1) Astronomers observed comet Hale-Bopp at 30.7 AU from the Sun using the ESO 2.2m telescope in Chile on December 4, 2010.
2) They detected the comet with a total brightness of R=23.3 mag, corresponding to an absolute brightness of R(1,1,0)=8.3 mag.
3) The profile of the comet was star-like without any evidence of an extended coma or tail, indicating a cessation of matter production from the comet. However, the measured brightness corresponds to a reflecting surface area nine times smaller than three years prior, suggesting some low-level activity may still be occurring.
Science with small telescopes - exoplanetsguest8aa6ebb
The search for extrasolar planets has become one of the most attractive problems in modern astrophysics. The biggest observatories in the world are involved in this task as well as little amateur instruments. There is also a huge variety of astronomical methods used for their investigation. Here I present the projects for searching for exoplanets by transit method and our observations of the planet WASP-2b. We observed a transit on 3/4 August 2008 with a 354 mm Schmidt-Cassegrain Celestron telescope and CCD SBIG STL 11000M camera. By precise photometry made using MaximDL software we obtained the light curve of the star system. Decrease of brightness by 0.02m is detected. Analyzing our data we estimate the radius of the planet and inclination of its orbit. Our results are in good correlation with the published information in literature.
SPECTROSCOPIC CONFIRMATION OF THE EXISTENCE OF LARGE, DIFFUSE GALAXIES IN THE...Sérgio Sacani
We recently identified a population of low surface brightness objects in the field of the z = 0.023 Coma cluster,
using the Dragonfly Telephoto Array. Here we present Keck spectroscopy of one of the largest of these “ultradiffuse
galaxies” (UDGs), confirming that it is a member of the cluster. The galaxy has prominent absorption
features, including the Ca II H+K lines and the G-band, and no detected emission lines. Its radial velocity of
cz=6280±120 km s−1 is within the 1σ velocity dispersion of the Coma cluster. The galaxy has an effective
radius of 4.3 ± 0.3 kpc and a Sérsic index of 0.89 ± 0.06, as measured from Keck imaging. We find no indications
of tidal tails or other distortions, at least out to a radius of ∼2re. We show that UDGs are located in a previously
sparsely populated region of the size—magnitude plane of quiescent stellar systems, as they are ∼6 mag fainter
than normal early-type galaxies of the same size. It appears that the luminosity distribution of large quiescent
galaxies is not continuous, although this could largely be due to selection effects. Dynamical measurements are
needed to determine whether the dark matter halos of UDGs are similar to those of galaxies with the same
luminosity or to those of galaxies with the same size.
Evidence for widespread hydrated minerals on asteroid (101955) BennuSérgio Sacani
Early spectral data from the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRISREx) mission reveal evidence for abundant hydrated minerals on the surface of near-Earth asteroid (101955) Bennu in the
form of a near-infrared absorption near 2.7 µm and thermal infrared spectral features that are most similar to those of aqueously altered CM-type carbonaceous chondrites. We observe these spectral features across the surface of Bennu, and there
is no evidence of substantial rotational variability at the spatial scales of tens to hundreds of metres observed to date. In the
visible and near-infrared (0.4 to 2.4 µm) Bennu’s spectrum appears featureless and with a blue (negative) slope, confirming
previous ground-based observations. Bennu may represent a class of objects that could have brought volatiles and organic
chemistry to Earth.
Science with small telescopes - exoplanetsguest8aa6ebb
The search for extrasolar planets has become one of the most attractive problems in modern astrophysics. The biggest observatories in the world are involved in this task as well as little amateur instruments. There is also a huge variety of astronomical methods used for their investigation. Here I present the projects for searching for exoplanets by transit method and our observations of the planet WASP-2b. We observed a transit on 3/4 August 2008 with a 354 mm Schmidt-Cassegrain Celestron telescope and CCD SBIG STL 11000M camera. By precise photometry made using MaximDL software we obtained the light curve of the star system. Decrease of brightness by 0.02m is detected. Analyzing our data we estimate the radius of the planet and inclination of its orbit. Our results are in good correlation with the published information in literature.
