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Topographic Constraints on the Origin of the Equatorial Ridge on Iapetus: Saturn's moon Iapetus has an equatorial ridge system, which may be as high as
20 km, that may have formed by endogenic forces, such as tectonic and
convective forces, or exogenic processes such as debris infall. We use
high-resolution topographic data to conduct a topographic analysis of the
ridge, which suggests a predominantly triangular morphology, with some ridge
face slopes reaching 40 degrees, allowing for an exogenic formation mechanism. | astro-ph_EP |
Mapping the Skies of Ultracool Worlds: Detecting Storms and Spots with
Extremely Large Telescopes: Extremely large telescopes (ELTs) present an unparalleled opportunity to
study the magnetism, atmospheric dynamics, and chemistry of very low mass stars
(VLMs), brown dwarfs, and exoplanets. Instruments such as the Giant Magellan
Telescope - Consortium Large Earth Finder (GMT/GCLEF), the Thirty Meter
Telescope's Multi-Objective Diffraction-limited High-Resolution Infrared
Spectrograph (TMT/MODHIS), and the European Southern Observatory's Mid-Infrared
ELT Imager and Spectrograph (ELT/METIS) provide the spectral resolution and
signal-to-noise (S/N) necessary to Doppler image ultracool targets' surfaces
based on temporal spectral variations due to surface inhomogeneities. Using our
publicly-available code, $Imber$, developed and validated in Plummer & Wang
(2022), we evaluate these instruments' abilities to discern magnetic star spots
and cloud systems on a VLM star (TRAPPIST-1); two L/T transition ultracool
dwarfs (VHS J1256$-$1257 b and SIMP J0136+0933); and three exoplanets (Beta Pic
b and HR 8799 d and e). We find that TMT/MODHIS and ELT/METIS are suitable for
Doppler imaging the ultracool dwarfs and Beta Pic b over a single rotation.
Uncertainties for longitude and radius are typically $\lesssim 10^{\circ}$, and
latitude uncertainties range from $\sim 10^{\circ} \ \rm{to} \ 30^{\circ}$.
TRAPPIST-1's edge-on inclination and low $\upsilon \sin i$ provide a challenge
for all three instruments while GMT/GCLEF and the HR 8799 planets may require
observations over multiple rotations. We compare the spectroscopic technique,
photometry-only inference, and the combination of the two. We find combining
spectroscopic and photometric observations can lead to improved Bayesian
inference of surface inhomogeneities and offers insight into whether ultracool
atmospheres are dominated by spotted or banded features. | astro-ph_EP |
Modeling a Transient Secondary Paleo-Lunar Atmosphere: 3-D Simulations
and Analysis: The lunar history of water deposition, loss, and transport post-accretion has
become an important consideration in relation to the possibility of a human
outpost on the Moon. Very recent work has shown that a secondary primordial
atmosphere of up to 10 mb could have been emplaced ~3.5 billion years ago due
to volcanic outgassing from the maria. Using a zero dimensional chemistry model
we demonstrate the temperature dependence of the resulting major atmospheric
components (CO or CO$_2$). We use a three dimensional general circulation model
to test the viability of such an atmosphere and derive its climatological
characteristics. Based on these results we then conjecture on its capability to
transport volatiles outgassed from the maria to the permanently shadowed
regions at the poles. Our preliminary results demonstrate that atmospheres as
low as 1 mb are viable and that permanent cold trapping of volatiles is only
possible at the poles. | astro-ph_EP |
Tracking Advanced Planetary Systems (TAPAS) with HARPS-N VII. Elder suns
with low-mass companions: We present the current status of and new results from our search for
exoplanets in a sample of solar-mass, evolved stars observed with the HARPS-N
and the 3.6-m Telescopio Nazionale Galileo (TNG), and the High Resolution
Spectrograph (HRS) and the 9.2-m Hobby Eberly Telescope (HET).
The aim of this project is to detect and characterise planetary-mass
companions to solar-mass stars in a sample of 122 targets at various stages of
evolution from the main sequence (MS) to the red giant branch (RGB), mostly
sub-gaints and giants, selected from the Pennsylvania-Toru\'n Planet Search
(PTPS) sample, and use this sample to study relations between stellar
properties, such as metallicity, luminosity, and the planet occurrence rate.
This work is based on precise radial velocity (RV) measurements. We have
observed the program stars for up to 11 years with the HET/HRS and the
TNG/HARPS-N.
We present the analysis of RV measurements with the HET/HRS and the
TNG/HARPS-N of four solar-mass stars, HD 4760, HD 96992 , BD+02 3313, and TYC
0434-04538-1. We found that: HD 4760 hosts a companion with a minimum mass of
13.9 MJ (a=1.14 au, e=0.23); HD 96992 is a host to a msin i=1.14 MJ companion
on a a=1.24 au and e=0.41 orbit, and TYC 0434-04538-1 hosts an msin i=6.1MJ
companion on a a=0.66 au and e=0.08$ orbit. In the case of BD+02 3313 we found
a correlation between the measured RVs and one of the stellar activity
indicators, suggesting that the observed RV variations may originate in either
stellar activity or be caused by the presence of an unresolved companion. We
also discuss the current status of the project and a statistical analysis of
the RV variations in our sample of target stars. | astro-ph_EP |
OSSOS XVIII: Constraining migration models with the 2:1 resonance using
the Outer Solar System Origins Survey: Resonant dynamics plays a significant role in the past evolution and current
state of our outer Solar System. The population ratios and spatial distribution
of Neptune's resonant populations are direct clues to understanding the history
of our planetary system. The orbital structure of the objects in Neptune's 2:1
mean-motion resonance (\emph{twotinos}) has the potential to be a tracer of
planetary migration processes. Different migration processes produce distinct
architectures, recognizable by well-characterized surveys. However, previous
characterized surveys only discovered a few twotinos, making it impossible to
model the intrinsic twotino population. With a well-designed cadence and nearly
100\% tracking success, the Outer Solar System Origins Survey (OSSOS)
discovered 838 trans-Neptunian objects, of which 34 are securely twotinos with
well-constrained libration angles and amplitudes. We use the OSSOS twotinos and
the survey characterization parameters via the OSSOS Survey Simulator to
inspect the intrinsic population and orbital distributions of twotino. The
estimated twotino population, 4400$^{+1500}_{-1100}$ with $H_r<8.66$
(diameter$\sim$100km) at 95\% confidence, is consistent with the previous
low-precision estimate. We also constrain the width of the inclination
distribution to a relatively narrow value of $\sigma_i$=6$^\circ$$^{+1}_{-1}$,
and find the eccentricity distribution is consistent with a Gaussian centered
on $e_\mathrm{c}=0.275$ with a width $e_\mathrm{w}=0.06$. We find a
single-slope exponential luminosity function with $\alpha=0.6$ for the
twotinos. Finally, we for the first time meaningfully constrain the fraction of
symmetric twotinos, and the ratio of the leading asymmetric islands; both
fractions are in a range of 0.2--0.6. These measurements rule out certain
theoretical models of Neptune's migration history. | astro-ph_EP |
Signals embedded in the radial velocity noise. Periodic variations in
the tau Ceti velocities: The abilities of radial velocity exoplanet surveys to detect the lowest-mass
extra-solar planets are currently limited by a combination of instrument
precision, lack of data, and "jitter". Jitter is a general term for any unknown
features in the noise, and reflects a lack of detailed knowledge of stellar
physics (asteroseismology, starspots, magnetic cycles, granulation, and other
stellar surface phenomena), as well as the possible underestimation of
instrument noise. We study an extensive set of radial velocities for the star
HD 10700 ($\tau$ Ceti) to determine the properties of the jitter arising from
stellar surface inhomogeneities, activity, and telescope-instrument systems,
and perform a comprehensive search for planetary signals in the radial
velocities. We perform Bayesian comparisons of statistical models describing
the radial velocity data to quantify the number of significant signals and the
magnitude and properties of the excess noise in the data. We reach our goal by
adding artificial signals to the "flat" radial velocity data of HD 10700 and by
seeing which one of our statistical noise models receives the greatest
posterior probabilities while still being able to extract the artificial
signals correctly from the data. We utilise various noise components to assess
properties of the noise in the data and analyse the HARPS, AAPS, and HIRES data
for HD 10700 to quantify these properties and search for previously unknown
low-amplitude Keplerian signals. ... | astro-ph_EP |
Analysis of the arm-like structure in the outer disk of PDS 70. Spiral
density wave or vortex?: Observing dynamical interactions between planets and disks is key to
understanding their formation and evolution. Two protoplanets have recently
been discovered within PDS 70's protoplanetary disk, along with an arm-like
structure towards the north-west of the star. Our aim is to constrain the
morphology and origin of this arm-like structure, and to assess whether it
could trace a spiral density wave caused by the dynamical interaction between
the planet PDS 70c and the disk. We analyze polarized and angular differential
imaging (PDI and ADI) data taken with VLT/SPHERE, spanning six years of
observations. PDI data sets are reduced using the IRDAP polarimetric data
reduction pipeline, while ADI data sets are processed using MUSTARD, a novel
inverse problem algorithm to tackle the geometrical biases spoiling the images
previously used for the analysis of this disk. We confirm the presence of the
arm-like structure in all PDI and ADI datasets. We do not observe a south-east
symmetric arm with respect to the disk minor axis, which seems to disfavor the
previous hypothesis that the arm is the footprint of a double-ring structure.
If the structure traces a spiral density wave following the motion of PDS 70c,
we would expect
$11\overset{\circ}{.}28^{+2\overset{\circ}{.}20}_{-0\overset{\circ}{.}86}$
rotation for the spiral in six years. However, we do not measure any
significant movement of the structure. If the arm-like structure is a
planet-driven spiral arm, the observed lack of rotation would suggest that the
assumption of rigid-body rotation may be inappropriate for spirals induced by
planets. We suggest that the arm-like structure may rather trace a vortex
appearing as a one-armed spiral in scattered light due to projection effects.
The vortex hypothesis accounts for both the lack of observed rotation and the
presence of a nearby sub-mm continuum asymmetry detected with ALMA. | astro-ph_EP |
Is the Solar System Stable ?: Since the formulation of the problem by Newton, and during three centuries,
astronomers and mathematicians have sought to demonstrate the stability of the
Solar System. Thanks to the numerical experiments of the last two decades, we
know now that the motion of the planets in the Solar System is chaotic, which
prohibits any accurate prediction of their trajectories beyond a few tens of
millions of years. The recent simulations even show that planetary collisions
or ejections are possible on a period of less than 5 billion years, before the
end of the life of the Sun. | astro-ph_EP |
Computing optical meteor flux using Global Meteor Network data: Meteor showers and their outbursts are the dominant source of meteoroid
impact risk to spacecraft on short time scales. Meteor shower prediction models
depend on historical observations to produce accurate forecasts. However, the
current lack of quality and persistent world-wide monitoring at optical
meteoroid sizes has left some recent major outbursts poorly observed. A novel
method of computing meteor shower flux is developed and applied to Global
Meteor Network data. The method is verified against previously published
observations of the Perseids and the Geminids. The complete mathematical and
algorithmic details of computing meteor shower fluxes from video observations
are described. As an example application of our approach, the flux measurements
of the 2021 Perseid outburst, the 2020-2022 Quadrantids, and 2020-2021 Geminids
are presented. The flux of the 2021 Perseids reached similar levels to the
1991-1994 and 2016 outbursts (ZHR $\sim$ 280). The flux of the Quadrantids
shows high year-to-year variability in the core of the stream while the longer
lasting background activity is less variable, consistent with an age difference
between the two components. The Geminids show a double peak in flux near the
time of peak. | astro-ph_EP |
The peculiar shapes of Saturn's small inner moons as evidence of mergers
of similar-sized moonlets: The Cassini spacecraft revealed the spectacular, highly irregular shapes of
the small inner moons of Saturn, ranging from the unique "ravioli-like" forms
of Pan and Atlas to the highly elongated structure of Prometheus. Closest to
Saturn, these bodies provide important clues regarding the formation process of
small moons in close orbits around their host planet, but their range of
irregular shapes has not been explained yet. Here we show that the spectrum of
shapes among Saturn's small moons is a natural outcome of merging collisions
among similar-sized moonlets possessing physical properties and orbits that are
consistent with those of the current moons. A significant fraction of such
merging collisions take place either at the first encounter or after 1-2
hit-and-run events, with impact velocities in the range of 1-5 times the mutual
escape velocity. Close to head-on mergers result in flattened objects with
large equatorial ridges, as observed on Atlas and Pan. With slightly more
oblique impact angles, collisions lead to elongated, Prometheus-like shapes.
These results suggest that the current forms of the small moons provide direct
evidence of the processes at the final stages of their formation, involving
pairwise encounters of moonlets of comparable size. Finally, we show that this
mechanism may also explain the formation of Iapetus' equatorial ridge, as well
as its oblate shape. | astro-ph_EP |
Induced Turbulence and the Density Structure of the Dust Layer in a
Protoplanetary Disk: We study the turbulence induced in the dust layer of a protoplanetary disk
based on the energetics of dust accretion due to gas drag. We estimate
turbulence strength from the energy supplied by dust accretion, using the
radial drift velocity of the dust particles in a laminar disk. Our estimate of
the turbulence strength agrees with previous analytical and numerical research
on the turbulence induced by Kelvin-Helmholtz and/or streaming instabilities
for particles whose stopping time is less than the Keplerian time. For such
small particles, the strongest turbulence is expected to occur when the
dust-to-gas ratio of the disk is ~C_eff^(1/2) (h_g / r) ~ 10^(-2), where C_eff
~ 0.2 represents the energy supply efficiency to turbulence and h_g / r ~ 5 x
10^(-2) is the aspect ratio of the gas disk. The maximum viscosity parameter is
alpha_max ~ C_eff T_s (h_g / r)^2 ~ 10^(-4) T_s, where T_s (<1) is the
non-dimensional stopping time of the dust particles. Modification in the
dust-to-gas ratio from the standard value, 10^(-2), by any process, results in
weaker turbulence and a thinner dust layer, and consequently may accelerate the
growth process of the dust particles. | astro-ph_EP |
Albedos of Small Hilda Group Asteroids as Revealed by Spitzer: We present thermal 24 $\mu$m observations from the \textit{Spitzer Space
Telescope} of 62 Hilda asteroid group members with diameters ranging from 3 to
12 kilometers. Measurements of the thermal emission when combined with reported
absolute magnitudes allow us to constrain the albedo and diameter of each
object. From our \textit{Spitzer} sample, we find the mean geometric albedo,
$p_{V} =$ 0.07 $\pm$ 0.05 for small (D $<$ 10 km) Hilda group asteroids. This
value of $p_{V}$ is greater than and spans a larger range in albedo space than
the mean albedo of large (D $\gtrsim$ 10 km) Hilda group asteroids which is
$p_{V} =$ 0.04 $\pm$ 0.01. Though this difference may be attributed to space
weathering, the small Hilda group population reportedly displays greater
taxonomic range from C-, D- and X-type whose albedo distributions are
commensurate with the range of determined albedos. We discuss the derived Hilda
size-frequency distribution, color-color space, and geometric albedo for our
survey sample in the context of the expected migration induced "seeding" of the
Hilda asteroid group with outer solar system proto-planetesimals as outlined in
the "Nice" formalism. | astro-ph_EP |
Uncertainty for calculating transport on Titan: a probabilistic
description of bimolecular diffusion parameters: Bimolecular diffusion coefficients are important parameters used by
atmospheric models to calculate altitude profiles of minor constituents in an
atmosphere. Unfortunately, laboratory measurements of these coefficients were
never conducted at temperature conditions relevant to the atmosphere of Titan.
Here we conduct a detailed uncertainty analysis of the bimolecular diffusion
coefficient parameters as applied to Titan's upper atmosphere to provide a
better understanding of the impact of uncertainty for this parameter on models.
