would demand a SD
progenitor, identifying CSM in a Type Ia system requires an SD scenario. However, various
possibilities for the companion star are still possible. The most likely suggestion is an evolved
star with a slow and massive wind (Vel´zquez et al. 2006; Blair et al. 2007; Chiotellis, Schure,
a
& Vink 2012; Williams et al. 2012), i.e., an Asymptotic Giant Branch (AGB) star. In a close
binary, such a star would be likely to produce an asymmetric wind, primarily in the orbital
plane. Thus characterizing the amount and spatial distribution of CSM in the remnant of a
Type Ia supernova has important implications for the nature of Type Ia progenitors. Here
we focus on the spatial distribution of CSM, which we identify using a powerful statistical
technique applied to the long Chandra observation of Kepler.
The excess of nitrogen in optical observations suggests mass loss from an evolved star.
Various authors have performed hydrodynamic modeling of a system with substantial mass
loss moving through the ISM with speeds of hundreds of km s−1 (Borkowski et al. 1992;
Vel´zquez et al. 2006; Chiotellis et al. 2012). This modeling is able to describe the observed
a
dense shell to the north, but in these cases the wind was assumed to be isotropic. We shall
show that our spatial localization of CSM requires an asymmetric wind.
2. Observations
Kepler was observed for 741 ks with the Chandra X-ray Observatory ACIS-S CCD
camera (S3 chip) between April and July 2006. Data were processed using CIAO v3.4 and
calibrated using CALDB v3.1.0. A large background region to the north of the remnant
(covering most of the remaining area on the S3 chip) was used for all spectra. Spectral
analysis was performed with XSPEC v.12 (Arnaud 1996). We used the nonequilibrium-
ionization (NEI) v2.0 thermal models, based on the APEC/APED spectral codes (Smith
et al. 2001) and augmented by the addition of inner-shell processes (Badenes et al. 2006).
3. –3–
There are a total of about 3 × 107 source counts, with fewer than 3% background (though
some of those may be dust-scattered source photons).
3. Gaussian Mixture decomposition
Our earlier study (Reynolds et al. 2007) showed that spectral variations occur over arc-
second scales, too small for full spectra of individual regions to have adequate statistics.
This also means that simple imaging in different spectral bands is incapable of producing
quantitative information characterizing regions of different spectral character. In order to
concentrate regions with similar spectra for detailed spectral analysis, we instead collect
regions of similar spectral character by describing each with four broad-band colors, and
clustering them in a four-dimensional color space using a collection of Gaussian probabil-
ity distributions (clusters). (Such a probabilistic description of multi-dimensional data is
known in the literature as a Gaussian mixture model.) The clusters are now of much higher
signal-to-noise than individual small regions (selected by eye, for instance, as in Reynolds et
al. 2007), and were selected with objective criteria. Gaussian mixture models are well-known
in many areas of science, and are becoming more widely used for a variety of applications in
astrophysics (e.g., Shang & Oh 2012; Lee et al. 2012; Hurley et al. 2012).
The integrated spectrum of Kepler is dominated by ejecta emission from Fe, Si, and S,
while O and Mg are expected to be primarily found in CSM. We therefore selected spectral
bands to feature the oxygen Lyα line (0.3 to 0.72 keV), the iron L shell region (0.72 to
1.3 keV), the magnesium Kα line (1.3 keV to 1.5 keV), and the silicon and sulfur Kα lines
(combined because of similar characteristics and behavior in the spectrum: 1.7 to 2.1 keV
and 2.3 to 2.7 keV respectively). Other elements can contribute in these bands, for instance
Ne Kα and Lyα in the Fe L region, but the integrated spectra show that these contributions
are less significant. Each of the previous energy bands was divided by the flux in unused
energy bands (1.5 to 1.7, 2.1 to 2.3, and 2.7 to 7.0 keV), so that the entire spectral range was
used. This gave the best signal-to-noise ratio for those fluxes, but at the cost of including
some line emission in the quasi-continuum bands. However, the quasi-continuum is broad
enough to allow maximum contrast with the line features we chose to highlight. (Insisting on
using a more line-free continuum, such as 4–6 keV, has two drawbacks: first, since far fewer
counts are present there, the ratios we seek to classify would have much higher statistical
noise; and second, one cluster in particular that we identify, that of synchrotron emission,
is actually less evident in 4–6 keV emission than in broader bands.) The strong contrast in
the clusters we identify, as seen in Figure 2, shows that we are not unduly hampered by the
presence of some line emission in our quasi-continuum band.