SPECTROSCOPIC CONFIRMATION OF THE EXISTENCE OF LARGE, DIFFUSE GALAXIES IN THE...Sérgio Sacani
We recently identified a population of low surface brightness objects in the field of the z = 0.023 Coma cluster,
using the Dragonfly Telephoto Array. Here we present Keck spectroscopy of one of the largest of these “ultradiffuse
galaxies” (UDGs), confirming that it is a member of the cluster. The galaxy has prominent absorption
features, including the Ca II H+K lines and the G-band, and no detected emission lines. Its radial velocity of
cz=6280±120 km s−1 is within the 1σ velocity dispersion of the Coma cluster. The galaxy has an effective
radius of 4.3 ± 0.3 kpc and a Sérsic index of 0.89 ± 0.06, as measured from Keck imaging. We find no indications
of tidal tails or other distortions, at least out to a radius of ∼2re. We show that UDGs are located in a previously
sparsely populated region of the size—magnitude plane of quiescent stellar systems, as they are ∼6 mag fainter
than normal early-type galaxies of the same size. It appears that the luminosity distribution of large quiescent
galaxies is not continuous, although this could largely be due to selection effects. Dynamical measurements are
needed to determine whether the dark matter halos of UDGs are similar to those of galaxies with the same
luminosity or to those of galaxies with the same size.
Evidence for widespread hydrated minerals on asteroid (101955) BennuSérgio Sacani
Early spectral data from the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRISREx) mission reveal evidence for abundant hydrated minerals on the surface of near-Earth asteroid (101955) Bennu in the
form of a near-infrared absorption near 2.7 µm and thermal infrared spectral features that are most similar to those of aqueously altered CM-type carbonaceous chondrites. We observe these spectral features across the surface of Bennu, and there
is no evidence of substantial rotational variability at the spatial scales of tens to hundreds of metres observed to date. In the
visible and near-infrared (0.4 to 2.4 µm) Bennu’s spectrum appears featureless and with a blue (negative) slope, confirming
previous ground-based observations. Bennu may represent a class of objects that could have brought volatiles and organic
chemistry to Earth.
The extremely high albedo of LTT 9779 b revealed by CHEOPSSérgio Sacani
Optical secondary eclipse measurements of small planets can provide a wealth of information about the reflective properties
of these worlds, but the measurements are particularly challenging to attain because of their relatively shallow depth. If such signals
can be detected and modeled, however, they can provide planetary albedos, thermal characteristics, and information on absorbers in
the upper atmosphere.
Aims. We aim to detect and characterize the optical secondary eclipse of the planet LTT 9779 b using the CHaracterising ExOPlanet
Satellite (CHEOPS) to measure the planetary albedo and search for the signature of atmospheric condensates.
Methods. We observed ten secondary eclipses of the planet with CHEOPS. We carefully analyzed and detrended the light curves using
three independent methods to perform the final astrophysical detrending and eclipse model fitting of the individual and combined light
curves.
Results. Each of our analysis methods yielded statistically similar results, providing a robust detection of the eclipse of LTT 9779 b
with a depth of 115±24 ppm. This surprisingly large depth provides a geometric albedo for the planet of 0.80+0.10
−0.17, consistent with
estimates of radiative-convective models. This value is similar to that of Venus in our own Solar System. When combining the eclipse
from CHEOPS with the measurements from TESS and Spitzer, our global climate models indicate that LTT 9779 b likely has a super
metal-rich atmosphere, with a lower limit of 400× solar being found, and the presence of silicate clouds. The observations also reveal
hints of optical eclipse depth variability, but these have yet to be confirmed.
Conclusions. The results found here in the optical when combined with those in the near-infrared provide the first steps toward
understanding the atmospheric structure and physical processes of ultrahot Neptune worlds that inhabit the Neptune desert.