Because temperature and pressure conditions are much lower than the laboratory
conditions in which bimolecular diffusion parameters were measured, we apply a
Bayesian framework, a problem-agnostic framework, to determine parameter
estimates and associated uncertainties. We solve the Bayesian calibration
problem using the open-source QUESO library which also performs a propagation
of uncertainties in the calibrated parameters to temperature and pressure
conditions observed in Titan's upper atmosphere. Our results show that, after
propagating uncertainty through the Massman model, the uncertainty in molecular
diffusion is highly correlated to temperature and we observe no noticeable
correlation with pressure. We propagate the calibrated molecular diffusion
estimate and associated uncertainty to obtain an estimate with uncertainty due
to bimolecular diffusion for the methane molar fraction as a function of
altitude. Results show that the uncertainty in methane abundance due to
molecular diffusion is in general small compared to eddy diffusion and the
chemical kinetics description. However, methane abundance is most sensitive to
uncertainty in molecular diffusion above 1200 km where the errors are
nontrivial and could have important implications for scientific research based
on diffusion models in this altitude range. | astro-ph_EP |
A Search for Wide Companions to the Extrasolar Planetary System HR 8799: The extrasolar planetary system around HR 8799 is the first multiplanet
system ever imaged. It is also, by a wide margin, the highest mass system with
>27 Jupiters of planetary mass past 25 AU. This is a remarkable system with no
analogue with any other known planetary system. In the first part of this paper
we investigate the nature of two faint objects imaged near the system. These
objects are considerably fainter (H=20.4, and 21.6 mag) and more distant
(projected separations of 612, and 534 AU) than the three known planetary
companions b, c, and d (68-24 AU). It is possible that these two objects could
be lower mass planets (of mass ~5 and ~3 Jupiters) that have been scattered to
wider orbits. We make the first direct comparison of newly reduced archival
Gemini adaptive optics images to archival HST/NICMOS images. With nearly a
decade between these epochs we can accurately assess the proper motion nature
of each candidate companion. We find that both objects are unbound to HR 8799
and are background. We estimate that HR 8799 has no companions of H<22 from
~5-15 arcsec. Any scattered giant planets in the HR 8799 system are >600 AU or
less than 3 Jupiters in mass. In the second part of this paper we carry out a
search for wider common proper motion objects. While we identify no bound
companions to HR 8799, our search yields 16 objects within 1 degree in the
NOMAD catalog and POSS DSS images with similar (+/-20 mas/yr) proper motions to
HR 8799, three of which warrant follow-up observations. | astro-ph_EP |
On the Effects of Planetary Oblateness on Exoplanet Studies: When studying transiting exoplanets it is common to assume a spherical planet
shape. However short rotational periods can cause a planet to bulge at its
equator, as is the case with Saturn whose equatorial radius is almost 10%
larger than its polar radius. As a new generation of instruments comes online,
it is important to continually assess the underlying assumptions of models to
ensure robust and accurate inferences. We analyze bulk samples of known
transiting planets and calculate their expected signal strength if they were to
be oblate. We find that for noise levels below 100ppm, as many as 100 planets
could have detectable oblateness. We also investigate the effects of fitting
spherical planet models to synthetic oblate lightcurves. We find that this
biases the retrieved parameters by several standard deviations for oblateness
values > 0.1-0.2. When attempting to fit an oblateness model to both spherical
and oblate lightcurves, we find that the sensitivity of such fits is correlated
with both the SNR as well as the time sampling of the data, which can mask the
oblateness signal. For typical values of these quantities for Kepler
observations, it is difficult to rule out oblateness values less than ~0.25.
This results in an accuracy wall of 10-15% for the density of planets which may
be oblate. Finally, we find that a precessing oblate planet has the ability to
mimic the signature of a long-period companion via transit timing variations,
inducing offsets at the level of 10s of seconds. | astro-ph_EP |
Dynamics of the TrES-2 system: The TrES-2 system harbors one planet which was discovered with the transit
technique. In this work we investigate the dynamical behavior of possible
additional, lower-mass planets. We identify the regions where such planets can
move on stable orbits and show how they depend on the initial eccentricity and
inclination. We find, that there are stable regions inside and outside the
orbit of TrES-2b where additional, smaller planets can move. We also show that
those planets can have a large orbital inclination which makes a detection with
the transit technique very difficult. | astro-ph_EP |
Changes in the physical environment of the inner coma of
67P/Churyumov-Gerasimenko with decreasing heliocentric distance: The Wide Angle Camera of the OSIRIS instrument on board the Rosetta
spacecraft is equipped with several narrowband filters that are centered on the
emission lines and bands of various fragment species. These are used to
determine the evolution of the production and spatial distribution of the gas
in the inner coma of comet 67P with time and heliocentric distance, here
between 2.6 - 1.3 AU pre-perihelion. Our observations indicate that the
emission observed in the OH, OI, CN, NH, and NH2 filters is mostly produced by
dissociative electron impact excitation of different parent species. We
conclude that CO2 rather than H2O is a significant source of the [OI] 630 nm
emission. A strong plume-like feature observed in the in CN and [OI] filters is
present throughout our observations. This plume is not present in OH emission
and indicates a local enhancement of the CO2/H2O ratio by as much as a factor
of 3. We observed a sudden decrease in intensity levels after March 2015, which
we attribute to decreased electron temperatures in the first kilometers above
the nucleus surface. | astro-ph_EP |
Laboratory Studies of Methane and Its Relationship to Prebiotic
Chemistry: In order to examine how the terrestrial life emerged, a number of laboratory
experiments have been conducted since the 1950s. Methane has been one of the
key molecules in these studies. In earlier studies, strongly reducing gas
mixtures containing methane and ammonia were mainly used to simulate possible
reactions in primitive Earth atmosphere, and amino acids and other organic
compounds were detected. Since the primitive Earth atmosphere was estimated to
be less reducing, contribution of extraterrestrial organics to the origin of
life is considered quite important. Extraterrestrial organic chemistry has been
experimentally and theoretically studied intensively, including laboratory
experiments simulating interstellar molecular reactions. Endogenous and
exogenous organics should have been supplied to the primitive ocean. Now
submarine hydrothermal systems are considered one of the plausible sites of
generation of life. Experiments simulating submarine hydrothermal systems where
methane played an important role are now intensively being conducted. We have
recognized the importance of such studies on possible reactions in other solar
system bodies to understand the origins of life. Titan and other icy bodies,
where methane plays significant roles, are especially good targets to be
studied. In the case of Titan, not only methane-containing atmospheres but also
liquidospheres composed of methane and other hydrocarbons have been used in
simulation experiments. This paper summarizes experiments simulating various
terrestrial and extraterrestrial environments, and possible roles of methane in
chemical evolution are discussed. | astro-ph_EP |
On the Vertical Shear Instability in Magnetized Protoplanetary Disks: The vertical shear instability (VSI) is a robust phenomenon in irradiated
protoplanetary disks (PPDs). While there is extensive literature on the VSI in
the hydrodynamic limit, PPDs are expected to be magnetized and their extremely
low ionization fractions imply that non-ideal magneto-hydrodynamic (MHD)
effects should be properly considered. To this end, we present linear analyses
of the VSI in magnetized disks with Ohmic resistivity. We primarily consider
toroidal magnetic fields, which are likely to dominate the field geometry in
PPDs. We perform vertically global and radially local analyses to capture
characteristic VSI modes with extended vertical structures. To focus on the
effect of magnetism, we use a locally isothermal equation of state. We find
that magnetism provides a stabilizing effect to dampen the VSI, with surface
modes, rather than body modes, being the first to vanish with increasing
magnetization. Subdued VSI modes can be revived by Ohmic resistivity, where
sufficient magnetic diffusion overcome magnetic stabilization, and hydrodynamic
results are recovered. We also briefly consider poloidal fields to account for
the magnetorotational instability (MRI), which may develop towards surface
layers in the outer parts of PPDs. The MRI grows efficiently at small radial
wavenumbers, in contrast to the VSI. When resistivity is considered, we find
the VSI dominates over the MRI for Ohmic Els\"{a}sser numbers $\lesssim 0.09$
at plasma beta parameter $\beta_Z \sim 10^4$. | astro-ph_EP |
Post-Impact Thermal Evolution of Porous Planetesimals: Impacts between planetesimals have largely been ruled out as a heat source in
the early Solar System, by calculations that show them to be an inefficient
heat source and unlikely to cause global heating. However, the long-term,
localized thermal effects of impacts on planetesimals have never been fully
quantified. Here, we simulate a range of impact scenarios between planetesimals
to determine the post-impact thermal histories of the parent bodies, and hence
the importance of impact heating in the thermal evolution of planetesimals. We
find on a local scale that heating material to petrologic type 6 is achievable
for a range of impact velocities and initial porosities, and impact melting is
possible in porous material at a velocity of > 4 km/s. Burial of heated
impactor material beneath the impact crater is common, insulating that material
and allowing the parent body to retain the heat for extended periods (~
millions of years). Cooling rates at 773 K are typically 1 - 1000 K/Ma,
matching a wide range of measurements of metallographic cooling rates from
chondritic materials. While the heating presented here is localized to the
impact site, multiple impacts over the lifetime of a parent body are likely to
have occurred. Moreover, as most meteorite samples are on the centimeter to
meter scale, the localized effects of impact heating cannot be ignored. | astro-ph_EP |
Evolution of the Dust Trail of Comet 17P/Holmes: The massive outburst of the comet 17P/Holmes in October 2007 is the largest
known outburst by a comet thus far. We present a new comprehensive model
describing the evolution of the dust trail produced in this phenomenon. The
model comprises of multiparticle Monte Carlo approach including the solar
radiation pressure effects, gravitational disturbance caused by Venus, Earth
and Moon, Mars, Jupiter and Saturn, and gravitational interaction of the dust
particles with the parent comet itself. Good accuracy of computations is
achieved by its implementation in Orekit, which executes Dormad-Prince
numerical integration methods with higher precision. We demonstrate performance
of the model by simulating particle populations with sizes from 0.001 mm to 1
mm with corresponding spherically symmetric ejection speed distribution, and
towards the Sun outburst modelling. The model is supplemented with and
validated against the observations of the dust trail in common nodes for 0.5
and 1 revolutions. In all cases, the predicted trail position showed a good
match to the observations. Additionally, the hourglass pattern of the trail was
observed for the first time within this work. By using variations of the
outburst model in our simulations, we determine that the assumption of the
spherical symmetry of the ejected particles leads to the scenario compatible
with the observed hourglass pattern. Using these data, we make predictions for
the two-revolution dust trail behavior near the outburst point that should be
detectable by using ground-based telescopes in 2022. | astro-ph_EP |
Kepler-93b: A Terrestrial World Measured to within 120 km, and a Test
Case for a New Spitzer Observing Mode: We present the characterization of the Kepler-93 exoplanetary system, based
on three years of photometry gathered by the Kepler spacecraft. The duration
and cadence of the Kepler observations, in tandem with the brightness of the
star, enable unusually precise constraints on both the planet and its host. We
conduct an asteroseismic analysis of the Kepler photometry and conclude that
the star has an average density of 1.652+/-0.006 g/cm^3. Its mass of
0.911+/-0.033 M_Sun renders it one of the lowest-mass subjects of asteroseismic
study. An analysis of the transit signature produced by the planet Kepler-93b,
which appears with a period of 4.72673978+/-9.7x10^-7 days, returns a
consistent but less precise measurement of the stellar density, 1.72+0.02-0.28
g/cm^3. The agreement of these two values lends credence to the planetary
interpretation of the transit signal. The achromatic transit depth, as compared
between Kepler and the Spitzer Space Telescope, supports the same conclusion.
We observed seven transits of Kepler-93b with Spitzer, three of which we
conducted in a new observing mode. The pointing strategy we employed to gather
this subset of observations halved our uncertainty on the transit radius ratio
R_p/R_star. We find, after folding together the stellar radius measurement of
0.919+/-0.011 R_Sun with the transit depth, a best-fit value for the planetary
radius of 1.481+/-0.019 R_Earth. The uncertainty of 120 km on our measurement
of the planet's size currently renders it one of the most precisely measured
planetary radii outside of the Solar System. Together with the radius, the
planetary mass of 3.8+/-1.5 M_Earth corresponds to a rocky density of 6.3+/-2.6
g/cm^3. After applying a prior on the plausible maximum densities of
similarly-sized worlds between 1--1.5 R_Earth, we find that Kepler-93b
possesses an average density within this group. | astro-ph_EP |
Innocent Bystanders: Orbital Dynamics of Exomoons during Planet-Planet
Scattering: Planet-planet scattering is the leading mechanism to explain the broad
eccentricity distribution of observed giant exoplanets. Here we study the
orbital stability of primordial giant planet moons in this scenario. We use
N-body simulations including realistic oblateness and evolving spin evolution
for the giant planets. We find that the vast majority (80-90% across all our
simulations) of orbital parameter space for moons is destabilized. There is a
strong radial dependence, as moons past 0.1 Hill radii are systematically
removed. Closer-in moons on Galilean-moon-like orbits (< 0.04 Hill radii) have
a good ( 20-40%) chance of survival. Destabilized moons may undergo a collision
with the star or a planet, be ejected from the system, be captured by another
planet, be ejected but still orbiting its free-floating host planet, or survive
on heliocentric orbits as "planets." The survival rate of moons increases with
the host planet mass but is independent of the planet's final (post-scattering)
orbits. Based on our simulations we predict the existence of an abundant
galactic population of free- floating (former) moons. | astro-ph_EP |
Atmospheric Reconnaissance of TRAPPIST-1 b with JWST/NIRISS: Evidence
for Strong Stellar Contamination in the Transmission Spectra: TRAPPIST-1 is a nearby system of seven Earth-sized, temperate, rocky
exoplanets transiting a Jupiter-sized M8.5V star, ideally suited for in-depth
atmospheric studies. Each TRAPPIST-1 planet has been observed in transmission
both from space and from the ground, confidently rejecting cloud-free,
hydrogen-rich atmospheres. Secondary eclipse observations of TRAPPIST-1 b with
JWST/MIRI are consistent with little to no atmosphere given the lack of heat
redistribution. Here we present the first transmission spectra of TRAPPIST-1 b
obtained with JWST/NIRISS over two visits. The two transmission spectra show
moderate to strong evidence of contamination from unocculted stellar
heterogeneities, which dominates the signal in both visits. The transmission
spectrum of the first visit is consistent with unocculted starspots and the
second visit exhibits signatures of unocculted faculae. Fitting the stellar
contamination and planetary atmosphere either sequentially or simultaneously,
we confirm the absence of cloud-free hydrogen-rich atmospheres, but cannot
assess the presence of secondary atmospheres. We find that the uncertainties
associated with the lack of stellar model fidelity are one order of magnitude
above the observation precision of 89 ppm (combining the two visits). Without
affecting the conclusion regarding the atmosphere of TRAPPIST-1 b, this
highlights an important caveat for future explorations, which calls for
additional observations to characterize stellar heterogeneities empirically
and/or theoretical works to improve model fidelity for such cool stars. This
need is all the more justified as stellar contamination can affect the search
for atmospheres around the outer, cooler TRAPPIST-1 planets for which
transmission spectroscopy is currently the most efficient technique. | astro-ph_EP |
A sub-Mercury-sized exoplanet: Since the discovery of the first exoplanet we have known that other planetary
systems can look quite unlike our own. However, until recently we have only
been able to probe the upper range of the planet size distribution. The high
precision of the Kepler space telescope has allowed us to detect planets that
are the size of Earth and somewhat smaller, but no previous planets have been
found that are smaller than those we see in our own Solar System. Here we
report the discovery of a planet significantly smaller than Mercury. This tiny
planet is the innermost of three planets that orbit the Sun-like host star,
which we have designated Kepler-37. Owing to its extremely small size, similar
to that of Earth's Moon, and highly irradiated surface, Kepler-37b is probably
a rocky planet with no atmosphere or water, similar to Mercury. | astro-ph_EP |
Measurement of the vertical atmospheric density profile from the X-ray
Earth occultation of the Crab Nebula with Insight-HXMT: In this paper, the X-ray Earth occultation (XEO) of the Crab Nebula is
investigated by using the Hard X-ray Modulation Telescope (Insight-HXMT). The
pointing observation data on the 30th September, 2018 recorded by the Low
Energy X-ray telescope (LE) of Insight-HXMT are selected and analyzed. The
extinction lightcurves and spectra during the X-ray Earth occultation process
are extracted. A forward model for the XEO lightcurve is established and the
theoretical observational signal for lightcurve is predicted. The atmospheric
density model is built with a scale factor to the commonly used MSIS density
profile within a certain altitude range. A Bayesian data analysis method is
developed for the XEO lightcurve modeling and the atmospheric density
retrieval. The posterior probability distribution of the model parameters is
derived through the Markov Chain Monte Carlo (MCMC) algorithm with the
NRLMSISE-00 model and the NRLMSIS 2.0 model as basis functions and the best-fit
density profiles are retrieved respectively. It is found that in the altitude
range of 105--200 km, the retrieved density profile is 88.8% of the density of
NRLMSISE-00 and 109.7% of the density of NRLMSIS 2.0 by fitting the lightcurve
in the energy range of 1.0--2.5 keV based on XEOS method. In the altitude range
of 95--125 km, the retrieved density profile is 81.0% of the density of
NRLMSISE-00 and 92.3% of the density of NRLMSIS 2.0 by fitting the lightcurve
in the energy range of 2.5--6.0 keV based on XEOS method. In the altitude range
of 85--110 km, the retrieved density profile is 87.7% of the density of
NRLMSISE-00 and 101.4% of the density of NRLMSIS 2.0 by fitting the lightcurve
in the energy range of 6.0--10.0 keV based on XEOS method. This study
demonstrates that the XEOS from the X-ray astronomical satellite Insight-HXMT
can provide an approach for the study of the upper atmosphere. | astro-ph_EP |
Moist Convection and the 2010-2011 Revival of Jupiter's South Equatorial
Belt: The transformation of Jupiter's South Equatorial Belt (SEB) from its faded,
whitened state in 2009-2010 to its normal brown appearance is documented via
comparisons of thermal-infrared (5-20 $\mu$m) and visible-light imaging between
November 2010 and November 2011. The SEB revival consisted of convective
eruptions triggered over $\sim100$ days, potentially powered by the latent heat
released by the condensation of water. The plumes rise from the water cloud
base and ultimately diverge and cool in the stably-stratified upper
troposphere. Thermal-IR images were acquired 2 days after the SEB disturbance
was first detected by amateur observers on November 9th 2010. Subsequent images
revealed the cold, putatively anticyclonic and cloudy plume tops surrounded by
warm, cloud-free conditions at their peripheries. The majority of the plumes
erupted from a single source near $140-160^\circ$W, coincident with the remnant
cyclonic circulation of a brown barge that had formed during the fade.