4. –4–
Fig. 1.— Top: Merged image between 0.3 and 8 keV (Reynolds et al. 2007). Red: 0.3 – 0.72
keV; green, 0.72 – 1.7 keV; blue, 1.7 – 8 keV. All three images were smoothed using platelet
smoothing (Willett 2007). Image size is 4.7 × 3.9. Bottom: Division of Kepler into clusters
(regions of similar spectral character). Colors are arbitrary. Region R (red) contains the
bulk of the CSM, while purple (P) is dominated by synchrotron emission. Most of the rest
is ejecta. See text for further details.
5. –5–
We divided the observation into about 5000 segments with sizes adjusted to produce
comparable numbers of counts (about 6000 each), and extracted the counts in the four line
bands plus continuum for each. We then produced a four-dimensional scatterplot of all 5000
segments using the logs of normalized band counts. Using the publicly available software
package mclust written in R (Fraley et al. 2012), we decomposed the four-dimensional dis-
tribution into several four-dimensional Gaussian clusters, each characterized by at most 15
parameters: its weight relative to other clusters, a center in each of four coordinates, and
a (symmetric) 4 × 4 covariance matrix. The number of Gaussians required was found by
the algorithm using the Bayesian Information Criterion (BIC), which was optimized for nine
clusters. However, the clusters corresponding to CSM emerged clearly for any total number
of clusters between five and ten.
In the Gaussian mixture model, each segment is associated with a set of probabilities of
belonging to each of the clusters. We will from now on refer to each cluster more narrowly
as a set of segments that have the highest probability of belonging to this cluster. Figure 1
shows the spatial distribution of the segments making up each cluster. The tendency of
segments of similar spectral character to form contiguous regions gives us confidence in the
method. Summed spectra of all segments in several clusters are shown in Figure 2. We
shall refer to the clusters by abbreviations for their colors in Fig. 1: R (red), O (orange),
Y (yellow), LG (light green), DG (dark green), LB (light blue), DB (dark blue), P (purple)
and B (black). The values of the count ratios that define the center of each cluster are given
in Table 1.
The clusters containing the most prominent emission in the ejecta-dominated bands are
regions LB, DG, LG, and Y. Figure 2 (top) shows region LG contrasted with CSM-dominated
regions, while the others are shown in Figure 2 (bottom). Each of their spectra contained
similar Fe L, Si Kα and Kβ, S Kα and Kβ, Ar Kα, and Ca Kα features with the exception
of region DG, whose features were much more muted. Region LG is the brightest of the
four, because of its close proximity to the bright north rim. Region Y contains the highest
concentration of iron in the remnant in a localized patch. The outer ejecta knots containing
lighter materials are contained primarily in region O as seen by the very high silicon to
iron ratios within the spectrum. Synchrotron radiation is represented by region P as the
spectrum is almost featureless. Regions R, DB, and B contain the clearest Mg feature,
suggesting CSM. These regions are compared in the two spectral plots of Figure 2. The
vast majority of the CSM is contained within region R. Confirmation of this separation’s
effectiveness is evidenced by comparing region R to the Spitzer 24 µm image of SN1604
(Fig. 3; Blair et al. 2007) which highlights the heated dust that is evidence of CSM. The two
images are extremely similar.
6. –6–
We can examine this issue more closely. Figure 2 shows the integrated spectrum of
region R (red, top), along with region LG (green, second from top), west-central portion of
region R (third from top) with local background subtracted, and finally the spectrum of a
small knot reproduced from Reynolds et al. (2007), also with a local background subtracted
(bottom). The west-central spectrum also shows the CSM component from a multicompo-
nent fit (dotted line; see below). The contrast between regions R and LG is evident: the
different peak energies result from a higher contribution from Ne in region R and from Fe
L in region LG. Region R also shows a feature between 0.6 and 0.7 keV, O Ly α, which is
more obvious in the lower two spectra. The obviously greater prominence of both O and Mg
in all of Region R (especially in the central emission) and the smaller contribution from Fe
confirm the association of those regions with CSM.
While we might expect CSM to be found around the outer edge of the SNR, its presence
across the central region is striking. Blair, Long, & Vancura (1991) first pointed out the
presence of nonradiative shocks there, indicating partially neutral upstream material. They
found that knots in the eastern part of the central emission showed redshifts of order 500
km s−1 while those in the western part were blueshifted by about −600 km s−1 , indicating
that the former were on the far side and the latter on the near side, projected against the
center. Our completely independent analysis locates the same regions, and we adopt the
same interpretation.