Measurements of the_near_nucleus_coma_of_comet_67_p_churyumov_gerasimenko_wit...Sérgio Sacani
Artigo descreve descoberta feita pelo instrumento Alice da sonda Rosetta no cometa 67P/Churyumov-Gerasimenko, das moléculas de água e dióxido de carbono quebradas que pairam pela atmosfera do cometa.
Spectroscopy and thermal modelling of the first interstellar object 1I/2017 U...Sérgio Sacani
During the formation and evolution of the Solar System, significant
numbers of cometary and asteroidal bodies were
ejected into interstellar space1,2. It is reasonable to expect that
the same happened for planetary systems other than our own.
Detection of such interstellar objects would allow us to probe
the planetesimal formation processes around other stars, possibly
together with the effects of long-term exposure to the
interstellar medium. 1I/2017 U1 ‘Oumuamua is the first known
interstellar object, discovered by the Pan-STARRS1 telescope
in October 2017 (ref. 3). The discovery epoch photometry
implies a highly elongated body with radii of ~ 200 × 20 m
when a comet-like geometric albedo of 0.04 is assumed. The
observable interstellar object population is expected to be
dominated by comet-like bodies in agreement with our spectra,
yet the reported inactivity of 'Oumuamua implies a lack
of surface ice. Here, we report spectroscopic characterization
of ‘Oumuamua, finding it to be variable with time but similar
to organically rich surfaces found in the outer Solar System.
We show that this is consistent with predictions of an insulating
mantle produced by long-term cosmic ray exposure4.
An internal icy composition cannot therefore be ruled out by
the lack of activity, even though ‘Oumuamua passed within
0.25 au of the Sun.
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.
Confirmation of the_ogle_planet_signature_and_its_characteristics_with_lens_s...Sérgio Sacani
O Telescópio Espacial Hubble e o Observatório W. M. Keck, no Havaí, fizeram confirmações independentes de um exoplaneta orbitando sua estrela central de uma distância bem grande. O planeta foi descoberto através de uma técnica chamada de microlente gravitacional.
Essa descoberta traz uma nova peça para o processo de caçada de exoplanetas: para descobrir planetas longe de suas estrelas, como Júpiter e Saturno estão do Sol. Os resultados obtidos pelo Hubble e pelo Keck apareceram em dois artigos da edição de 30 de Julho de 2015 do The Astrophysical Journal.
A grande maioria dos exoplanetas catalogados são aqueles localizados bem perto de suas estrelas, isso acontece porque as técnicas atuais de se caçar exoplanetas favorecem a descoberta de planetas com curtos períodos orbitais. Mas esse não é o caso da técnica de microlente gravitacional, que pode encontrar planetas mais frios e mais distantes com órbitas de longo período que outros métodos não são capazes de detectar.
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
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Frozen to death
1. Astronomy & Astrophysics manuscript no. hb c ESO 2011
April 25, 2011
Frozen to death? – Detection of comet Hale-Bopp at 30.7 AU
Gy. M. Szab´1,2,3 , K. S´rneczky1 , L. L. Kiss1,4
o a
1
Konkoly Observatory, Konkoly-Thege Mikl´s ut 15-17, H1121 Budapest, Hungary
o ´
2
E¨tv¨s Fellow at the Astronomy Dept., University of Texas at Austin, 78712 TX, USA
o o
3
Dept. of Exp. Physics & Astronomical Observatory, University of Szeged, 6720 Szeged, Hungary
4
Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
ABSTRACT
arXiv:1104.4351v1 [astro-ph.EP] 21 Apr 2011
Context. Comet Hale–Bopp (C/1995 O1) has been the single most significant comet encountered by modern astronomy,
still having displayed significant activity at 25.7 AU solar distance in late 2007. It is a puzzling question when and
where this activity will finally cease.