Additional plumes erupted from the leading edge of the central disturbance
immediately east of the source. The tropospheric plumes excited stratospheric
thermal waves over the SEB, showing a direct connection between moist
convection and stratospheric wave activity. The subsidence of dry, unsaturated
air warmed the troposphere and removed the white aerosols. The aerosol-free air
was redistributed throughout the SEB by the zonal flow, following a
westward-moving southern branch and an eastward-moving northern branch that
revived the brown colouration over $\sim200$ days. The last stage of the
revival was the re-establishment of normal convective activity northwest of the
GRS in September 2011. Moist convection may therefore play an important role in
controlling the timescale and atmospheric variability during the SEB life
cycle. [Abridged] | astro-ph_EP |
Bifurcation of planetary building blocks during Solar System formation: Geochemical and astronomical evidence demonstrate that planet formation
occurred in two spatially and temporally separated reservoirs. The origin of
this dichotomy is unknown. We use numerical models to investigate how the
evolution of the solar protoplanetary disk influenced the timing of protoplanet
formation and their internal evolution. Migration of the water snow line can
generate two distinct bursts of planetesimal formation that sample different
source regions. These reservoirs evolve in divergent geophysical modes and
develop distinct volatile contents, consistent with constraints from accretion
chronology, thermo-chemistry, and the mass divergence of inner and outer Solar
System. Our simulations suggest that the compositional fractionation and
isotopic dichotomy of the Solar System was initiated by the interplay between
disk dynamics, heterogeneous accretion, and internal evolution of forming
protoplanets. | astro-ph_EP |
Homogeneous model for the TRAPPIST-1e planet with an icy layer: In this work we investigate whether a multilayered planet can be approximated
as a homogeneous planet, and in particular how well the dissipation rate of a
multilayered planet can be reproduced with a homogeneous rheology. We study the
case of a stratified body with an icy crust that, according to recent studies,
displays a double peak feature in the tidal response that cannot be reproduced
with a homogeneous planet with an Andrade rheology. We revisit the problem with
a slightly more complex rheology for the homogeneous body, the Sundberg-Cooper
rheology, which naturally has a double peak feature, and apply the model to the
TRAPPIST-1e planet. Our results compare very well with the results obtained
when employing a multilayered model, showing that it is possible to approximate
the behavior of a multilayer icy planet with a homogeneous planet using the
Sundberg-Cooper rheology. This highlights the fact that we do not need the
complexity of the multilayer planet model in order to estimate the tidal
dissipation of an icy planet. | astro-ph_EP |
Thermophysical modeling of NEOWISE observations of DESTINY+ targets
Phaethon and 2005 UD: Thermophysical models allow for improved constraints on the physical and
thermal surface properties of asteroids beyond what can be inferred from more
simple thermal modeling, provided a sufficient number of observations is
available. We present thermophysical modeling results of observations from the
NEOWISE mission for two near-Earth asteroids which are the targets of the
DESTINY+ flyby mission: (3200) Phaethon and (155140) 2005 UD. Our model assumes
a rotating, cratered, spherical surface, and employs a Monte Carlo Markov Chain
to explore the multi-dimensional parameter space of the fit. We find an
effective spherical diameter for Phaethon of $4.6^{+0.2}_{-0.3}~$km, a
geometric albedo of $p_V=0.16\pm0.02$, and a thermal inertia $\Gamma=880$
$^{+580}_{-330}$, using five epochs of NEOWISE observations. The best model fit
for (155140) 2005 UD was less well constrained due to only having two NEOWISE
observation epochs, giving a diameter of $1.2\pm0.4~$km and a geometric albedo
of $p_V=0.14\pm0.09$. | astro-ph_EP |
Detection of a Satellite of the Trojan Asteroid (3548) Eurybates -- A
Lucy Mission Target: We describe the discovery of a satellite of the Trojan asteroid (3548)
Eurybates in images obtained with the Hubble Space Telescope. The satellite was
detected on three separate epochs, two in September 2018 and one in January
2020. The satellite has a brightness in all three epochs consistent with an
effective diameter of d2 =1.2+/-0.4 km. The projected separation from Eurybates
was s~1700-2300 km and varied in position, consistent with a large range of
possible orbits. Eurybates is a target of the Lucy Discovery mission and the
early detection of a satellite provides an opportunity for a significant
expansion of the scientific return from this encounter. | astro-ph_EP |
The flipped orbit of KELT-19Ab inferred from the symmetric TESS transit
light curves: Dozens of planets are now discovered with large orbital obliquity, and have
become the proof for the dynamical evolution of planetary orbits. In the
current samples, there is an apparent clustering of planets around $90^\circ$,
and also an absence of planets around $180^\circ$ although the latter is
expected by some theories. Statistical extrapolation using Hierarchical
Bayesian Analysis have recently refuted the significant clustering around
$90^\circ$ and suggested that the distribution may actually be broader. In this
work, the symmetric TESS transit light curve of KELT-19Ab is analyzed using
gravity darkening to measure its true obliquity. Its large sky projected
obliquity $\lambda = -179.7^{\circ+3.7^\circ}_{\,\,-3.8^\circ}$ makes KELT-19Ab
the only currently known planet with obliquity potentially close to
$180^\circ$. We apply spectroscopic constraints on $v\mathrm{sin}i$ and
$\lambda$ as well as theoretical constraints on the limb-darkening coefficients
to find that the KELT-19Ab's obliquity is $\psi =
155^{\circ+17^\circ}_{\,\,-21^\circ}$, in favor of a flipped orbit. The result
is consistent with the statistically inferred uniformity of obliquity
distribution, and also highlights the applicability of the gravity darkening
technique to symmetric light curves. | astro-ph_EP |
The evolution of photo-evaporating viscous discs in binaries: A large fraction of stars are in binary systems, yet the evolution of
proto-planetary discs in binaries has been little explored from the theoretical
side. In this paper we investigate the evolution of the discs surrounding the
primary and secondary components of binary systems on the assumption that this
is driven by photoevaporation induced by X-rays from the respective star. We
show how for close enough separations (20-30 AU for average X-ray luminosities)
the tidal torque of the companion changes the qualitative behaviour of disc
dispersal from inside out to outside in. Fewer transition discs created by
photoevaporation are thus expected in binaries. We also demonstrate that in
close binaries the reduction in viscous time leads to accelerated disc clearing
around both components, consistent with $\textit{unresolved}$ observations.
When looking at the $\textit{differential}$ disc evolution around the two
components, in close binaries discs around the secondary clear first due to the
shorter viscous timescale associated with the smaller outer radius. In wide
binaries instead the difference in photo-evaporation rate makes the secondaries
longer lived, though this is somewhat dependent on the assumed scaling of
viscosity with stellar mass. We find that our models are broadly compatible
with the growing sample of $\textit{resolved}$ observations of discs in
binaries. We also predict that binaries have higher accretion rates than single
stars for the same disc mass. Thus binaries probably contribute to the observed
scatter in the relationship between disc mass and accretion rate in young
stars. | astro-ph_EP |
Polar stellar-spots and grazing planetary transits: possible explanation
for the low number of discovered grazing planets: We assess a physically feasible explanation for the low number of discovered
(near-)grazing planetary transits through all ground and space based transit
surveys. We performed simulations to generate the synthetic distribution of
detectable planets based on their impact parameter, and found that a larger
number of (near-)grazing planets should have been detected than have been
detected. Our explanation for the insufficient number of (near-)grazing planets
is based on a simple assumption that a large number of (near-)grazing planets
transit host stars which harbor dark giant polar spot, and thus the transit
light-curve vanishes due to the occultation of grazing planet and the polar
spot. We conclude by evaluating the properties required of polar spots in order
to make disappear the grazing transit light-curve, and we conclude that their
properties are compatible with the expected properties from observations. | astro-ph_EP |
Spectroscopic Coronagraphy for Planetary Radial Velocimetry of
Exoplanets: We propose the application of coronagraphic techniques to the spectroscopic
direct detection of exoplanets via the Doppler shift of planetary molecular
lines. Even for an unresolved close-in planetary system, we show that the
combination of a visible nuller and an extreme adaptive optics system can
reduce the photon noise of a main star and increase the total signal-to-noise
ratio (S/N) of the molecular absorption of the exoplanetary atmosphere: it
works as a spectroscopic coronagraph. Assuming a 30 m telescope, we demonstrate
the benefit of these high-contrast instruments for nearby close-in planets that
mimic 55 Cnc b ($0.6 \lambda/D$ of the angular separation in the K band). We
find that the tip-tilt error is the most crucial factor; however, low-order
speckles also contribute to the noise. Assuming relatively conservative
estimates for future wavefront control techniques, the spectroscopic
coronagraph can increase the contrast to $ \sim 50-130 $ times and enable us to
obtain $\sim 3-6 $ times larger S/N for warm Jupiters and Neptunes at 10 pc
those without it. If the tip-tilt error can be reduced to $\lesssim 0.3$ mas
(rms), it gains $\sim 10-30$ times larger S/N and enables us to detect warm
super-Earths with an extremely large telescope. This paper demonstrates the
concept of spectroscopic coronagraphy for future spectroscopic direct
detection. Further studies of the selection of coronagraphs and tip-tilt
sensors will extend the range of application of the spectroscopic direct
detection beyond the photon collecting area limit. | astro-ph_EP |
A Consistent Reduced Network for HCN Chemistry in Early Earth and Titan
Atmospheres: Quantum Calculations of Reaction Rate Coefficients: HCN is a key ingredient for synthesizing biomolecules such as nucleobases and
amino acids. We calculate 42 reaction rate coefficients directly involved with
or in competition with the production of HCN in the early Earth or Titan
atmospheres. These reactions are driven by methane and nitrogen radicals
produced via UV photodissociation or lightning. For every reaction in this
network, we calculate rate coefficients at 298 K using canonical variational
transition state theory (CVT) paired with computational quantum chemistry
simulations at the BHandHLYP/augcc-pVDZ level of theory. We also calculate the
temperature dependence of the rate coefficients for the reactions that have
barriers from 50 to 400 K. We present 15 new reaction rate coefficients with no
previously known value; 93% of our calculated coefficients are within an order
of magnitude of the nearest experimental or recommended values. Above 320 K,
the rate coefficient for the new reaction H2CN -> HCN + H dominates. Contrary
to experiments, we find the HCN reaction pathway, N + CH3 -> HCN + H2, to be
inefficient and suggest that the experimental rate coefficient actually
corresponds to an indirect pathway, through the H2CN intermediate. We present
CVT using energies computed with density functional theory as a feasible and
accurate method for calculating a large network of rate coefficients of
small-molecule reactions. | astro-ph_EP |
Layered semi-convection and tides in giant planet interiors - I.
Propagation of internal waves: Layered semi-convection is a possible candidate to explain Saturn's
luminosity excess and the abnormally large radius of some hot Jupiters. In
giant planet interiors, it could lead to the creation of density staircases,
which are convective layers separated by thin stably stratified interfaces. We
study the propagation of internal waves in a region of layered semi-convection,
with the aim to predict energy transport by internal waves incident upon a
density staircase. The goal is then to understand the resulting tidal
dissipation when these waves are excited by other bodies such as moons in giant
planets systems. We use a local Cartesian analytical model, taking into account
the complete Coriolis acceleration at any latitude, thus generalizing previous
works. We find transmission of incident internal waves to be strongly affected
by the presence of a density staircase, even if these waves are initially pure
inertial waves (which are restored by the Coriolis acceleration). In
particular, low-frequency waves of all wavelengths are perfectly transmitted
near the critical latitude. Otherwise, short-wavelength waves are only
efficiently transmitted if they are resonant with a free mode (interfacial
gravity wave or short-wavelength inertial mode) of the staircase. In all other
cases, waves are primarily reflected unless their wavelengths are longer than
the vertical extent of the entire staircase (not just a single step). We expect
incident internal waves to be strongly affected by the presence of a density
staircase in a frequency-, latitude- and wavelength-dependent manner. First,
this could lead to new criteria to probe the interior of giant planets by
seismology; and second, this may have important consequences for tidal
dissipation and our understanding of the evolution of giant planet systems. | astro-ph_EP |
Evidence that the Directly-Imaged Planet HD 131399 Ab is a Background
Star: We present evidence that the recently discovered, directly-imaged planet HD
131399 Ab is a background star with non-zero proper motion. From new JHK1L'
photometry and spectroscopy obtained with the Gemini Planet Imager, VLT/SPHERE,
and Keck/NIRC2, and a reanalysis of the discovery data obtained with
VLT/SPHERE, we derive colors, spectra, and astrometry for HD 131399 Ab. The
broader wavelength coverage and higher data quality allow us to re-investigate
its status. Its near-infrared spectral energy distribution excludes spectral
types later than L0 and is consistent with a K or M dwarf, which are the most
likely candidates for a background object in this direction at the apparent
magnitude observed. If it were a physically associated object, the projected
velocity of HD 131399 Ab would exceed escape velocity given the mass and
distance to HD 131399 A. We show that HD 131399 Ab is also not following the
expected track for a stationary background star at infinite distance. Solving
for the proper motion and parallax required to explain the relative motion of
HD 131399 Ab, we find a proper motion of 12.3 mas/yr. When compared to
predicted background objects drawn from a galactic model, we find this proper
motion to be high, but consistent with the top 4% fastest-moving background
stars. From our analysis we conclude that HD 131399 Ab is a background K or M
dwarf. | astro-ph_EP |
Observations of Mass Loss from the Transiting Exoplanet HD 209458b: Using the new Cosmic Origins Spectrograph (COS) on the {\it Hubble Space
Telescope (HST)}, we obtained moderate-resolution, high signal/noise
ultraviolet spectra of HD 209458 and its exoplanet HD 209458b during transit,
both orbital quadratures, and secondary eclipse. We compare transit spectra
with spectra obtained at non-transit phases to identify spectral features due
to the exoplanet's expanding atmosphere. We find that the mean flux decreased
by $7.8\pm 1.3$% for the C II 1334.5323\AA\ and 1335.6854\AA\ lines and by
$8.2\pm 1.4$% for the Si III 1206.500\AA\ line during transit compared to
non-transit times in the velocity interval --50 to +50 km s$^{-1}$. Comparison
of the C II and Si III line depths and transit/non-transit line ratios shows
deeper absorption features near --10 and +15 km s$^{-1}$ and less certain
features near --40 and +30--70 km s$^{-1}$, but future observations are needed
to verify this first detection of velocity structure in the expanding
atmosphere of an exoplanet. Our results for the C II lines and the
non-detection of Si IV 1394.76\AA\ absorption are in agreement with
\citet{Vidal-Madjar2004}, but we find absorption during transit in the Si III
line contrary to the earlier result. The $8\pm 1$% obscuration of the star
during transit is far larger than the 1.5% obscuration by the exoplanet's disk.