4. Quantitative analysis of CSM
The integrated spectrum of Region R clearly shows distinctive features consistent with
its identification as CSM. Ideally, we would use oxygen as a tracer of CSM, as relatively
little O is present in most SN Ia models (e.g., Maeda et al. 2010). However, while an
inflection at the energy of O Lyα (0.65 keV) can be seen in some of the spectra of Figure 2,
the substantial absorbing column density makes quantitative analysis difficult. The clearest
signal of an element not expected to be synthesized in much quantity in a SN Ia (e.g.,
Maeda et al. 2010) comes from Mg Kα (1.34 keV), with the absence of Mg Lyα (1.47 keV)
as a constraint. The regions we identify as CSM have weak, but well-isolated and easily
characterized, emission features for Mg Kα, and we use that line to quantify CSM (Figure
3, center).
We subdivided the CSM-dominated region R into 14 subregions, and extracted spectra
from each. We subtracted background from a large region to the north of the remnant. Spec-
tra were fitted with two Gaussians for the two lines and power-law model for the continuum,
between 1.2 and 1.6 keV, yielding line strengths. The Lyα feature was often negligible. For
7. –7–
Fig. 2.— Top: Spectra of primary CSM regions compared to ejecta-dominated Region LG.
Top: Total of Region R. Next: Region LG. Third: West central part of Region R, with
local background subtracted; dotted curve is the CSM component from the multicomponent
spectral fit described in the text. Bottom: Small knot in central region with local background
subtracted, reproduced from Reynolds et al. (2007). Notice the clear O and Mg features in all
three CSM spectra. Bottom: Spectra of primary ejecta regions compared to the synchrotron
region and Region R. Comparable in strength to Region R is Region LB; note the difference in
shape near the peak, due to Fe L dominance of Region LB. In order of decreasing brightness
at 1 keV: Regions LG, DG, DB, Y, B, P (synchrotron) and O.
8. –8–
10 9.1 7.6
14 10
8.9
3.7 18
5.4 23
6.4
10 5.3
6.7
5.4
Fig. 3.— Top: Region R (contours) plus an IR-bright central subregion of Region LG
superposed on the 24 µm Spitzer image (Blair et al. 2007). Color scale is in MJy sr−1 .
Middle: Soft X-ray image (ct pixel−1 in 0.3 – 0.72 keV) from Chandra, with contours of
subregions of CSM-dominated Region R (contours) plus the IR-bright central LG subregion.
Numbers: Mg line-strength surface brightnesses (units: 10−8 photons cm−2 s−1 arcsec −2 ).
Bottom: Region R contours superposed on a smoothed image from 6.2 to 6.8 keV (Fe K).
The IR-bright LG subregion overlaps with the peak of central Fe K emission. Note the poor
correlation between strong Fe emission and CSM, especially in the west-central region.
9. –9–
each subregion, we calculated the line-strength surface brightness (ph cm−2 s−1 arcsec−2 ).
These values are shown in Figure 3 (center).
The west-central portion of region R, with a surface area of 410 arcsec2 and a Mg Kα
flux of 4.2×10−5 ph cm−2 s−1 , rivals the bright northern rim in Mg Kα surface brightness. In
order to learn more about its plasma properties, we modeled its spectrum with a simple plane
shock (vpshock model in XSPEC). As in Reynolds et al. (2007), we assumed an absorbing
column density NH = 5.2 × 1021 cm−2 . Wilms et al. (2000) solar abundances were adopted
for all elements except for N (fixed to 3× solar) and Ne and Mg (that we allowed to vary in
the fitting process). The ejecta contribution was modeled by a pure Fe, NEI v1.1 vpshock
model plus a separate single-ionization-timescale model vnei containing intermediate-mass
(Si, S, Ar, and Ca) elements. (We also added a Gaussian line at 0.73 keV in order to account
for missing Fe lines in the NEI v1.1 atomic code.) Ejecta contribute to the spectrum mostly
in Fe L- and K-shell lines, and in Kα lines of Si, S, and Ar (see Fig. 2). The temperature
and ionization age of the dominant CSM component are 1.2 keV and 1.1 × 1011 cm−3 s, and
the fitted Ne and Mg abundances are near solar (0.7 and 1.2, respectively). This component
is plotted as the dotted curve in Figure 2; it accounts for almost all the emission except for
obvious Fe L and K emission. Fits with a more elaborate vnpshock model with unequal ion
and electron temperatures reproduce these results. For an assumed shock velocity of ∼ 1500
km s−1 (corresponding to the mean temperature of 2.7 keV), typical for Balmer-dominated
shocks in Kepler’s SNR (Blair et al. 1991), electrons are heated first to 0.8 keV at the
shock front, and then gain energy through Coulomb collision with ions. Average properties
of the shocked CSM in the west-central portion of region R appear typical of Kepler as a
whole, although its emission measure of 0.32M d2 cm−3 is only a small fraction of the total
4
2 −3
emission measure of ∼ 10M d4 cm (see Blair et al. 2007 for a discussion of the CSM
plasma properties as derived from the spatially-integrated XMM-Newton RGS spectrum).