Aims. Here we present new observations with the ESO 2.2m telescope at La Silla to check the activity of Hale–Bopp at
30.7 AU solar distance.
Methods. On 2010-12-04, 26 CCD images were taken with 180 s exposure times for photometry and morphology.
m m
Results. The comet was detected in R and had a total brightness of 23. 3±0. 2, referring to an absolute brightness of
m ′′
R(1,1,0)=8. 3. The profile of the coma was star-like at a seeing of 1. 9, without any evidence of a coma or tail extending
farther than 2. 5 (=55,000 km in projection) and exceeding 26.5 mag/arcs2 surface brightness.
′′
Conclusions. The measured total brightness corresponds to a relative total reflecting surface, aR C, of 485 km2 , nine
times less than three years before. The calculated aR C value would imply a nucleus with 60–65 km radius assuming 4%
albedo. This size estimate is in significant contradiction with the previous results scattering around 35 km. Therefore
we suggest that the comet may still be in a low-level activity, despite the lack of a prominent coma. Alternatively, if the
nucleus is already dormant, the albedo should be as high as 13%, assuming a radius of 35 km. With this observation,
Hale–Bopp has been the most distant comet ever observed, far beyond the orbit of Neptune.
1. Introduction. paper we present a photometrical and morphological anal-
ysis of these observations.
According to the paradigm of cometary activity, matter
production decreases fast after the rapid decrease of water
sublimation at 3 AU (Fern´ndez 2005). A few exceptional
a 2. Observations
comets, however, displayed activity far beyond 3 AU (e.g.
Lowry et al. 1999; Lowry & Fitzsimmons 2001, 2005; Lowry We made new RC filtered observations with the 2.2 m ESO
& Wiessman 2003; Snodgrass et al. 2006, 2008; Mazzotta telescope at the La Silla site in Chile, on December 4, 2010,
Epifani et al. 2006, 2007). There are known examples of ac- with sidereal tracking. We took 26×180 s exposures with an
′′ ′′
tive long period comets (Szab´ et al. 2001a, 2002, Meech et
o image scale of 0. 24/pixel. The seeing was ≈1. 9, mostly due
al. 2004, Mazzotta Epifani et al. 2009, Ivanova et al. 2011) to the airmass of 1.9–2.1, which was caused by the comet’s
and Centaur objects (e.g. Meech et al. 1997, Rousselot 2008, unfavourable celestial position close to the Southern pole.
Jewitt 2009). The exact location of the snow line is very im- The images were corrected in a standard fashion, includ-
portant in understanding the water reservoirs in the Solar ing bias and flat field correction. We aligned and co-added
System, and therefore the structure and evolution of extra- the images by fitting a coordinate grid to the stars, yield-
solar systems, too. ing a “star field” image for photometric calibrations. The
images were then re-aligned with respect to the proper mo-
In Szab´ et al. (2008) we have shown that C/1995 O1
o tion of the comet, to get untrailed “comet” images. In this
(Hale–Bopp) was the most distant active comet ever ob- step, the MPC ephemerides at the time of each observation
served, with a prominent coma at 25.7 AU solar distance. were used to match the individual frames. Fig. 1 shows the
This surprising observation raised the question where this “comet” images on December 4, 2010, after co-adding the
activity will cease. Observing Hale–Bopp in a completely images taken between 5:41–6:48 UT (top panel), and be-
frozen state would be also extremely important because a tween 7:08–8:10 UT (lower panel). The proper motion of
thick coma was constantly present during the entire ap- the comet confirms its identification.
parition. The coma obscured the nucleus which was not Hale–Bopp was detected very slightly northwest from
observed directly. Lacking photometric data of the bared the positions predicted for the La Silla site at the time
nucleus, its size, one of the most important input parame- of observations (Table 1, astrometry part; Fig. 1 lowest
ter in activity models remained uncertain. panel). The mean deviations and their rms scatter in right
′′ ′′ ′′ ′′
Here we present new observations of Hale–Bopp at 30.7 ascension and declination are 0. 59±0. 06 and 0. 25±0. 20,
AU solar distance. In late 2010, the comet exhibited evi- respectively. The confidence interval of the measured po-
dence for the cessation of the matter production. In this sitions was tested by astrometry of artificial sources: with
2. 2 Gy. M. Szab´, K. S´rneczky,L. L. Kiss: Detection of comet Hale-Bopp at 30.7 AU
o a
addstar routine in IRAF, we added 18 artificial comets to Table 1. Ephemerides and astronetry of Hale-Bopp on
the image with the same PSF and brightness as the comet 2010-12-04.