Absorption during transit at velocities between --50 and +50 km s$^{-1}$ in the
C II and Si III lines requires high-velocity ion absorbers, but models that
assume that the absorbers are high-temperature thermal ions are inconsistent
with the COS spectra. Assuming hydrodynamic model values for the gas
temperature and outflow velocity at the limb of the outflow as seen in the C II
lines, we find mass-loss rates in the range (8--40)$\times 10^{10}$ g s$^{-1}$. | astro-ph_EP |
Dissipative Divergence of Resonant Orbits: A considerable fraction of multi-planet systems discovered by the
observational surveys of extrasolar planets reside in mild proximity to
first-order mean motion resonances. However, the relative remoteness of such
systems from nominal resonant period ratios (e.g. 2:1, 3:2, 4:3) has been
interpreted as evidence for lack of resonant interactions. Here we show that a
slow divergence away from exact commensurability is a natural outcome of
dissipative evolution and demonstrate that libration of critical angles can be
maintained tens of percent away from nominal resonance. We construct an
analytical theory for the long-term dynamical evolution of dissipated resonant
planetary pairs and confirm our calculations numerically. Collectively, our
results suggest that a significant fraction of the near-commensurate extrasolar
planets are in fact resonant and have undergone significant dissipative
evolution. | astro-ph_EP |
Modelling the Spectra of Planets, Brown Dwarfs and Stars using VSTAR: We describe a new software package capable of predicting the spectra of
solar-system planets, exoplanets, brown dwarfs and cool stars. The Versatile
Software for Transfer of Atmospheric Radiation (VSTAR) code combines a
line-by-line approach to molecular and atomic absorption with a full multiple
scattering treatment of radiative transfer. VSTAR is a modular system
incorporating an ionization and chemical equilibrium model, a comprehensive
treatment of spectral line absorption using a database of more than 2.9 billion
spectral lines, a scattering package and a radiative transfer module. We test
the methods by comparison with other models and benchmark calculations. We
present examples of the use of VSTAR to model the spectra of terrestrial and
giant planet in our own solar system, brown dwarfs and cool stars. | astro-ph_EP |
The Peculiar Photometric Properties of 2010 WG9: A Slowly-Rotating
Trans-Neptunian Object from the Oort Cloud: We present long-term BVRI observations of 2010 WG9, an ~100-km diameter
trans-Neptunian object (TNO) with an extremely high inclination of 70 deg
discovered by the La Silla - QUEST southern sky survey. Most of the
observations were obtained with ANDICAM on the SMARTS 1.3m at Cerro Tololo,
Chile from Dec 2010 to Nov 2012. Additional observations were made with EFOSC2
on the 3.5-m NTT telescope of the European Southern Observatory at La Silla,
Chile in Feb 2011. The observations reveal a sinusoidal light curve with
amplitude 0.14 mag and period 5.4955 +/- 0.0025d, which is likely half the true
rotation period. Such long rotation periods have previously been observed only
for tidally-evolved binary TNOs, suggesting that 2010 WG9 may be such a system.
We predict a nominal separation of at least 790 km, resolvable with HST and
ground-based systems. We measure B-R = 1.318 +/- 0.029 and V-R = 0.520 +/-
0.018, consistent with the colors of modestly red Centaurs and Damocloids. At
I-band wavelengths, we observe an unusually large variation of color with
rotational phase, with R-I ranging from 0.394 +/- 0.025 to 0.571 +/- 0.044. We
also measure an absolute R-band absolute magnitude of 7.93 +/- 0.05 and solar
phase coefficient 0.049 +/- 0.019 mag/deg. | astro-ph_EP |
The effect of late giant collisions on the atmospheres of protoplanets
and the formation of cold sub-Saturns: We investigate the origins of cold sub-Saturns (CSS), an exoplanetary
population inferred from microlensing surveys. If confirmed, these planets
would rebut a theorised gap in planets' mass distribution between those of
Neptune and Jupiter caused by the rapid runaway accretion of super-critical
cores. In an attempt to resolve this theoretical-observational disparity, we
examine the outcomes of giant collisions between sub-critical protoplanets. Due
to the secular interaction among protoplanets, these events may occur in
rapidly depleting discs. We show that impactors ~ 5% the mass of near-runaway
envelopes around massive cores can efficiently remove these envelopes entirely
via a thermally-driven super-Eddington wind emanating from the core itself, in
contrast with the stellar Parker winds usually considered. After a brief
cooling phase, the merged cores resume accretion. But, the evolution timescale
of transitional discs is too brief for the cores to acquire sufficiently
massive envelopes to undergo runaway accretion despite their large combined
masses. Consequently, these events lead to the emergence of CSS without their
transformation into gas giants. We show that these results are robust for a
wide range of disc densities, grain opacities and silicate abundance in the
envelope. Our fiducial case reproduces CSS with heavy (>= 30 M_Earth) cores and
less massive (a few M_Earth) sub-critical envelopes. We also investigate the
other limiting cases, where continuous mergers of comparable-mass cores yield
CSS with wider ranges of core-to-envelope mass ratios and envelope opacities.
Our results indicate that it is possible for CSS and Uranus and Neptune to
emerge within the framework of well studied processes and they may be more
common than previously postulated. | astro-ph_EP |
Sub-Seasonal Variation in Neptune's Mid-Infrared Emission: We present an analysis of all currently available ground-based imaging of
Neptune in the mid-infrared. Dating between 2003 and 2020, the images reveal
changes in Neptune's mid-infrared ($\sim 8-25\mu$m) emission over time in the
years surrounding Neptune's 2005 southern summer solstice. Images sensitive to
stratospheric ethane ($\sim12\mu$m), methane ($\sim8\mu$m), and CH$_3$D
($\sim9\mu$m) display significant sub-seasonal temporal variation on regional
and global scales. Comparison with H$_2$ S(1) hydrogen-quadrupole
($\sim17.035\mu$m) spectra suggests these changes are primarily related to
stratospheric temperature changes. The stratosphere appears to have cooled
between 2003 and 2009 across multiple filtered wavelengths, followed by a
dramatic warming of the south pole between 2018 and 2020. Conversely,
upper-tropospheric temperatures -- inferred from $\sim 17-25$-micron imaging --
appear invariant during this period, except for the south pole, which appeared
warmest between 2003 and 2006. We discuss the observed variability in the
context of seasonal forcing, tropospheric meteorology, and the solar cycle.
Collectively, these data provide the strongest evidence to date that processes
produce sub-seasonal variation on both global and regional scales in Neptune's
stratosphere. | astro-ph_EP |
Time and phase resolved optical spectra of potentially hazardous
asteroid 2014 JO25: The asteroid 2014 JO25, considered to be "potentially hazardous" by the Minor
Planet Center, was spectroscopically followed during its close-Earth encounter
on 19th and 20th of April 2017. The spectra of the asteroid were taken with the
low resolution spectrograph (LISA), mounted on the 1.2-m telescope at the Mount
Abu Infrared Observatory, India. Coming from a region close to the Hungaria
population of asteroids, this asteroid follows a comet-like orbit with a
relatively high inclination and large eccentricity. Hence, we carried out
optical spectroscopic observations of the asteroid to look for comet-like
molecular emissions or outbursts. However, the asteroid showed a featureless
spectrum, devoid of any comet-like features. The asteroid's light curve was
analyzed using V band magnitudes derived from the spectra and the most likely
solution for the rotation of the asteroid was obtained. The absolute magnitude
$H$ and the slope parameter $G$ were determined for the asteroid in V filter
band using the IAU accepted standard two parameter H-G model. A peculiar,
rarely found result from these observations is its phase bluing trend. The
relative B-V color index seems to decrease with increasing phase angle, which
indicates a phase bluing trend. Such trends have seldom been reported in
literature. However, phase reddening in asteroids is very common. The asymmetry
parameter $g$ and the single scattering albedo $w$ were estimated for the
asteroid by fitting the Hapke phase function to the observed data. The asteroid
shows relatively large value for the single scattering albedo and a highly back
scattering surface. | astro-ph_EP |
Breakup of the Synchronous State of Binary Asteroid Systems: This paper continues the authors' previous work and presents a coplanar
averaged ellipsoid-ellipsoid model of synchronous binary asteroid system (BAS)
plus thermal and tidal effects. Using this model, we analyze the breakup
mechanism of the synchronous BAS. Different from the classical spin-orbit
coupling model which neglects the rotational motion's influence on the orbital
motion, our model considers simultaneously the orbital motion and the
rotational motions. Our findings are following. (1) Stable region of the
secondary's synchronous state is mainly up to the secondary's shape. The
primary's shape has little influence on it. (2) The stable region shrinks
continuously with the increasing value of the secondary's shape parameter
$a_B/b_B$. Beyond the value of $a_B/b_B=\sqrt{2}$, the planar stable region for
the secondary's synchronous rotation is small but not zero. (3) Considering the
BYORP torque, our model shows agreement with the 1-degree of freedom adiabatic
invariance theory in the outwards migration process, but an obvious difference
in the inwards migration process. In particular, our studies show that the
so-called 'long-term' stable equilibrium between the BYORP torque and the tidal
torque is never a real equilibrium state, although the binary asteroid system
can be captured in this state for quite a long time. (4) In case that the
primary's angular velocity gradually reduces due to the YORP effect, the
secondary's synchronous state may be broken when the primary's rotational
motion crosses some major spin-orbit resonances. | astro-ph_EP |
Cosmogenic nuclide enhancement via deposition from long-period comets as
a test of the Younger Dryas impact hypothesis: We explore the idea that detectable excursions in 26Al may arise from direct
deposition by any bolide, and excursions in 14C and 10Be abundances in the
atmosphere may result from long-period comet impacts. This is very different
from the usual processes of production by cosmic rays within Earths atmosphere.
Long-period comets experience greatly increased cosmic ray flux beyond the
protection of the suns magnetic field. We report the computed amount of 14C,
10Be, and 26Al present on long-period comets as a function of comet mass. We
find that the amount of nuclide mass on large long-period comets entering the
Earths atmosphere may be sufficient for creating anomalies in the records of
14C and 10Be from past impacts. In particular, the estimated mass of the
proposed Younger Dryas comet is consistent with its having deposited sufficient
isotopes to account for recorded 14C and 10Be increases at that time. The
26Al/10Be ratio is much larger in extraterrestrial objects than in the
atmosphere, and so, we note that measuring this ratio in ice cores is a
suitable definitive test for the Younger Dryas impact hypothesis, even if the
hypothetical bolide is not a long-period comet and/or did not contribute to the
14C and 10Be increases. | astro-ph_EP |
Probing the turbulent mixing strength in protoplanetary disks across the
stellar mass range: no significant variations: Dust settling and grain growth are the first steps in the planet-formation
process in protoplanetary disks. These disks are observed around stars with
different spectral types, and there are indications that the disks around lower
mass stars are significantly flatter, which could indicate that they settle and
evolve faster, or in a different way.
We aim to test this assumption by modeling the median spectral energy
distributions (SEDs) of three samples of protoplanetary disks: around Herbig
stars, T Tauri stars and brown dwarfs. We focus on the turbulent mixing
strength to avoid a strong observational bias from disk and stellar properties
that depend on stellar mass.
We generated SEDs with the radiative transfer code MCMax, using a hydrostatic
disk structure and settling the dust in a self-consistent way with the
alpha-prescription to probe the turbulent mixing strength.
We are able to fit all three samples with a disk with the same input
parameters, scaling the inner edge to the dust evaporation radius and disk mass
to millimeter photometry. The Herbig stars require a special treatment for the
inner rim regions, while the T-Tauri stars require viscous heating, and the
brown dwarfs lack a good estimate of the disk mass because only few millimeter
detections exist.
We find that the turbulent mixing strength does not vary across the stellar
mass range for a fixed grain size distribution and gas-to-dust ratio. Regions
with the same temperature have a self-similar vertical structure independent of
stellar mass, but regions at the same distance from the central star appear
more settled in disks around lower mass stars. We find a relatively low
turbulent mixing strength of alpha = 10^(-4) for a standard grain size
distribution, but our results are also consistent with alpha = 0.01 for a grain
size distribution with fewer small grains or a lower gas-to-dust ratio. | astro-ph_EP |
Massive Protostellar Disks as a Hot Laboratory of Silicate Grain
Evolution: Typical accretion disks around massive protostars are hot enough for water
ice to sublimate. We here propose to utilize the massive protostellar disks for
investigating the collisional evolution of silicate grains with no ice mantle,
which is an essential process for the formation of rocky planetesimals in
protoplanetary disks around lower-mass stars. We for the first time develop a
model of massive protostellar disks that includes the coagulation,
fragmentation, and radial drift of dust. We show that the maximum grain size in
the disks is limited by collisional fragmentation rather than by radial drift.
We derive analytic formulas that produce the radial distribution of the maximum
grain size and dust surface density in the steady state. Applying the analytic
formulas to the massive protostellar disk of GGD27-MM1, where the grain size is
constrained from a millimeter polarimetric observation, we infer that the
silicate grains in this disk fragment at collision velocities above ~ 10 m/s.