Blue-shifted optical emission found at this location by Blair et al. (1991) indicates that this
material was expelled toward us by the SN progenitor. We also performed a spectral analysis
of the adjacent central portion of region R farther south and east that contains red-shifted
optical emission from material expelled away from us by the SN progenitor. We found the
same Ne and Mg abundances there but a slightly higher temperature of 1.5 keV, a somewhat
shorter ionization age of 7.1 × 1010 cm−3 s, and a significantly smaller emission measure of
0.078M d2 cm−3 . These results may be interpreted as a modest density effect, where the
4
blue-shifted west-central portion of the CSM is somewhat denser than the red-shifted portion
farther south and east.
Although the spatial correlation between region R and the IR emission is reasonably
good, it is far from perfect. One reason for this might be superposition of ejecta and CSM
along the line of sight. We examined X-ray spectra at an IR bright east-central location
10. – 10 –
(see Fig. 3) that is part of the ejecta-dominated LG region. While the Fe L-shell emission
dominates, a strong Mg Kα line is also present, with a surface brightness as high as found in
central portions of region R. An image in the 6.2 – 6.8 keV energy range shows a prominent Fe
Kα-emitting filament intersecting the central CSM band at this location (Fig. 3). Apparently,
both the CSM and Fe-rich ejecta contribute significantly to the X-ray spectrum of this region.
They may even arise from physically adjacent regions, as the collision of Fe-rich ejecta with
denser than average CSM might be expected to enhance Fe L- and K-shell emission. But
the overall spatial correlation between Fe-rich ejecta (as traced by the Fe Kα emission) and
the CSM is quite poor in the central regions of the remnant, presumably because of the
asymmetric distribution of Fe within the SN ejecta.
5. Hydrodynamic simulations
The presence of the band of CSM across the center of Kepler (echoed by the presence
of nonradiative Hα emission) indicates that this material is seen in projection in front and
in back of the remnant. Such a morphology could result from a pre-SN CSM distribution
that is predominantly disk-like, as expected for mass loss from AGB stars (see Section 6
below for references), with the line of sight roughly in the plane of the disk. To examine
this possibility more closely, we performed 2-D hydrodynamic simulations of an ejecta-driven
blast wave expanding into an azimuthally varying stellar wind, with density varying by a
factor of 10 from pole to equator. Half the wind mass is within 10◦ of the equator. We ignored
the wind speed as it is a few tens of km s−1 , negligible compared to the blast-wave speed. We
did not attempt to model the north-south density gradient (Blair et al. 2007), presumably
the result of system motion to the north or northwest (Bandiera 1987). We modeled the
SN ejecta with an exponential profile appropriate for Type Ia explosions (Dwarkadas &
Chevalier 1998). We used the well-tested code VH-1, a conservative, finite-volume code
for evolving the Euler equations describing an ideal, compressible gas. The details of the
numerical simulation follow the procedure described in Warren & Blondin (2012).
Figure 4 shows a stage at which the swept-up CSM mass is about equal to the ejected
mass (1.4M ). A torus of shocked CSM occupies the equatorial plane. The lower panel
of Figure 4 shows a 3-D projection of the model, integrating the square of the density
(proportional to emission measure) along lines of sight, with the symmetry axis tilted by 10◦
to the plane of the sky. The result is an incomplete bar of shocked CSM across the remnant
center as observed, whose radius is about half that of the extent to the north and south,
in rough agreement with the central bar of CSM we observe. While the simulation is only
suggestive, it indicates that more detailed study of such a model may lead to an improved
11. – 11 –
Fig. 4.— Top: Hydrodynamic simulation of density as a blast wave encounters an equatorial
wind. At this time, the swept-up mass is comparable to the ejected mass of 1.4M . The
wind speed has been neglected compared to the blast-wave speed. Bottom: Line-of-sight
integration of density squared from the hydrodynamic simulation. The symmetry axis is
tilted out of the sky plane by 10◦ .
12. – 12 –
understanding of Kepler’s dynamics.