image itself. Because of the noise and sampling, the astrom-
etry of these artificial comets resulted in slightly different Ephemerides
positions than where they were inserted. The standard error λ [◦ ] β [◦ ] R [AU] ∆ [AU] E [◦ ] α [◦ ]
′′
of these differences resulted to be 0. 23 in both coordinates, 278.8 −66.9 30.7 31.0 69.5 1.8
which can be considered as the 1-sigma confidence level er- Astrometry
ror of the measured positions of Hale-Bopp. NASA JPL UT RA (00h 38m ) Dec (-86◦ 32′ )
s Pred. Meas. Pred. Meas.
Horizon reports 3-σ uncertainties on the position of 0.45
s s ′′ ′′
′′
and 0. 10 in RA and DEC, respectively. The measured very 6:15 42. 94 42. 28 53. 9 53. 8
s s ′′ ′′
marginal differences may be attributed to non-gravitational 6:57 41. 17 40. 60 50. 9 50. 5
s s ′′ ′′
forces from the distant activity of the comet. 7:39 39. 42 38. 73 47. 8 47. 0
For the photometry, we used local comparison stars
taken from the USNO-B catalog. To check the consistency
past 3 years. The measured total magnitude of Hale–Bopp
of the magnitude zero-point, another method was also used, m m
was R =23. 3±0. 2 in 2010, referring to an absolute bright-
based on synthetic RC magnitudes from 2MASS photom- m
ness of mR (1,1,0)=8. 3. This brightness can be converted to
etry. For calculating the synthetic RC magnitudes, we col-
aR C, the total reflecting surface in the aperture (Eddington
lected 2MASS photometry of all stars in the Landolt (1992)
1910),
catalog and followed the method of least-square fitting of
the color indices (see Bilir et al. 2008, who give transforma- 0.4(m⊙ −mcomet )
2.22 × 1022 πR2 ∆2 10
tions from 2MASS magnitudes to Sloan gr and ri colours; aR C = . (2)
we followed the same procedure to determine a transfor- 10−0.4αβ
mation from 2MASS magnitudes to RC magnitude). We m
Here m⊙ = −27. 11, the apparent RC brightness of the
excluded all stars with K > 13 from the fitting, and also Sun, and the β phase coefficient is usually assumed to
restricted J − K < 0.4 where the color-color distributions be 0.04. Substituting the measured total brightness yields
were narrow and linear. We then determined aR C ≈ 485 km2 . This value is only 11% that of measured in
late 2007, when Hale–Bopp was at 25.7 AU solar distance.
RC = J − 0.766(J − H) + 2.364(J − K) + 0.213 ± 0.055, (1) Unlike most comets with distant activity, the inner coma
of Hale–Bopp followed a distinct, flat profile (Szab´ et al.