The inferred fragmentation threshold velocity is lower than the maximum grain
collision velocity in typical protoplanetary disks around low-mass stars,
implying that coagulation alone may not lead to the formation of rocky
planetesimals in those disks. With future measurements of grain sizes in
massive protostellar disks, our model will provide more robust constraints on
the sticking property of silicate grains. | astro-ph_EP |
An accurate, extensive, and practical line list of methane for the
HITEMP database: A methane line list for the HITEMP spectroscopic database, covering 0-13,400
cm$^{-1}$ ($>$746 nm), is presented. To create this compilation, ab initio line
lists of $^{12}$CH$_{4}$ from Rey et al. (2017) ApJ, 847, 105 (provided at
separate temperatures in the TheoReTS information system), are now combined
with HITRAN2016 methane data to produce a single line list suitable for
high-temperature line-by-line calculations up to 2000 K. An
effective-temperature interpolation model was created in order to represent
continuum-like features at any temperature of interest. This model is
advantageous to previously-used approaches that employ so-called
``super-lines'', which are suitable only at a given temperature and require
separate line lists for different temperatures. The resultant HITEMP line list
contains $\sim$32 million lines and is significantly more flexible than
alternative line lists of methane, while accuracy required for astrophysical or
combustion applications is retained. Comparisons against experimental
observations of methane absorption at high temperatures have been used to
demonstrate the accuracy of the new work. The line list includes both strong
lines and quasi-continuum features and is provided in the common user-friendly
HITRAN/HITEMP format, making it the most practical methane line list for
radiative transfer modeling at high-temperature conditions. | astro-ph_EP |
Tidal Downsizing Model. IV. Destructive feedback in planets: I argue that feedback is as important to formation of planets as it is to
formation of stars and galaxies. Energy released by massive solid cores puffs
up pre-collapse gas giant planets, making them vulnerable to tidal disruptions
by their host stars. I find that feedback is the ultimate reason for some of
the most robust properties of the observed exoplanet populations: the rarity of
gas giants at all separations from $\sim 0.1$ to $\sim 100$~AU, the abundance
of $\sim 10 M_\oplus$ cores but dearth of planets more massive than $\sim 20
M_\oplus$. Feedback effects can also explain (i) rapid assembly of massive
cores at large separations as needed for Uranus, Neptune and the suspected HL
Tau planets; (ii) the small core in Jupiter yet large cores in Uranus and
Neptune; (iii) the existence of rare "metal monster" planets such as CoRoT-20b,
a gas giant made of heavy elements by up to $\sim 50$\%. | astro-ph_EP |
How does Background Air Pressure Influence the Inner Edge of the
Habitable Zone for Tidally Locked Planets in a 3D View?: We examine the effect of varying background N2 surface pressure (labelled as
pN2) on the inner edge of the habitable zone for 1:1 tidally locked planets
around M dwarfs, using the three-dimensional (3D) atmospheric general
circulation model (AGCM) ExoCAM. In our experiments, the rotation period is
fixed when varying the stellar flux, in order to more clearly isolate the role
of pN2. We find that the stellar flux threshold for the runaway greenhouse is a
non-monotonous function of pN2. This is due to the competing effects of five
processes: pressure broadening, heat capacity, lapse rate, relative humidity,
and clouds. These competing processes increase the complexity in predicting the
location of the inner edge of the habitable zone. For a slow rotation orbit of
60 Earth days, the critical stellar flux for the runaway greenhouse onset is
1700--1750, 1900--1950, and 1750--1800 W m$^{-2}$ under 0.25, 1.0, and 4.0 bar
of pN2, respectively, suggesting that the magnitude of the effect of pN2 is
within ~13%. For a rapid rotation orbit, the effect of varying pN2 on the inner
edge is smaller, within a range of ~7%. Moreover, we show that Rayleigh
scattering effect as varying pN2 is unimportant for the inner edge due to the
masking effect of cloud scattering and to the strong shortwave absorption by
water vapor under hot climates. Future work using AGCMs having different cloud
and convection schemes and cloud-resolving models having explicit cloud and
convection are required to revise this problem. | astro-ph_EP |
The heating history of Vesta and the onset of differentiation: In this work we study the link between the evolution of the internal
structure of Vesta and thermal heating due to 26Al and 60Fe and long-lived
radionuclides, taking into account the chemical differentiation of the body and
the affinity of 26Al with silicates. Differentiation takes place in all
scenarios in which Vesta completes its accretion in less than 1.4 Ma after the
injection of 26Al into the Solar Nebula. In all those scenarios where Vesta
completes its formation in less than 1 Ma from the injection of 26Al, the
degree of silicate melting reaches 100 vol. % throughout the whole asteroid. If
Vesta completed its formation between 1 and 1.4 Ma after 26Al injection, the
degree of silicate melting exceeds 50 vol. % over the whole asteroid but
reaches 100 vol. % only in the hottest, outermost part of the mantle in all
scenarios where the porosity is lower than 5 vol. %. If the formation of Vesta
occurred later than 1.5 Ma after the injection of 26Al, the degree of silicate
melting is always lower than 50 vol. % and is limited only to a small region of
the asteroid. The radiation at the surface dominates the evolution of the crust
which ranges in thickness from 8 to about 30 km after 5 Ma: a layer about 3-20
km thick is composed of primitive unmelted chondritic material while a layer of
about 5-10 km is eucritic. | astro-ph_EP |
Testing the Early Mars H2-CO2 Greenhouse Hypothesis with a 1-D
Photochemical Model: A recent study by Ramirez et al. (2014) demonstrated that an atmosphere with
1.3-4 bar of CO2 and H2O, in addition to 5-20% H2, could have raised the mean
annual and global surface temperature of early Mars above the freezing point of
water. Such warm temperatures appear necessary to generate the rainfall (or
snowfall) amounts required to carve the ancient martian valleys. Here, we use
our best estimates for early martian outgassing rates, along with a 1-D
photochemical model, to assess the conversion efficiency of CO, CH4, and H2S to
CO2, SO2, and H2. Our outgassing estimates assume that Mars was actively
recycling volatiles between its crust and interior, as Earth does today. H2
production from serpentinization and deposition of banded iron-formations is
also considered. Under these assumptions, maintaining an H2 concentration of
~1-2% by volume is achievable, but reaching 5% H2 requires additional H2
sources or a slowing of the hydrogen escape rate below the diffusion limit. If
the early martian atmosphere was indeed H2-rich, we might be able to see
evidence of this in the rock record. The hypothesis proposed here is consistent
with new data from the Curiosity Rover, which show evidence for a long-lived
lake in Gale Crater near Mt. Sharp. It is also consistent with measured oxygen
fugacities of martian meteorites, which show evidence for progressive mantle
oxidation over time. | astro-ph_EP |
The size-frequency distribution of H>13 NEOs and ARM targets detected by
Pan-STARRS1: We determine the absolute magnitude (H) distribution (or size-frequency
distribution, SFD; $N(H) \propto 10^{\alpha H}$ where $\alpha$ is the slope of
the distribution) for near-Earth objects (NEO) with $13<H<30$ and Asteroid
Retrieval Mission (ARM) targets with $27<H<31$ that were detected by the 1\st\
telescope of the Panoramic Survey Telescope and Rapid Response System -
Pan-STARRS1 (e.g. Kaiser et al. 2002, Kaiser 2004, Hodapp et al. 2004). The NEO
and ARM target detection efficiencies were calculated using the Greenstreet et
al. (2012) NEO orbit distribution. The debiased Pan-STARRS1 NEO absolute
magnitude distribution is more complex than a single slope power law - it shows
two transitions - at H$\sim$16 from steep to shallow slope, and in the
$21<H<23$ interval from a shallow to steep slope, which is consistent with
other recent works (e.g. Mainzer et al. 2011c, Brown et al. 2013, Harris and
D`Abramo 2015). We fit $\alpha = 0.48\pm0.02$ for NEOs with $13<H<16$, $\alpha
= 0.33\pm0.01$ for NEOs with $16<H<22$, and $\alpha = 0.62\pm0.03$ for the
smaller objects with $H>22$. There is also another change in slope from steep
to shallow around H=27. The three ARM target candidates detected by Pan-STARRS1
in one year of surveying have a corrected SFD with slope $\alpha =
0.40^{+0.33}_{-0.45}$.
We also show that the window for follow up observations of small
(H$\gtrsim$22) NEOs with the NASA IRTF telescope and Arecibo and Goldstone
radars are extremely short - on order of days, and procedures for fast response
must be implemented in order to measure physical characteristics of small
Earth-approaching objects. CFHT's MegaCam and Pan-STARRS1 have longer observing
windows and are capable of following-up more NEOs due to their deeper limiting
magnitudes and wider fields of view. | astro-ph_EP |
Exoplanet Nodal Precession Induced by Rapidly Rotating Stars: Impacts on
Transit Probabilities and Biases: For the majority of short period exoplanets transiting massive stars with
radiative envelopes, the spin angular momentum of the host star is greater than
the planetary orbital angular momentum. In this case, the orbits of the planets
will undergo nodal precession, which can significantly impact the probability
that the planets transit their parent star. In particular, for some
combinations of the spin-orbit angle $\psi$ and the inclination of the stellar
spin $i_*$, all such planets will eventually transit at some point over the
duration of their precession period. Thus, as the time over which the sky has
been monitored for transiting planets increases, the frequency of planets with
detectable transits will increase, potentially leading to biased estimates of
exoplanet occurrence rates, especially orbiting more massive stars.
Furthermore, due to the dependence of the precession period on orbital
parameters such as spin-orbit misalignment, the observed distributions of such
parameters may also be biased. We derive the transit probability of a given
exoplanet in the presence of nodal precession induced by a rapidly spinning
host star. We find that the effect of nodal precession has already started to
become relevant for some short-period planets, i.e., Hot Jupiters, orbiting
massive stars, by increasing transit probabilities by of order a few percent
for such systems within the original $Kepler$ field. We additionally derive
simple expressions to describe the time evolution of the impact parameter $b$
for applicable systems, which should aid in future investigations of exoplanet
nodal precession and spin-orbit alignment. | astro-ph_EP |
Detection of the hydrogen Balmer lines in the ultra-hot Jupiter WASP-33b: Ultra-hot Jupiters (UHJs) are highly irradiated giant exoplanets with
extremely high day-side temperatures, which lead to thermal dissociation of
most of the molecular species. It is expected that the neutral hydrogen atom is
one of the main species in the upper atmospheres of ultra-hot Jupiters. Neutral
hydrogen has been detected in several UHJs by observing its Balmer line
absorption. Here, we report four transit observations of the ultra-hot Jupiter
WASP-33b, performed with the CARMENES and HARPS-North spectrographs, and the
detection of the H${\alpha}$, H${\beta}$, and H${\gamma}$ lines in the
planetary transmission spectrum. The combined H$\alpha$ transmission spectrum
of the four transits has an absorption depth of 0.99$\pm$0.05 %, which
corresponds to an effective radius of 1.31$\pm$0.01 Rp . The strong H${\alpha}$
absorption indicates that the line probes the high-altitude thermosphere. We
further fitted the three Balmer lines using the PAWN model, assuming that the
atmosphere is hydrodynamic and in LTE. We retrieved a thermosphere temperature
$12200^{+1300}_{-1000}$ K and a mass-loss rate ${\rm
\dot{M}}=10^{11.8^{+0.6}_{-0.5}}$ g/s. The retrieved large mass-loss rate is
compatible with the "Balmer-driven" atmospheric escape scenario, in which the
stellar Balmer continua radiation in the near-ultraviolet is substantially
absorbed by the excited hydrogen atoms in the planetary thermosphere. | astro-ph_EP |
Properties of the irregular satellite system around Uranus inferred from
K2, Herschel and Spitzer observations: In this paper we present visible range light curves of the irregular Uranian
satellites Sycorax, Caliban, Prospero, Ferdinand and Setebos taken with Kepler
Space Telescope in the course of the K2 mission. Thermal emission measurements
obtained with the Herschel/PACS and Spitzer/MIPS instruments of Sycorax and
Caliban were also analysed and used to determine size, albedo and surface
characteristics of these bodies. We compare these properties with the
rotational and surface characteristics of irregular satellites in other giant
planet systems and also with those of main belt and Trojan asteroids and
trans-Neptunian objects. Our results indicate that the Uranian irregular
satellite system likely went through a more intense collisional evolution than
the irregular satellites of Jupiter and Saturn. Surface characteristics of
Uranian irregular satellites seems to resemble the Centaurs and trans-Neptunian
objects more than irregular satellites around other giant planets, suggesting
the existence of a compositional discontinuity in the young Solar system inside
the orbit of Uranus. | astro-ph_EP |
Accurate reference spectra of HD in H$_2$/He bath for planetary
applications: The hydrogen deuteride (HD) molecule is an important deuterium tracer in
astrophysical studies. The atmospheres of gas giants are dominated by molecular
hydrogen, and simultaneous observation of H$_2$ and HD lines provides reliable
information on the D/H ratios on these planets. The reference spectroscopic
parameters play a crucial role in such studies. Under thermodynamic conditions
encountered in these atmospheres, the spectroscopic studies of HD require not
only the knowledge of line intensities and positions but also accurate
reference data on pressure-induced line shapes and shifts. Our aim is to
provide accurate collision-induced line-shape parameters for HD lines that
cover any thermodynamic conditions relevant to the atmospheres of giant
planets, i.e., any relevant temperature, pressure, and perturbing gas (the
H$_2$/He mixture) composition. We perform quantum-scattering calculations on a
new highly accurate ab initio potential energy surface, and we use scattering
S-matrices obtained this way to determine the collision-induced line-shape
parameters. We use the cavity ring-down spectroscopy for validation of our
theoretical methodology. We report accurate collision-induced line-shape
parameters for the pure rotational R(0), R(1), and R(2) lines, the most
relevant HD lines for the investigations of atmospheres of the giant planets.
Besides the basic Voigt-profile collisional parameters (i.e. the broadening and
shift parameters), we also report their speed dependences and the complex Dicke
parameter, which can influence the effective width and height of the HD lines
up to almost a factor of 2 for giant planet conditions. The sub-percent-level
accuracy, reached in this work, considerably improves the previously available
data. All the reported parameters are consistent with the HITRAN database
format, hence allowing for the use of HAPI for generating the beyond-Voigt
spectra of HD. | astro-ph_EP |
Physical properties of near-Earth asteroid (2102) Tantalus from
multi-wavelength observations: Between 2010 and 2017 we have collected new optical and radar observations of
the potentially hazardous asteroid (2102)~Tantalus from the ESO NTT and Danish
telescopes at the La Silla Observatory and from the Arecibo planetary radar.
The object appears to be nearly spherical, showing a low amplitude light-curve
variation and limited large-scale features in the radar images. The spin-state
is difficult to constrain with the available data; including a certain
light-curve subset significantly changes the spin-state estimates, and the
uncertainties on period determination are significant. Constraining any change
in rotation rate was not possible, despite decades of observations. The convex
lightcurve-inversion model, with rotational pole at
${\lambda}=210{\pm}41${\deg} and ${\beta}=-30{\pm}35${\deg}, is more flattened
than the two models reconstructed by including radar observations: with
prograde (${\lambda}=36{\pm}23${\deg}, ${\beta}=30{\pm}15${\deg}), and with
retrograde rotation mode (${\lambda}=180{\pm}24${\deg},
${\beta}=-30{\pm}16${\deg}). Using data from WISE we were able to determine
that the prograde model produces the best agreement in size determination
between radar and thermophysical modelling. Radar measurements indicate
possible variation in surface properties, suggesting one side might have lower
radar albedo and be rougher at centimetre-to-decimetre scale than the other.
However, further observations are needed to confirm this. Thermophysical
analysis indicates a surface covered in fine-grained regolith, consistent with
radar albedo and polarisation ratio measurements. Finally, geophysical
investigation of the spin-stability of Tantalus shows that it could be
exceeding its critical spin-rate via cohesive forces. | astro-ph_EP |
Effects of the Planetary Temperature on the Circumplanetary Disk and on
the Gap: Circumplanetary disks regulate the late accretion to the giant planet and
serve as the birthplace for satellites. Understanding their characteristics via
simulations also helps to prepare for their observations. Here we study disks
around 1, 3, 5, 10 $\mathrm{M_{Jup}}$ planets with three dimensional, global
radiative hydrodynamic simulations with sub-planet peak resolution, and various
planetary temperatures. We found that as the 1 $\mathrm{M_{Jup}}$ planet
radiates away its formation heat, the circumplanetary envelope transitions to a
disk between $T_p = 6000$ K and 4000 K. In the case of 3-10 $\mathrm{M_{Jup}}$
planets a disk always forms. The temperature profile of the circumplanetary
disks is very steep, the inner 1/6th is over the silicate condensation
temperature and the entire disk is above water freezing point, making satellite
formation impossible in this early stage ($<$1 Myr). Satellites might form much
later and first in the outer parts of the disk migrating inwards later on. Our
disk masses are $1, 7, 20, 40 \times 10^{-3}\mathrm{M_{Jup}}$ for the 1, 3, 5,
10 $\mathrm{M_{Jup}}$ gas giants respectively, and we provide an empirical
formula to estimate the subdisk masses based on the planet- and circumstellar
disk mass. Our finding is that the cooler the planet, the lower the temperature
of the subdisk, the higher the vertical influx velocities, and the planetary
gap is both deeper and wider. We also show that the gaps in 2D and 3D are
different. The subdisk eccentricity increases with planetary mass and violently
interacts with the circumstellar disk, making satellite-formation less likely,
if $\mathrm{M_p} \gtrsim 5 \mathrm{M_{Jup}}$. | astro-ph_EP |
Search for serendipitous TNO occultation in X-rays: To study the population properties of small, remote objects beyond Neptune's
orbit in the outer solar system, of kilometer size or smaller, serendipitous
occultation search is so far the only way. For hectometer-sized Trans-Neptunian
Objects (TNOs), optical shadows actually disappear because of diffraction.