6. Discussion
Our observations of CSM toward the center of Kepler are naturally explained by a disk
seen edge-on, while the bright northern rim results from northward motion of the system
(Bandiera 1987). Such an asymmetric distribution of CSM is expected from a binary system,
where an equatorial disk of enhanced mass loss is likely. Inferences of CSM around at least
a few Type Ia SNe continue to accumulate (e.g., SN 2008J; Taddia et al. 2012; PTF11kx;
Dilday et al. 2012). In particular, Dilday et al. (2012) infer a substantially asymmetric CSM
distribution, suggesting a symbiotic binary progenitor system with mass loss concentrated
in the orbital plane. In an SD model for Kepler, as required by the CSM, we expect the
companion to have been an AGB star. Some AGB stars show dense envelopes accessible
to study through molecular emission such as CO; on scales of 10 − 20 , the emission is
relatively symmetric (Neri et al. 1998). A few systems seem to require asymmetry, though the
incomplete sampling of the 3-element IRAM interferometer used by Neri et al. (1998) make
detailed imaging impossible. Additional observational evidence for asymmetric winds from
evolved stars is presented by Chiu et al. (2009) and Huggins (2007). On theoretical grounds,
we expect that the winds from AGB stars in detached binaries can, through gravitational
focusing by the companion, produce highly asymmetric CSM, with density contrasts of 10
or more found in numerical simulations by Mastrodemos & Morris (1999) and characterized
by Huggins, Mauron, & Wirth (2009). Politano & Taam (2011) find that several percent of
AGB systems should show such strong asymmetries. In a particular case, 3D simulations of
RS Oph (Walder, Folini, & Shore 2008) show very large equator-to-pole density variations
on the scale of the orbital separation, averaging to factors of 2 – 3 on much larger scales.
These simulations do not consider the possibility of a wind from the white-dwarf companion
(Hachisu et al. 1996), which would tend to evacuate material perpendicular to the plane of
the disk and enhance the equator-to-pole density contrast.
The distribution of strong Fe emission in Kepler is worthy of note. Most Type Ia SN
models produce highly stratified ejecta, so most Fe should be in the remnant interior. Where
the ejecta impact the dense wind in the equatorial plane, then, one might expect enhanced
Fe emission – so in a plane roughly coincident with the plane of central CSM. This does not
appear to be the case. Fe Kα emission toward Kepler’s interior is asymmetrically distributed,
with one patch near the east-central CSM emission, but less near the west-central CSM. We
speculate that one cause of Fe asymmetry might be the “shadow” in Fe cast by the companion
star, blocking the ejection of material in that direction. Pan et al. (2012) estimate that a
13. – 13 –
RG companion can shadow up to 18% of the solid angle from the ejecta of a Type Ia SN,
while Garc´ıa-Senz et al. (2012) show that morphological effects of a companion’s shadow can
survive for hundreds of years. Further study of Fe in particular in Kepler will allow us to
examine this possibility in more detail.
7. Conclusions
We have used Gaussian mixture decompositions of multicolor X-ray spectral data from
the long Chandra observation of Kepler’s supernova remnant to identify and characterize
regions of shocked circumstellar material, as distinct from the ejecta that dominate the
integrated spectrum. We find that shocked CSM is co-located with both nonradiative shocks
identified by Hα emission, and with 24 µm dust continuum emission seen with Spitzer. We
suggest that the central band of CSM is the remnant of a circumstellar disk seen edge-
on. Our 2-D hydrodynamic simulation shows that a blast wave encountering an equatorial
wind from a companion can naturally produce this morphology. The asymmetry we observe
requires a binary progenitor system in which the donor is an evolved (AGB) star.
This work was supported by the National Science Foundation through award AST-
0708224 and by NASA through grant NNX11AB14G.
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This preprint was prepared with the AAS L TEX macros v5.2.
A
16. – 16 –
Table 1. Cluster Centers
Energy Band B P DB LB DG LG Y O R
O 0.89 0.73 1.01 1.13 1.1 1.11 1.23 0.87 1.05
Fe 1.2 1.07 1.35 1.49 1.44 1.47 1.68 1.31 1.39
Mg 0.9 0.87 0.96 1.02 0.97 1.03 1.07 0.89 1.05
Si/S 1.1 0.97 1.11 1.19 1.16 1.17 1.19 1.13 1.14
Note. — Entries are the logarithms of ratios of counts in the four energy
bands to continuum at the centers of each of the nine clusters identified by
the Gaussian Mixture Method. Bands are described more fully in the text.