o
valid for stars (J − K < 0.4). We identified 15 appropri-
2008). In 2010 we did not detect such a coma, and the pro-
ate stars in our images which had 2MASS photometry. Eq
file of Hale–Bopp was not broader than that of the stars
1 has been applied to these stars, and finally the photo-
(Fig. 2). This indicated the lack of a bright, extended in-
metric zero-point of the average of 180 s images resulted
m m ner coma. The measured intensity peak of the comet had an
to be 24. 42±0. 06 magnitudes. This result is absolutely
m apparent surface brightness of 24.3 mag/arcs2. For compar-
consistent with a zero-point of 24. 45±0.05, derived from
ison, the peak intensity was 20.6 mag/arcs2 3 years before,
the R photometry presented in the USNO-B catalog. The ′′
and that time a flat intensity plateau extended to ≈ 2. 5
4% difference of the two different zero-points are much less
from the nucleus. By late 2010, the bright flat coma van-
than the photon noise of the comet image itself. We finally
ished, suggesting that the matter production in 2010 was
adopted a zero-point of 24.43 magnitudes at the average
at least an order of magnitude less than in 2007. This es-
airmass of ≈2 where the observations were taken. This ZP
timate is also compatible with the measured decrement of
is consitent with the standard value of WFI of ≈24.4–24.6
aR C by an order of magnitude. In 2010, the comet also
in the Zenith (chip #6), and the average R extinction co- ′′
lacked coma or tail extending farther than 2. 5 and exceed-
efficient of 0.07–0.12. 2
ing 26.5 mag/arcs surface brightness (which is the limit of
The comet and the comparison stars were measured
′′ a 2-σ detection of extended sources above the background
in an aperture which had 2. 9 diameter, i.e. 1.5-times
noise). Since coma-like feature is not present in the images,
the FWHM of stellar profiles. This is quite close to the
we conclude that Hale-Bopp had a very faint outer coma,
aperture of the optimal S/N (1.3×FWHM, Szab´ et al.o
if any, at 30,7 AU solar distance.
2001b), and contains 85% of the starlight. The total pho-
m Because aR C of 485 km2 is in a contradiction with
ton noise (star+background) was 0. 08 (1-σ confidence in-
the expectations based on previous size estimates, we can-
terval). Because the comparison stars were measured with
not exclude that a loose coma component was still present
the same aperture, there was no need to aperture correc-
m around Hale–Bopp at the time of observation. The value
tion. Estimating conservatively an error of ≈0. 1 in both
of aR C would refer to a total cross section of 12,000 km2
ZP and in the systematics from aperture effects, the to-
m of the reflecting matter, assuming 4% albedo. A bared nu-
tal error results to be 0. 2. Surface photometry has been
cleus could only support this value if it would be as large
applied to the each pixels of the raw “comet” image, i.e.
as 62 km (radius), which is incompatible to the previous
without deconvolution to the seeing. Errors were estimated
size estimates (e.g. 35 km, Weaver and Lamy 1997; 30 km,
based on the background scatter of 1.27 ADU/pixel, or 0.07
Fern´ndez 2003; 37 km, Lamy et al. 2004). Therefore, some
a
ADU/arcs2 . The surface brightness profile is plotted in Fig.
part of the reflecting matter must be present in the form of
2.
a loose coma. This may be an indirect evidence for a low-
level activity of Hale–Bopp, beyond the orbit of Neptune.
3. Results Another possibility is that the comet is already dormant,
but there might still be dust grains around the nucleus. If
Compared to our previous observations, the general ap- there is plenty dust in the aperture, this can significantly
pearance of the comet has drastically changed during the contribute to the total light.
3. Gy. M. Szab´, K. S´rneczky,L. L. Kiss: Detection of comet Hale-Bopp at 30.7 AU
o a 3
ρ (arcsec)
0.2 0.5 1 2 5
24.0
24.5 * *
**
25.0
**
µ (mag/arcs2)
* **
25.5 * **
* Hale−Bopp
*
26.0 Stellar profile ** *
** *
* *** * *
26.5
Surface detection limit
27.0 * * ** *
**
* 50* 100
5 10 20
ρ (103 km)
Fig. 2. Surface brightness profile of Hale–Bopp (stars),
compared to the average profile of faint stars (dashed line).
The dotted line shows the 2-σ limit of surface brightness
detection in the comet image.