Observations at shorter wave lengths are needed. Here we report the effort of
TNO occultation search in X-rays using RXTE/PCA data of Sco X-1 taken from June
2007 to October 2011. No definite TNO occultation events were found in the 334
ks data. We investigate the detection efficiency dependence on the TNO size to
better define the sensible size range of our approach and suggest upper limits
to the TNO size distribution in the size range from 30 m to 300 m. A list of
X-ray sources suitable for future larger facilities to observe is proposed. | astro-ph_EP |
A radiative-convective equilibrium model for young giant exoplanets:
Application to beta Pictoris b: We present a radiative-convective equilibrium model for young giant
exoplanets. Model predictions are compared with the existing photometric
measurements of planet beta Pictoris b in the J, H, Ks, L', NB 4.05, M' bands .
This model will be used to interpret future photometric and spectroscopic
observations of exoplanets with SPHERE, mounted at the VLT with a first light
expected mid-2014. | astro-ph_EP |
An evidence for solar activity influence on the meteorological processes
in the south polar region of Mars during the great opposition in AD 1924: A time series of the Martian south ice polar cap mean diameter for the period
July-December 1924 is investigated. The data are based on the high quality
pictures, which are obtained by visual observations of 60 cm telescope in
Hamburg Observatory during the great opposition of Mars in AD 1924. After
removing of the seasonal trend (caused by the springtime regression of the cap)
quasi 36 and 80-82 days cycles in residuals has been obtained. The sunspot
activity spectra for the corresponding period is almost the same one. The local
maximums of polar cap area residuals has been occured of about 10 days after
the corresponding minimums of sunspot activity. The so obtained results are
briefly discussed. | astro-ph_EP |
Do we really know the dust? Systematics and uncertainties of the
mid-infrared spectral analysis methods: The spectral region around 10 micrometer, showing prominent dust emission
bands, is commonly used to derive the chemical composition of protoplanetary
dust. Different analysis methods have been proposed for this purpose, but so
far, no comparative test has been performed to test their validity. We
calculated model spectra of disk models with different geometries and central
sources, using a 2D radiative transfer code. These spectra were then fitted in
a blind test using four different spectral decomposition methods. We studied
the effect of disk structure (flared vs. flat), inclination angle, size of the
inner disk hole and stellar luminosity on the fitted chemical composition. Our
results show that the derived dust compositions by all methods deviate
systematically from the real chemical composition. Out of the four tested
spectral decomposition methods, our new two-layer temperature distribution
method, differs the least from the input dust composition and the results show
the weakest systematic effects. The reason for the deviations of the results
given by other methods lies in their simplifying assumptions (e.g. single
average grain temperature or one component continuum). We also tested the
influence of different noise levels on the results of the spectral
decomposition methods. We find that, for ground-based observations (8-13
micrometer) the expected uncertainty in the value of the crystallinity is about
11% for a signal-to-noise ratio of 100, while for space-based observations
(7-17 micrometer) the uncertainty is about 5%. On the basis of our results, we
propose a recipe for the analysis and interpretation of dust spectroscopy data
in the mid-infrared which should be especially valuable for analysing Spitzer
data and ground-based infrared spectroscopy data in the 10 micrometer window. | astro-ph_EP |
Rapid Mid-Infrared Variability in Protostellar Disks: Spectral energy distribution (SED) in protostellar disks is determined by the
disks'internal dissipation and reprocessing of irradiation from their host
stars. Around T Tauri stars, most mid-infrared (MIR) radiation (in a few to a
few ten {\mu}m wavelength range) emerge from regions around a fraction to a few
AU's. This region is interesting because it contains both the habitable zone
and the snow line. Recent observations reveal that SED variations, in the MIR
wavelength range. These variations are puzzling because they occur on time
scale (a few days) which is much shorter than the dynamical (months to years)
time scale at 1AU to a few AU's. They are probably caused by shadows casted by
inner onto outer disk regions. Interaction between disks and their misaligned
magnetized host stars can lead to warped structure and periodic SED
modulations. Rapid aperiodic SED variations may also be induced by observed
X-ray flares from T Tauri stars. These flares can significantly modulate the
ionization fraction of the gas and the net charge carried by the grains near
the surface of the inner disk. The newly charged grains may be accelerated by
the stellar or disk magnetic field and adjust their distances from the
midplane. Shadows casted by these grains attenuates the flux of stellar photons
irradiated onto regions at several AU's from the central stars. We use this
model to account for the observed rapid aperiodic SED variabilities. We suggest
regular monitoring of SED variations will not only provide valuable information
on the distribution of the disk aspect ratio near the habitable zone but also
provide a probe on the interaction between the inner regions of the disk with
the magnetosphere of their host stars. | astro-ph_EP |
Ghost in the time series: no planet for Alpha Cen B: We re-analyse the publicly available radial velocity (RV) measurements for
Alpha Cen B, a star hosting an Earth-mass planet candidate, Alpha Cen Bb, with
3.24 day orbital period. We demonstrate that the 3.24 d signal observed in the
Alpha Cen B data almost certainly arises from the window function (time
sampling) of the original data. We show that when stellar activity signals are
removed from the RV variations, other significant peaks in the power spectrum
of the window function are coincidentally suppressed, leaving behind a spurious
yet apparently-significant 'ghost' of a signal that was present in the window
function's power spectrum to begin with. Even when fitting synthetic data with
time sampling identical to the original data, but devoid of any genuine
periodicities close to that of the planet candidate, the original model used to
infer the presence of Alpha Cen Bb leads to identical conclusions: viz., the
3$\sigma$ detection of a half-a-metre-per-second signal with 3.236 day period.
Our analysis underscores the difficulty of detecting weak planetary signals in
RV data, and the importance of understanding in detail how every component of
an RV data set, including its time sampling, influences final statistical
inference. | astro-ph_EP |
Wall emission in circumbinary disks: the case of CoKu Tau/4: A few years ago, the mid-IR spectrum of a Weak Line T Tauri Star, CoKu Tau/4,
was explained as emission from the inner wall of a circumstellar disk, with the
inner disk truncated at ~10 AU. Based on the SED shape and the assumption that
it was produced by a single star and its disk, CoKu Tau/4 was classified as a
prototypical transitional disk, with a clean inner hole possibly carved out by
a planet, some other orbiting body, or by photodissociation. However, recently
it has been discovered that CoKu Tau/4 is a close binary system. This implies
that the observed mid-IR SED is probably produced by the circumbinary disk. The
aim of the present paper is to model the SED of CoKu Tau/4 as arising from the
inner wall of a circumbinary disk, with parameters constrained by what is known
about the central stars and by a dynamical model for the interaction between
these stars and their surrounding disk. In order to fit the Spitzer IRS SED,
the binary orbit should be almost circular, implying a small mid-IR variability
(10%) related to the variable distances of the stars to the inner wall of the
circumbinary disk. Our models suggest that the inner wall of CoKu Tau/4 is
located at 1.7a, where a is the semi-major axis of the binary system (a~8AU). A
small amount of optically thin dust in the hole (<0.01 lunar masses) helps to
improve the fit to the 10microns silicate band. Also, we find that water ice
should be absent or have a very small abundance (a dust to gas mass ratio
<5.6X10^{-5}). In general, for a binary system with eccentricity e>0, the model
predicts mid-IR variability with periods similar to orbital timescales,
assuming that thermal equilibrium is reached instantaneously. | astro-ph_EP |
Cleaning our Hazy Lens: Exploring Trends in Transmission Spectra of Warm
Exoplanets: Relatively little is understood about the atmospheric composition of
temperate to warm exoplanets (equilibrium temperature $T_{\rm eq}<$ 1000 K), as
many of them are found to have uncharacteristically flat transmission spectra.
Their flattened spectra are likely due to atmospheric opacity sources such as
planet-wide photochemical hazes and condensation clouds. We compile the
transmission spectra of 25 warm exoplanets previously observed by the Hubble
Space Telescope and quantify the haziness of each exoplanet using a normalized
amplitude of the water absorption feature ($A_{\rm H}$). By examining the
relationships between $A_{\rm H}$ and various planetary and stellar forcing
parameters, we endeavor to find correlations of haziness associated with
planetary properties. We adopt new statistical correlation tests that are more
suitable for the small, non-normally distributed warm exoplanet sample. Our
analysis shows that none of the parameters hold statistically significant
correlation with $A_{\rm H}$ ($p \le 0.01$) with the addition of new exoplanet
data, including the previously identified linear trends between $A_{\rm H}$ and
$T_{\rm{eq}}$ or hydrogen-helium envelope mass fraction (f$_{\rm{HHe}}$). This
suggests that haziness in warm exoplanets is not simply controlled by any
single planetary/stellar parameter. Among all the parameters we investigated,
planet gravity ($g_{\rm p}$), atmospheric scale height ($H$), planet density
($\rho_{\rm p}$), orbital eccentricity ($e$), and age of the star ($t_{\rm
age}$) hold tentative correlations with $A_{\rm H}$. Specifically, lower $H$,
higher $g_{\rm p}$, $\rho_{\rm p}$, $e$, or $t_{\rm age}$ may lead to clearer
atmospheres. We still need more observations and laboratory experiments to
fully understand the complex physics and chemistry involved in creating hazy
warm exoplanets. | astro-ph_EP |
Earths are not Super-Earths, Saturns are not Jupiters: Imprints of
pressure-bump planet formation on planetary architectures: In protoplanetary disks, sufficiently massive planets excite pressure bumps,
which can then be preferred locations for forming new planet cores. We discuss
how this loop may affect the architecture of multi-planet systems, and compare
our predictions with observation. Our main prediction is that low-mass planets
and giant planets can each be divided into two subpopulations with different
levels of mass uniformity. Low-mass planets that can and cannot reach the
pebble isolation mass (the minimum mass required to produce a pressure bump)
develop into intra-similar "Super-Earths" and more diverse "Earths",
respectively. Gas giants that do and do not accrete envelope quickly develop
into intra-similar "Jupiters" and more diverse "Saturns", respectively.
Super-Earths prefer to form long chains via repeated pressure-bump planet
formation, while Jupiter formation is usually terminated at pairs or triplets
due to dynamical instability. These predictions are broadly consistent with
observations. In particular, we discover a previously overlooked mass
uniformity dichotomy among the observed populations of both low-mass planets
(Earths vs. Super-Earths) and gas giants (Saturns vs. Jupiters). For low-mass
planets, planets well below the pebble isolation mass ($\lesssim 3M_\oplus$ or
$\lesssim 1.5 R_\oplus$ for sun-like stars) show significantly higher
intra-system pairwise mass difference than planets around the pebble isolation
mass. For gas giants, the period ratios of intra-system pairs show a bimodal
distribution, which can be interpreted as two subpopulations with different
levels of mass uniformity. These findings suggest that pressure-bump planet
formation could be an important ingredient in shaping planetary architectures. | astro-ph_EP |
Finding regions of bounded motion in binary asteroid environment using
Lagrangian descriptors: Trajectory design in highly-perturbed environments like binary asteroids is
challenging. It typically requires using realistic, non-autonomous dynamical
models in which periodic solutions derived in autonomous systems vanish. In
this work, Lagrangian descriptors are employed in the perturbed planar
bi-elliptic restricted four-body problem to find regions of bounded motion over
a finite horizon about Dimorphos, the secondary body of the (65803) Didymos
binary system. Results show that Lagrangian descriptors successfully reveal
phase space organizing structures both in the unperturbed and perturbed planar
bi-elliptic restricted four-body problem. With no solar radiation pressure,
regions of bounded motion are visually identified, so granting access to a vast
selection of bounded orbits about Dimorphos. Conversely, the presence of solar
radiation pressure breaks down the majority of structures, leading to a large
region of unstable motion with rare exceptions. Lagrangian descriptors are
computationally inexpensive dynamical indicators that could be conveniently
applied to astrodynamics. | astro-ph_EP |
Fate of the runner in hit-and-run collisions: In similar-sized planetary collisions, a significant part of the impactor
often misses the target and continues downrange. We follow the dynamical
evolution of "runners" from giant impacts to determine their ultimate fate.
Surprisingly, runners re-impact their target planets only about half of the
time, for realistic collisional and dynamical scenarios. Otherwise they remain
in orbit for tens of millions of years (the limit of our N-body calculations)
and longer, or sometimes collide with a different planet than the first one.
When the runner does return to collide again with the same arget planet, its
impact velocity is mainly constrained by the outcome of the prior collision.
Impact angle and orientation, however, are unconstrained by the prior
collision. | astro-ph_EP |
Characterization of the HD 108236 system with CHEOPS and TESS.
Confirmation of a fifth transiting planet: The HD108236 system was first announced with the detection of four small
planets based on TESS data. Shortly after, the transit of an additional planet
with a period of 29.54d was serendipitously detected by CHEOPS. In this way,
HD108236 (V=9.2) became one of the brightest stars known to host five small
transiting planets (R$_p$<3R$_{\oplus}$). We characterize the planetary system
by using all the data available from CHEOPS and TESS space missions. We use the
flexible pointing capabilities of CHEOPS to follow up the transits of all the
planets in the system, including the fifth transiting body. After updating the
host star parameters by using the results from Gaia eDR3, we analyzed 16 and 43
transits observed by CHEOPS and TESS, respectively, to derive the planets
physical and orbital parameters. We carried out a timing analysis of the
transits of each of the planets of HD108236 to search for the presence of
transit timing variations. We derived improved values for the radius and mass
of the host star (R$_{\star}$=0.876$\pm$0.007 R$_{\odot}$ and
M$_{\star}$=0.867$_{-0.046}^{+0.047}$ M$_{\odot}$). We confirm the presence of
the fifth transiting planet f in a 29.54d orbit. Thus, the system consists of
five planets of R$_b$=1.587$\pm$0.028, R$_c$=2.122$\pm$0.025,
R$_d$=2.629$\pm$0.031, R$_e$=3.008$\pm$0.032, and R$_f$=1.89$\pm$0.04
[R$_{\oplus}$]. We refine the transit ephemeris for each planet and find no
significant transit timing variations for planets c, d, and e. For planets b
and f, instead, we measure significant deviations on their transit times (up to
22 and 28 min, respectively) with a non-negligible dispersion of 9.6 and 12.6
min in their time residuals. We confirm the presence of planet f and find no
significant evidence for a potential transiting planet in a 10.9d orbital
period, as previously suggested. Full abstract in the PDF file. | astro-ph_EP |
Prospecting for exo-Earths in multiple planet systems with a gas giant: In this work, we hunt for the best places to find exo-Earths in the currently
known exoplanet population. While it is still unclear whether Jupiter had a
beneficial or detrimental effect on the creation of the right environment for a
habitable Earth to develop, we focus on the 51 multiple planet systems that
have at least one Jupiter-like planet and aim to identify which would be good
candidates to host an exo-Earth. We conduct a series of numerical simulations
to identify dynamically stable regions of the habitable zone of the multiple
exoplanet systems capable of hosting an Earth-mass planet. We produce a
candidate list of 16 systems that could host such a stable exo-Earth in their
habitable zone, and for which the induced radial velocity signal of a
hypothetical one, two or four Earth-mass planet on the host star would be
detectable with the ESPRESSO spectrograph. We find that whilst the
gravitational interactions with the massive planet nearest the habitable zone
are critical in determining stability, the secular resonant interactions
between multiple planets can also have a dramatic influence on the overall
stability of the habitable zone. | astro-ph_EP |
On the coexistence of the streaming instability and the vertical shear
instability in protoplanetary disks: The streaming instability is a leading candidate mechanism to explain the
formation of planetesimals. Yet, the role of this instability in the driving of
turbulence in protoplanetary disks, given its fundamental nature as a linear
hydrodynamical instability, has so far not been investigated in detail. We
study the turbulence that is induced by the streaming instability as well as
its interaction with the vertical shear instability. For this purpose, we
employ the FLASH Code to conduct two-dimensional axisymmetric global disk
simulations spanning radii from $1$ au to $100$ au, including the mutual drag
between gas and dust as well as the radial and vertical stellar gravity. If the
streaming instability and the vertical shear instability start their growth at
the same time, we find the turbulence in the dust mid-plane layer to be
primarily driven by the streaming instability. It gives rise to vertical gas
motions with a Mach number of up to ${\sim}10^{-2}$. The dust scale height is
set in a self-regulatory manner to about $1\%$ of the gas scale height. In
contrast, if the vertical shear instability is allowed to saturate before the
dust is introduced into our simulations, then it continues to be the main
source of the turbulence in the dust layer. The vertical shear instability
induces turbulence with a Mach number of ${\sim}10^{-1}$ and thus impedes dust
sedimentation. Nonetheless, we find the vertical shear instability and the
streaming instability in combination to lead to radial dust concentration in
long-lived accumulations which are significantly denser than those formed by
the streaming instability alone. Thus, the vertical shear instability may
promote planetesimal formation by creating weak overdensities that act as seeds
for the streaming instability. | astro-ph_EP |
Stellar Spin-Orbit Misalignment in a Multiplanet System: Stars hosting hot Jupiters are often observed to have high obliquities,
whereas stars with multiple co-planar planets have been seen to have low
obliquities. This has been interpreted as evidence that hot-Jupiter formation
is linked to dynamical disruption, as opposed to planet migration through a
protoplanetary disk. We used asteroseismology to measure a large obliquity for
Kepler-56, a red giant star hosting two transiting co-planar planets. These
observations show that spin-orbit misalignments are not confined to hot-Jupiter
systems. Misalignments in a broader class of systems had been predicted as a
consequence of torques from wide-orbiting companions, and indeed
radial-velocity measurements revealed a third companion in a wide orbit in the
Kepler-56 system. | astro-ph_EP |
Latitudinal Variation of Clouds' Structure Responsible for Venus' Cold
Collar: Global Climate Models (GCM) are very useful tools to study theoretically the
general dynamics and specific phenomena in planetary atmospheres. In the case
of Venus, several GCMs succeeded in reproducing the atmosphere's superrotation
and the global temperature field. However, the highly variable polar
temperature and the permanent cold collar have not been reproduced
satisfactorily yet. Here we improve the radiative transfer scheme of the
Institut Pierre Simon Laplace Venus GCM in order to numerically simulate the
polar thermal features in Venus atmosphere. The main difference with the
previous model is that we now take into account the latitudinal variation of
the cloud structure. Both solar heating rates and infrared cooling rates have
been modified to consider the cloud top's altitude decrease toward the poles
and the variation in latitude of the different particle modes' abundances. A
new structure that closely resembles the observed cold collar appears in the
average temperature field at $2\times10^{4} - 4\times10^{3}$~Pa ($\sim62 -
66$~km) altitude range and $60^{\circ} - 90^{\circ}$ latitude band. It is not
isolated from the pole as in the observation-based maps, but the obtained
temperature values (220~K) are in good agreement with observed values.
Temperature polar maps across this region show an inner warm region where the
polar vortex is observed, but the obtained 230~K average value is colder than
the observed mean value and the simulated horizontal structure does not show
the fine-scale features present within the vortex. Our study shows that the
cloud structure is essential in the cold collar formation. Although our
analysis focuses on the improvement of the radiative forcing and the variations
it causes in the thermal structure, polar dynamics is definitely affected by
this modified environment and a noteworthy upwelling motion is found in the
cold collar area. | astro-ph_EP |
Inner Boundary Condition in Quasi-Lagrangian Simulations of Accretion
Disks: In simulations of viscously evolving accretion disks, the inner boundary
condition is particularly important. If treated incorrectly, it induces
incorrect behavior very quickly, because the viscous time is shortest near the
inner boundary. Recent work has determined the correct inner boundary in
Eulerian simulations. But in quasi-Lagrangian simulations (e.g., SPH, moving
mesh, and mesh-less), where the inner boundary is modeled by removing mass
within a finite zone, the inner density profile typically becomes anomalously
depleted. Here we show how the boundary condition should be applied in such
codes, via a simple modification of the usual approach: when one removes mass,
one must speed up the remaining material so that the disk's angular momentum is
unchanged. We show with both 1D and 2D moving-mesh (AREPO) simulations that
this scheme works as desired in viscously evolving disks. It produces no
spurious density depletions and is independent of the mass removal rate,
provided that the disk is adequately resolved and that the mass removal rate is
not so extreme as to trigger instabilities. This "torque-free" mass removal
technique permits the use of quasi-Lagrangian codes to simulate viscously
evolving disks, while including a variety of additional effects. As an example,
we apply our scheme to a 2D simulation of an accretion disk perturbed by a very
massive planet, in which the disk is evolved to viscous steady state. | astro-ph_EP |
Constraining the gap size in the disk around HD 100546 in the
mid-infrared: We refine the gap size measurements of the disk surrounding the Herbig Ae
star HD 100546 in the N band. Our new mid-infrared interferometric (MIDI) data
have been taken with the UT baselines and span the full range of orientations.
The correlated fluxes show a wavy pattern in which the minima separation links
to a geometrical structure in the disk. We fit each correlated flux measurement
with a spline function, deriving the corresponding spatial scale, while
assuming that the pattern arises interferometrically due to the bright emission
from the inner disk and the opposing sides of the wall of the outer disk. We
then fit an ellipse to the derived separations at their corresponding position
angles, thereby using the observations to constrain the disk inclination to i
=47 +/- 1 degree and the disk position angle to PA =135.0 +/- 2.5 degree East
of North, both of which are consistent with the estimated values in previous
studies. We also derive the radius of the ellipse to 15.7 +/- 0.8 au. To
confirm that the minima separations translate to a geometrical structure in the
disk, we model the disk of HD 100546 using a semi-analytical approach taking
into account the temperature and optical depth gradients. Using this model, we
simultaneously reproduce the level and the minima of the correlated fluxes and
constrain the gap size of the disk for each observation. The values obtained
for the projected gap size in different orientations are consistent with the
separation found by the geometrical model. | astro-ph_EP |
Five key exoplanet questions answered via the analysis of 25 hot Jupiter
atmospheres in eclipse: Population studies of exoplanets are key to unlocking their statistical
properties. So far the inferred properties have been mostly limited to
planetary, orbital and stellar parameters extracted from, e.g., Kepler, radial
velocity, and GAIA data. More recently an increasing number of exoplanet
atmospheres have been observed in detail from space and the ground. Generally,
however, these atmospheric studies have focused on individual planets, with the
exception of a couple of works which have detected the presence of water vapor
and clouds in populations of gaseous planets via transmission spectroscopy.
Here, using a suite of retrieval tools, we analyse spectroscopic and
photometric data of 25 hot Jupiters, obtained with the Hubble and Spitzer Space
Telescopes via the eclipse technique. By applying the tools uniformly across
the entire set of 25 planets, we extract robust trends in the thermal structure
and chemical properties of hot Jupiters not obtained in past studies. With the
recent launch of JWST and the upcoming missions Twinkle, and Ariel, population
based studies of exoplanet atmospheres, such as the one presented here, will be
a key approach to understanding planet characteristics, formation, and
evolution in our galaxy. | astro-ph_EP |
Debris Disks: Seeing Dust, Thinking of Planetesimals and Planets: Debris disks are optically thin, almost gas-free dusty disks observed around
a significant fraction of main-sequence stars older than about 10 Myr. Since
the circumstellar dust is short-lived, the very existence of these disks is
considered as evidence that dust-producing planetesimals are still present in
mature systems, in which planets have formed - or failed to form - a long time
ago. It is inferred that these planetesimals orbit their host stars at asteroid
to Kuiper-belt distances and continually supply fresh dust through mutual
collisions. This review outlines observational techniques and results on debris
disks, summarizes their essential physics and theoretical models, and then
places them into the general context of planetary systems, uncovering
interrelations between the disks, dust parent bodies, and planets. It is shown
that debris disks can serve as tracers of planetesimals and planets and shed
light on the planetesimal and planet formation processes that operated in these
systems in the past. | astro-ph_EP |
Tidal Evolution of the Evection Resonance/Quasi-Resonance and the
Angular Momentum of the Earth-Moon System: Forming the Moon by a high-angular momentum impact may explain the Earth-Moon
isotopic similarities, however, the post-impact angular momentum needs to be
reduced by a factor of 2 or more to the current value (1 L_EM) after the Moon
forms. Capture into the evection resonance, occurring when the lunar perigee
precession period equals one year, could remove the angular momentum excess.
However the appropriate angular momentum removal appears sensitive to the tidal
model and chosen tidal parameters. In this work, we use a constant-time delay
tidal model to explore the Moon's orbital evolution through evection. We find
that exit from formal evection occurs early and that subsequently, the Moon
enters a quasi-resonance regime, in which evection still regulates the lunar
eccentricity even though the resonance angle is no longer librating. Although
not in resonance proper, during quasi-resonance angular momentum is
continuously removed from the Earth-Moon system and transferred to Earth's
heliocentric orbit. The final angular momentum, set by the timing of
quasi-resonance escape, is a function of the ratio of tidal strength in the
Moon and Earth and the absolute rate of tidal dissipation in the Earth. We
consider a physically-motivated model for tidal dissipation in the Earth as the
mantle cools from a molten to a partially molten state. We find that as the
mantle solidifies, increased terrestrial dissipation drives the Moon out of
quasi-resonance. For post-impact systems that contain >2 L_EM, final angular
momentum values after quasi-resonance escape remain significantly higher than
the current Earth-Moon value. | astro-ph_EP |
UV Surface Environment of Earth-like Planets Orbiting FGKM Stars Through
Geological Evolution: The UV environment of a host star affects the photochemistry in the
atmosphere, and ultimately the surface UV environment for terrestrial planets
and therefore the conditions for the origin and evolution of life. We model the
surface UV radiation environment for Earth-sized planets orbiting FGKM stars at
the 1AU equivalent distance for Earth through its geological evolution. We
explore four different types of atmospheres corresponding to an early Earth
atmosphere at 3.9 Gyr ago and three atmospheres covering the rise of oxygen to
present day levels at 2.0 Gyr ago, 0.8 Gyr ago and modern Earth (Following
Kaltenegger et al. 2007). In addition to calculating the UV flux on the surface
of the planet, we model the biologically effective irradiance, using DNA damage
as a proxy for biological damage. We find that a pre-biotic Earth (3.9 Gyr ago)
orbiting an F0V star receives 6 times the biologically effective radiation as
around the early Sun and 3520 times the modern Earth-Sun levels. A pre-biotic
Earth orbiting GJ 581 (M3.5V) receives 300 times less biologically effective
radiation, about 2 times modern Earth-Sun levels. The UV fluxes calculated here
provide a grid of model UV environments during the evolution of an Earth-like
planet orbiting a range of stars. These models can be used as inputs into
photo-biological experiments and for pre-biotic chemistry and early life
evolution experiments. | astro-ph_EP |
Kelvin-Helmholtz Instabilities in Multi-Sized Dust Layers: We examine the effect of the dust size distribution on Kelvin-Helmholtz
instabilities in the protoplanetary disk with dust sedimentation. With newly
taking into account the dust size distribution, the growth rate of the
Kelvin-Helmholtz instability is calculated using the linear stability analysis
with the dust density distribution consistent with sedimentation. Dust
abundance required for gravitational instabilities before the Kelvin-Helmholtz
instability is derived from the linear stability analysis, and it is found that
the required dust abundance significantly coincides with that estimated from
the Richardson number. It is also found that when the dust size distribution is
taken into account, the critical Richardson number for the onset of the
Kelvin-Helmholtz instability tends to increase with dust abundance. This result
is different from that in the case without the dust size distribution. | astro-ph_EP |
Orbital Evolution of Moons in Weakly Accreting Circumplanetary Disks: We investigate the formation of hot and massive circumplanetary disks (CPDs)
and the orbital evolution of satellites formed in these disks. Because of the
comparatively small size-scale of the sub-disk, quick magnetic diffusion
prevents the magnetorotational instability (MRI) from being well-developed at
ionization levels that would allow MRI in the parent protoplanetary disk. In
the absence of significant angular momentum transport, continuous mass supply
from the parental protoplanetary disk leads to the formation of a massive CPD.
We have developed an evolutionary model for this scenario and have estimated
the orbital evolution of satellites within the disk. We find, in a certain
temperature range, that inward migration of a satellite can be stopped by a
change in the structure due to the opacity transitions. Moreover, by capturing
second and third migrating satellites in mean motion resonances, a compact
system in Laplace resonance can be formed in our disk models. | astro-ph_EP |
Eccentric Companions to Kepler-448b and Kepler-693b: Clues to the
Formation of Warm Jupiters: I report the discovery of non-transiting close companions to two transiting
warm Jupiters (WJs), Kepler-448/KOI-12b (orbital period
$P=17.9\,\mathrm{days}$, radius $R_{\rm p}=1.23^{+0.06}_{-0.05}\,R_{\rm Jup}$)
and Kepler-693/KOI-824b ($P=15.4\,\mathrm{days}$, $R_{\rm
p}=0.91\pm0.05\,R_{\rm Jup}$), via dynamical modeling of their transit timing
and duration variations (TTVs and TDVs). The companions have masses of
$22^{+7}_{-5}\,M_{\rm Jup}$ (Kepler-448c) and $150^{+60}_{-40}\,M_{\rm Jup}$
(Kepler-693c), and both are on eccentric orbits ($e=0.65^{+0.13}_{-0.09}$ for
Kepler-448c and $e=0.47^{+0.11}_{-0.06}$ for Kepler-693c) with periastron
distances of $1.5\,\mathrm{au}$. Moderate eccentricities are detected for the
inner orbits as well ($e=0.34^{+0.08}_{-0.07}$ for Kepler-448b and
$e=0.2^{+0.2}_{-0.1}$ for Kepler-693b). In the Kepler-693 system, a large
mutual inclination between the inner and outer orbits
($53^{+7}_{-9}\,\mathrm{deg}$ or $134^{+11}_{-10}\,\mathrm{deg}$) is also
revealed by the TDVs. This is likely to induce a secular oscillation of the
inner WJ's eccentricity that brings its periastron close enough to the host
star for tidal star-planet interactions to be significant. In the Kepler-448
system, the mutual inclination is weakly constrained and such an eccentricity
oscillation is possible for a fraction of the solutions. Thus these WJs may be
undergoing tidal migration to become hot Jupiters (HJs), although the migration
via this process from beyond the snow line is disfavored by the close-in and
massive nature of the companions. This may indicate that WJs can be formed in
situ and could even evolve into HJs via high-eccentricity migration inside the
snow line. | astro-ph_EP |
Orbit classification in the planar circular Pluto-Charon system: We numerically investigate the orbital dynamics of a spacecraft, or a comet,
or an asteroid in the Pluto-Charon system in a scattering region around Charon
using the planar circular restricted three-body problem. The test particle can
move in bounded orbits around Charon or escape through the necks around the
Lagrangian points $L_1$ and $L_2$ or even collide with the surface of Charon.
We explore four of the five possible Hill's regions configurations depending on
the value of the Jacobi constant which is of course related with the total
orbital energy. We conduct a thorough numerical analysis on the phase space
mixing by classifying initial conditions of orbits and distinguishing between
three types of motion: (i) bounded, (ii) escaping and (iii) collisional. In
particular, we locate the different basins and we relate them with the
corresponding spatial distributions of the escape and collision times. Our
results reveal the high complexity of this planetary system. Furthermore, the
numerical analysis shows a strong dependence of the properties of the
considered basins with the total orbital energy, with a remarkable presence of
fractal basin boundaries along all the regimes. Our results are compared with
earlier ones regarding the Saturn-Titan planetary system. | astro-ph_EP |
Planet populations inferred from debris discs: insights from 178 debris
systems in the ISPY, LEECH and LIStEN planet-hunting surveys: We know little about the outermost exoplanets in planetary systems, because
our detection methods are insensitive to moderate-mass planets on wide orbits.