14
46
7:39 UT
16
48
18
DEC (−86:32:)
20
50
6:57 UT
mag
22
Hale-Bopp
24
52
C/1987 H1
6:15 UT 26 C/1987 F1
C/1984 K1
54
28 C/1983 O1 young Oort-cloud comets
44 43 42 41 40 39 38
RA (00:38:) 30
5 10 15 20 25 30 35 40
Fig. 1. Upper images: comet Hale–Bopp at 30.7 AU so- R (AU)
lar distance, on December 4, 2010. The field of view is
′ ′
2. 67×1. 57; north is up, east is to the left. The brightest
Fig. 3. The light curve of Hale–Bopp compared to obser-
star in the image is 2MASS J00395120-8632270, which has
vations of 6 dynamically young Oort-cloud comets (Meech
an R magnitude of 12.93±0.06. Lowest panel: Comparison
et al. 2004; the last five point are upper limits). The top
of the measured astrometric positions (open dots with er-
dashed curve shows the prediction of distant activity by
rors) and the JPL predictions (solid dots). Error bars de-
Capria et al. (2002). The middle solid curve is the bright-
note the 2-sigma level confidence intervals of the measured
ness prediction if the nucleus is already dormant. The lower
positions.
dashed curve plots the expected brightness of a 37 km size
nucleus (radius) with 4% albedo (Lamy et al. 2004).
In Fig. 3, we plot the observed brightness of Hale–Bopp
(collected from ICQ and MPC bulletins) against the so-
lar distance. For comparison, data for 6 dynamically young observations, Hale–Bopp was consistently 3–5 magnitudes
Oort-comets are also plotted (Meech et al. 2004). We also brighter than these Oort-comets at distances <15 AU. By
show a theoretical light curve predicted from the CO pro- 2010, the total brightness of the comet has diminished by
duction curve by Capria et al. (2002), assuming water ice 1.5–2 magnitudes. The observed sudden decrease of the
crystallization heating the nucleus. In the case of previous total brightness, together with the disappearance of the
4. 4 Gy. M. Szab´, K. S´rneczky,L. L. Kiss: Detection of comet Hale-Bopp at 30.7 AU
o a
prominent coma shows that Hale–Bopp is now on the way somewhat larger than one would expect for a nucleus
of freezing out: the cessation of matter production. with 30–40 km radius (unless its albedo is very high),
so we suggest some light is scattered by a faint coma
which is still present.
4. Discussion and summary 3. The comet has a star-like appearance, therefore we con-
Our observation is worth comparing to the observations of cluded the comet lacks the presence of the thick coma
1P/Halley at large heliocentric distances (Hainaut et al. which was characteristic to this comet at all observed
2004). Observed at 28.1 AU, comet Halley had been the solar distances. This situation will finally enable a re-
most distant comet ever observed until our present obser- liable coma-nucleus separation and the photometry of
vations of Hale–Bopp. With the VLT an integration time of the nucleus with large telescopes such as VLT.
almost 9 hours has been applied, leading to the detection
of Halley’s nucleus with S/N=8. That time, comet Halley Acknowledgments
was a 28.1 magnitude atmosphereless body. The significant
difference between these comets (over the simple fact that This project has been supported by the Hungarian OTKA
Hale–Bopp is larger and brighter) is how far out comet Grants K76816, K83790 and MB08C 81013, the “Lend¨ let”
u
Hale–Bopp could maintain an observable matter produc- Program of the Hungarian Academy of Sciences, and the
tion. The activity of comet Halley ceased at around 11 AU E¨tv¨s Fellowship of the Hungarian State.