However, debris discs can probe the outer-planet population, because dynamical
modelling of observed discs can reveal properties of perturbing planets. We use
four sculpting and stirring arguments to infer planet properties in 178
debris-disc systems from the ISPY, LEECH and LIStEN planet-hunting surveys.
Similar analyses are often conducted for individual discs, but we consider a
large sample in a consistent manner. We aim to predict the population of
wide-separation planets, gain insight into the formation and evolution
histories of planetary systems, and determine the feasibility of detecting
these planets in the near future. We show that a `typical' cold debris disc
likely requires a Neptune- to Saturn-mass planet at 10-100 au, with some
needing Jupiter-mass perturbers. Our predicted planets are currently
undetectable, but modest detection-limit improvements (e.g. from JWST) should
reveal many such perturbers. We find that planets thought to be perturbing
debris discs at late times are similar to those inferred to be forming in
protoplanetary discs, so these could be the same population if newly formed
planets do not migrate as far as currently thought. Alternatively, young
planets could rapidly sculpt debris before migrating inwards, meaning that the
responsible planets are more massive (and located further inwards) than
debris-disc studies assume. We combine self-stirring and size-distribution
modelling to show that many debris discs cannot be self-stirred without having
unreasonably high masses; planet- or companion-stirring may therefore be the
dominant mechanism in many (perhaps all) debris discs. Finally, we provide
catalogues of planet predictions, and identify promising targets for future
planet searches. | astro-ph_EP |
Dust Growth and Magnetic Fields: from Cores to Disks (even down to
Planets): The recent rapid progress in observations of circumstellar disks and
extrasolar planets has reinforced the importance of understanding an intimate
coupling between star and planet formation. Under such a circumstance, it may
be invaluable to attempt to specify when and how planet formation begins in
star-forming regions and to identify what physical processes/quantities are the
most significant to make a link between star and planet formation. To this end,
we have recently developed a couple of projects. These include an observational
project about dust growth in Class 0 YSOs and a theoretical modeling project of
the HL Tauri disk. For the first project, we utilize the archive data of radio
interferometric observations, and examine whether dust growth, a first step of
planet formation, occurs in Class 0 YSOs. We find that while our observational
results can be reproduced by the presence of large ($\sim$ mm) dust grains for
some of YSOs under the single-component modified blackbody formalism, an
interpretation of no dust growth would be possible when a more detailed model
is used. For the second project, we consider an origin of the disk
configuration around HL Tauri, focusing on magnetic fields. We find that
magnetically induced disk winds may play an important role in the HL Tauri
disk. The combination of these attempts may enable us to move towards a
comprehensive understanding of how star and planet formation are intimately
coupled with each other. | astro-ph_EP |
SWEET-Cat updated. New homogenous spectroscopic parameters: Context: Exoplanets have now been proven to be very common. The number of its
detections continues to grow following the development of better instruments
and missions. One key step for the understanding of these worlds is their
characterization, which mostly depend on their host stars. Aims:We perform a
significant update of the Stars With ExoplanETs CATalog (SWEET-Cat), a unique
compilation of precise stellar parameters for planet-host stars provided for
the exoplanet community. Methods: We made use of high-resolution spectra for
planet-host stars, either observed by our team or found in several public
archives. The new spectroscopic parameters were derived for the spectra
following the same homogeneous process (ARES+MOOG). The host star parameters
were then merged together with the planet properties listed in exoplanet.eu to
perform simple data analysis. Results: We present new spectroscopic homogeneous
parameters for 106 planet-host stars. Sixty-three planet hosts are also
reviewed with new parameters. We also show that there is a good agreement
between stellar parameters derived for the same star but using spectra obtained
from different spectrographs. The planet-metallicity correlation is reviewed
showing that the metallicity distribution of stars hosting low-mass planets
(below 30 M$_{\oplus}$) is indistinguishable from that from the solar
neighborhood sample in terms of metallicity distribution. | astro-ph_EP |
"TNOs are Cool": A survey of the trans-Neptunian region XIV. Size/albedo
characterization of the Haumea family observed with Herschel and Spitzer: A group of trans-Neptunian objects (TNO) are dynamically related to the dwarf
planet 136108 Haumea. Ten of them show strong indications of water ice on their
surfaces, are assumed to have resulted from a collision, and are accepted as
the only known TNO collisional family. Nineteen other dynamically similar
objects lack water ice absorptions and are hypothesized to be dynamical
interlopers. We have made observations to determine sizes and geometric albedos
of six of the accepted Haumea family members and one dynamical interloper. Ten
other dynamical interlopers have been measured by previous works. We compare
the individual and statistical properties of the family members and
interlopers, examining the size and albedo distributions of both groups. We
also examine implications for the total mass of the family and their ejection
velocities. We use far-infrared space-based telescopes to observe the target
TNOs near their thermal peak and combine these data with optical magnitudes to
derive sizes and albedos using radiometric techniques. We determine the
power-law slope of ejection velocity as a function of effective diameter. The
detected Haumea family members have a diversity of geometric albedos $\sim$
0.3-0.8, which are higher than geometric albedos of dynamically similar objects
without water ice. The median geometric albedo for accepted family members is
$p_V=0.48_{-0.18}^{+0.28}$, compared to 0.08$_{-0.05}^{+0.07}$ for the
dynamical interlopers. In the size range $D=175-300$ km, the slope of the
cumulative size distribution is $q$=3.2$_{-0.4}^{+0.7}$ for accepted family
members, steeper than the $q$=2.0$\pm$0.6 slope for the dynamical interlopers
with D$< $500 km. The total mass of Haumea's moons and family members is 2.4%
of Haumea's mass. The ejection velocities required to emplace them on their
current orbits show a dependence on diameter, with a power-law slope of
0.21-0.50. | astro-ph_EP |
Thermophysical Characteristics of OSIRIS-REx Target Asteroid (101955)
Bennu: In this work, we investigate the thermophysical properties, including thermal
inertia, roughness fraction and surface grain size of OSIRIS-REx target
asteroid (101955) Bennu by using a thermophysical model with the recently
updated 3D radar-derived shape model (\cite[Nolan et al., 2013]{Nolan2013}) and
mid-infrared observations (\cite[M$\ddot{u}$ller et al, 2012]{Muller2012},
\cite[Emery et al., 2014]{Emery2014}). We find that the asteroid bears an
effective diameter of $510^{+6}_{-40}$ m, a geometric albedo of
$0.047^{+0.0083}_{-0.0011}$, a roughness fraction of $0.04^{+0.26}_{-0.04}$,
and thermal inertia of $240^{+440}_{-60}\rm~Jm^{-2}s^{-0.5}K^{-1}$ for our
best-fit solution. The best-estimate thermal inertia suggests that fine-grained
regolith may cover a large portion of Bennu's surface, where a grain size may
vary from $1.3$ to $31$~mm. Our outcome suggests that Bennu is suitable for the
OSIRIS-REx mission to return samples to Earth. | astro-ph_EP |
A Distinct Population of Small Planets: Sub-Earths: The sizes of small planets have been known to be bi-modal, with a gap
separating planets that have lost their primordial atmospheres (super-Earths),
and the ones that retain them (mini-Neptunes). Here, we report evidences for
another distinct population at smaller sizes. By focussing on planets orbiting
around GK-dwarfs inward of 16 days, and correcting for observational
completeness, we find that the number of super-Earths peak around 1.4 Earth
radii and disappear shortly below this size. Instead, a new population of
planets (sub-Earths) appear to dominate at sizes below ~ 1 Earth radius, with
an occurrence that rises with decreasing size. This pattern is also observed in
ultra-short-period planets.
The end of super-Earths supports earlier claims that super-Earths and
mini-Neptunes, planets that likely form in gaseous proto-planetary disks, have
a narrow mass distribution. The sub-Earths, in contrast, can be described by a
power-law mass distribution and may be explained by the theory of terrestrial
planet formation. We therefore speculate that they are formed well after the
gaseous disks have dissipated. The extension of these sub-Earths towards longer
orbital periods, currently invisible, may be the true terrestrial analogues.
This strongly motivates new searches. | astro-ph_EP |
Topographic Constraints on the Origin of the Equatorial Ridge on Iapetus: Saturn's moon Iapetus has an equatorial ridge system, which may be as high as
20 km, that may have formed by endogenic forces, such as tectonic and
convective forces, or exogenic processes such as debris infall. We use
high-resolution topographic data to conduct a topographic analysis of the
ridge, which suggests a predominantly triangular morphology, with some ridge
face slopes reaching 40 degrees, allowing for an exogenic formation mechanism. | astro-ph_EP |
Ring Formation in Protoplanetary Disks Driven by an Eccentric
Instability: We find that, under certain conditions, protoplanetary disks may
spontaneously generate multiple, concentric gas rings without an embedded
planet through an eccentric cooling instability. Using both linear theory and
non-linear hydrodynamics simulations, we show that a variety of background
states may trap a slowly precessing, one-armed spiral mode that becomes
unstable when a gravitationally-stable disk rapidly cools. The angular momentum
required to excite this spiral comes at the expense of non-uniform mass
transport that generically results in multiple rings. For example, one
long-term hydrodynamics simulation exhibits four long-lived, axisymmetric gas
rings. We verify the instability evolution and ring formation mechanism from
first principles with our linear theory, which shows remarkable agreement with
the simulation results. Dust trapped in these rings may produce observable
features consistent with observed disks. Additionally, direct detection of the
eccentric gas motions may be possible when the instability saturates, and any
residual eccentricity leftover in the rings at later times may also provide
direct observational evidence of this mechanism. | astro-ph_EP |
A Simple Phenomenological Model for Grain Clustering in Turbulence: We propose a simple model for density fluctuations of aerodynamic grains,
embedded in a turbulent, gravitating gas disk. The model combines a calculation
for the behavior of a group of grains encountering a single turbulent eddy,
with a hierarchical approximation of the eddy statistics. This makes analytic
predictions for a range of quantities including: distributions of grain
densities, power spectra and correlation functions of fluctuations, and maximum
grain densities reached. We predict how these scale as a function of grain drag
time t_stop, spatial scale, grain-to-gas mass ratio, strength of turbulence
(alpha), and detailed disk properties. We test these against numerical
simulations with various turbulence-driving mechanisms. The simulations agree
well with the predictions, spanning t_stop*Omega ~ 1e-4 - 10, alpha ~ 1e-10 -
1e-2, and grain-to-gas mass ratio ~0-3. Results from 'turbulent concentration'
simulations and laboratory experiments are also predicted as a special case.
Vortices on a wide range of scales disperse and concentrate grains
hierarchically. For small grains this is most efficient in eddies with turnover
time comparable to the stopping time, but fluctuations are also damped by local
gas-grain drift. For large grains, shear and gravity lead to a much broader
range of eddy scales driving fluctuations, with most power on the largest
scales. The grain density distribution has a log-Poisson shape, with
fluctuations for large grains up to factors >1000. We provide simple analytic
expressions for the predictions, and discuss implications for planetesimal
formation, grain growth, and the structure of turbulence. | astro-ph_EP |
Climate Modeling of a Potential ExoVenus: The planetary mass and radius sensitivity of exoplanet discovery capabilities
has reached into the terrestrial regime. The focus of such investigations is to
search within the Habitable Zone where a modern Earth-like atmosphere may be a
viable comparison. However, the detection bias of the transit and radial
velocity methods lies close to the host star where the received flux at the
planet may push the atmosphere into a runaway greenhouse state. One such
exoplanet discovery, Kepler-1649b, receives a similar flux from its star as
modern Venus does from the Sun, and so was categorized as a possible exoVenus.
Here we discuss the planetary parameters of Kepler-1649b with relation to Venus
to establish its potential as a Venus analog. We utilize the general
circulation model ROCKE-3D to simulate the evolution of the surface temperature
of Kepler-1649b under various assumptions, including relative atmospheric
abundances. We show that in all our simulations the atmospheric model rapidly
diverges from temperate surface conditions towards a runaway greenhouse with
rapidly escalating surface temperatures. We calculate transmission spectra for
the evolved atmosphere and discuss these spectra within the context of the
James Webb Space Telescope (JWST) Near-Infrared Spectrograph (NIRSpec)
capabilities. We thus demonstrate the detectability of the key atmospheric
signatures of possible runaway greenhouse transition states and outline the
future prospects of characterizing potential Venus analogs. | astro-ph_EP |
Physical properties, transmission and emission spectra of the WASP-19
planetary system from multi-colour photometry: We present new ground-based, multi-colour, broad-band photometric
measurements of the physical parameters, transmission and emission spectra of
the transiting extrasolar planet WASP-19b. The measurements are based on
observations of 8 transits and four occultations using the 1.5m Danish
Telescope, 14 transits at the PEST observatory, and 1 transit observed
simultaneously through four optical and three near-infrared filters, using the
GROND instrument on the ESO 2.2m telescope. We use these new data to measure
refined physical parameters for the system. We find the planet to be more
bloated and the system to be twice as old as initially thought. We also used
published and archived datasets to study the transit timings, which do not
depart from a linear ephemeris. We detected an anomaly in the GROND transit
light curve which is compatible with a spot on the photosphere of the parent
star. The starspot position, size, spot contrast and temperature were
established. Using our new and published measurements, we assembled the
planet's transmission spectrum over the 370-2350 nm wavelength range and its
emission spectrum over the 750-8000 nm range. By comparing these data to
theoretical models we investigated the theoretically-predicted variation of the
apparent radius of WASP-19b as a function of wavelength and studied the
composition and thermal structure of its atmosphere. We conclude that: there is
no evidence for strong optical absorbers at low pressure, supporting the common
idea that the planet's atmosphere lacks a dayside inversion; the temperature of
the planet is not homogenized, because the high warming of its dayside causes
the planet to be more efficient in re-radiating than redistributing energy to
the night side; the planet seems to be outside of any current classification
scheme. | astro-ph_EP |
Outbursting Comet P/2010 V1 (Ikeya-Murakami): A Miniature Comet Holmes: Short-period comet P/2010 V1 (Ikeya-Murakami, hereafter V1) was discovered
visually by two amateur astronomers. The appearance of the comet was peculiar,
consisting of an envelope, a spherical coma near the nucleus and a tail
extending in the anti-solar direction. We investigated the brightness and the
morphological development of the comet by taking optical images with
ground-based telescopes. Our observations show that V1 experienced a
large-scale explosion between UT 2010 October 31 and November 3. The color of
the comet was consistent with the Sun (g'-RC=0.61+-0.20, RC-IC=0.20+-0.20, and
B-RC=0.93+-0.25), suggesting that dust particles were responsible for the
brightening. We used a dynamical model to understand the peculiar morphology,
and found that the envelope consisted of small grains (0.3-1 micron) expanding
at a maximum speed of 500+-40 m/s, while the tail and coma were composed of a
wider range of dust particle sizes (0.4-570 micron) and expansion speeds 7-390
m/s. The total mass of ejecta is ~5x10^8 kg and kinetic energy ~5x10^12 J.
These values are much smaller than in the historic outburst of 17P/Holmes in
2007, but the energy per unit mass (1x10^4 J/kg) is comparable. The energy per
unit mass is about 10% of the energy released during the crystallization of
amorphous water ice suggesting that crystallization of buried amorphous ice can
supply the mass and energy of the outburst ejecta. | astro-ph_EP |
Results from a set of three-dimensional numerical experiments of a hot
Jupiter atmosphere: We present highlights from a large set of simulations of a hot Jupiter
atmosphere, nominally based on HD 209458b, aimed at exploring both the
evolution of the deep atmosphere, and the acceleration of the zonal flow or
jet. We find the occurrence of a super-rotating equatorial jet is robust to
changes in various parameters, and over long timescales, even in the absence of
strong inner or bottom boundary drag. This jet is diminished in one simulation
only, where we strongly force the deep atmosphere equator-to-pole temperature
gradient over long timescales. Finally, although the eddy momentum fluxes in
our atmosphere show similarities with the proposed mechanism for accelerating
jets on tidally-locked planets, the picture appears more complex. We present
tentative evidence for a jet driven by a combination of eddy momentum transport
and mean flow. | astro-ph_EP |
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