o o
solar distance. Interestingly, an outburst was observed in
1991 when comet Halley suddely brightened 5 magnitudes,
when the comet was at around 14.2 AU solar distance (West References
et al. 1991). A’Hearn, M.F. et al., 1984, AJ, 89, 579
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Capria, M.T., Coradini, A., De Sanctis, M.C., 2002, EMP, 90, 217
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an alternative explanation of the measured brightness. A 417, 1159
bared nucleus with 35 km radius can exhibit the observed Jewitt, David, 2009, AJ, 137, 4296
Landolt, A.U., 1992, AJ, 104, 340
brightness if it has a higher albedo, approximately 13%. Ivanova, O. V., Skorov, Y. V., Korsun, P. P., Afanasiev, V. L., Blum,
This is significantly higher than the albedo of cometary J., 2011, Icarus, 211, 559
nuclei in general and that of derived for Hale–Bopp espe- Lamy, P. L., T´th, I., Fern´ndez, Y. R., Weaver, H. A., 2004, in:
o a
cially (4%, Lamy et al. 2004). But considering a possible Comets II (eds. M. C. Festou, H. U. Keller, and H. A. Weaver),
University of Arizona Press, Tucson, p.223
re-condensation of the gases to the surface during the dis- Lowry, S.C., Fitzsimmons, A., Cartwright I.M., Williams, I.P., 1999,
tant activity, it would be even plausible that the surface A&A, 349, 649
has been covered with a thin ice film that could increase Lowry, S.C., Fitzsimmons, A., 2001, A&A, 365, 204
the albedo. In the case of this scenario, the light curve of Lowry, S.C., Weissman P.R., 2003, Icarus, 164, 492
Hale–Bopp will follow the path of our brighter prediction. Lowry, S.C., Fitzsimmons, A., 2005, MNRAS, 358, 641
Mazzotta Epifani, E., Palumbo, P., Capria, M.T., Cremonese, G.,
If the measured excess light is due to low-level activity, the Fulle, M., Colangeli, L., 2006, A&A, 460, 935
light curve will converge to the fainter prediction in the Mazzotta Epifani, E., Palumbo, P., Capria, M.T., Cremonese, G.,
future. Fulle, M., Colangeli, L., 2007, MNRAS, 381, 713
In Szab´ et al. (2008) we estimated the basic parameters
o Mazzotta Epifani, E., Palumbo, P., Capria, M. T., Cremonese, G.,
Fulle, M., Colangeli, L., 2009, A&A, 502, 355
of the activity of Hale–Bopp at 25,7 AU solar distance. We Meech, K.J., Buie, M.W., Samarasinha, N.H., Mueller, B.E.A.,
estimated its temperature was 53.1 K. This was slightly Belton, M.J.S., 1997, AJ, 113, 844
less than 54.8 K for a blackbody, due to the sublimation Meech, K.J., Hainaut, O.R., Marsden, B.G., 2004, Icarus, 170, 463
of 2 × 1019 CO molecules/m2 /s. In 30.7 AU solar distance, Rousselot, P., 2008, A&A, 480, 543
the equivalent temperature of a blackbody is 50.1 K. Since Snodgrass, C., Lowry, S.C., Fitzsimmons, A., 2006, MNRAS, 373,
1590
Hale–Bopp seems to turn into inactive state, we infer that Snodgrass, C., Lowry, S.C., Fitzsimmons, A., 2008, MNRAS, 385, 737
the temperature of activity cessation is somewhere between Szab´, Gy.M., Cs´k, B., S´rneczky, K., Kiss, L.L., 2001a, A&A, 374,
o a a
50–53 K for a Hale–Bopp type comet. 712
The main results of this paper can be summarized as Szab´, Gy.M., Cs´k, B., S´rneczky, K., Kiss, L.L., 2001b, A&A, 375,
o a a
285
follows: Szab´, Gy.M., Kiss, L.L., S´rneczky, K., Szil´di, K., 2002, A&A, 384,
o a a
702
1. We detected comet Hale–Bopp at 30.7 AU, which is the Szab´, Gy.M., Kiss, L.L., S´rneczky, K., 2008, ApJ, 677, L121
o a
most distant detection of a comet so far. Weaver, H.A. et al., 1997, Science, 275, 1900
2. The absolute brightness of Hale–Bopp was West, R. M., Hainaut, O., Smette, A., A&A, 246, 77
m
R(1,1,0)=8. 3. The total relative reflecting surface
of the 40,000 km environment around the nucleus,
aR C is 485 km2 . This is a factor of 9 less than that
we observed in 2007, and suggests the comet is near
to ceasing its activity. However, this cross section is