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On the Need for a Classification System for Consistent Characterization
of the Composition of Planetary Bodies: A classification system is presented for characterizing the composition of
planetary bodies. From mass-radius and mass-density relationships, planets may
be broadly grouped into five composition classes identified as: Gas Giant,
Rock-Ice Giant, gas-rich Terrestrial, Rock Terrestrial, and Rock-Ice
Terrestrial based upon the mass fractions of H-He gas, rock, and ice. For each
of these broad composition classes, specific bulk composition classes are
defined and characterized with Solar System analog names. The classification
system allows for both general and detailed characterization of exoplanets
based upon planetary mass-radius-composition models and provides rationale for
distinguishing gas-rich super-Earths from mini-Neptunes. | astro-ph_EP |
A shortcut to calculate SPAM limb-darkening coefficients: We release a new grid of stellar limb-darkening coefficients (LDCs, using the
quadratic, power-2 and claret-4 laws) and intensity profiles for the Kepler, U,
B, V and R passbands, based on STAGGER model atmospheres. The data can be
downloaded from Zenodo (doi:10.5281/zenodo.5593162). We compare the
newly-released LDCs, computed by ExoTETHyS, with previously published values,
based on the same atmospheric models using a so-called "SPAM" procedure. The
SPAM method relies on synthetic light curves in order to compute the LDCs that
best represent the photometry of exoplanetary transits. We confirm that
ExoTETHyS achieves the same objective with a much simpler algorithm. | astro-ph_EP |
The Oblique Orbit of WASP-107b from K2 Photometry: Observations of nine transits of WASP-107 during the {\it K2} mission reveal
three separate occasions when the planet crossed in front of a starspot. The
data confirm the stellar rotation period to be 17 days --- approximately three
times the planet's orbital period --- and suggest that large spots persist for
at least one full rotation. If the star had a low obliquity, at least two
additional spot crossings should have been observed. They were not observed,
giving evidence for a high obliquity. We use a simple geometric model to show
that the obliquity is likely in the range 40-140$^\circ$, i.e., both spin-orbit
alignment and anti-alignment can be ruled out. WASP-107 thereby joins the small
collection of relatively low-mass stars hosting a giant planet with a high
obliquity. Most such stars have been observed to have low obliquities; all the
exceptions, including WASP-107, involve planets with relatively wide orbits
("warm Jupiters", with $a_{\rm min}/R_\star \gtrsim 8$). This demonstrates a
connection between stellar obliquity and planet properties, in contradiction to
some theories for obliquity excitation. | astro-ph_EP |
Shape models and spin states of Jupiter Trojans: Testing the streaming
instability formation scenario: The leading theory for the origin of Jupiter Trojans (JTs) assumes that JTs
were captured to their orbits near the Lagrangian points of Jupiter during the
early reconfiguration of the giant planets. The natural source region for the
majority of JTs would then be the population of planetesimals born in a massive
trans-Neptunian disk. If true, JTs represent the most accessible stable
population of small Solar System bodies that formed in the outer regions of the
Solar System. For this work, we compiled photometric datasets for about 1000
JTs and applied the convex inversion technique in order to assess their shapes
and spin states. We obtained full solutions for $79$ JTs, and partial solutions
for an additional $31$ JTs. We found that the observed distribution of the pole
obliquities of JTs is broadly consistent with expectations from the streaming
instability, which is the leading mechanism for the formation of planetesimals
in the trans-Neptunian disk. The observed JTs' pole distribution has a slightly
smaller prograde vs. retrograde asymmetry (excess of obliquities $>130^\circ$)
than what is expected from the existing streaming instability simulations.
However, this discrepancy can be plausibly reconciled by the effects of the
post-formation collisional activity. Our numerical simulations of the
post-capture spin evolution indicate that the JTs' pole distribution is not
significantly affected by dynamical processes such as the eccentricity
excitation in resonances, close encounters with planets, or the effects of
nongravitational forces. However, a few JTs exhibit large latitude variations
of the rotation pole and may even temporarily transition between prograde- and
retrograde-rotating categories. | astro-ph_EP |
Debris disks as signposts of terrestrial planet formation: Circumstantial evidence suggests that most known extra-solar planetary
systems are survivors of violent dynamical instabilities. Here we explore how
giant planet instabilities affect the formation and survival of terrestrial
planets. We simulate planetary system evolution around Sun-like stars from
initial conditions that comprise: an inner disk of planetesimals and planetary
embryos, three giant planets at Jupiter-Saturn distances, and a massive outer
planetesimal disk. We then calculate dust production rates and debris disk SEDs
assuming that each planetesimal particle represents an ensemble of smaller
bodies in collisional equilibrium. We predict a strong correlation between the
presence of terrestrial planets and debris disks, mediated by the giant
planets. Strong giant planet instabilities destroy all rocky material -
including fully-formed terrestrial planets if the instabilities occur late -
along with the icy planetesimals. Stable or weakly unstable systems allow
terrestrial planets to accrete and significant dust to be produced in their
outer regions. Stars older than ~100 Myr with bright cold dust emission (at ~70
microns) signpost the dynamically calm environments conducive to efficient
terrestrial accretion. We predict that while the typical eccentricities of
terrestrial planets are small, there should exist a novel class of terrestrial
planet system whose single planet undergoes large amplitude oscillations in
eccentricity and inclination. By scaling to the observed semimajor axis
distribution of giant exoplanets, we estimate that terrestrial exoplanets in
the same systems should be a few times more abundant at 0.5 AU than giant or
terrestrial exoplanets at 1 AU. Finally, we discuss the Solar System, which
appears to be unusual in combining a rich terrestrial planet system with a low
dust content. | astro-ph_EP |
Radial Transport of Large-Scale Magnetic Fields in Accretion Disks. I.
Steady Solutions and an Upper Limit on the Vertical Field Strength: Large-scale magnetic fields are key ingredients of magnetically driven disk
accretion. We study how large-scale poloidal fields evolve in accretion disks,
with the primary aim of quantifying the viability of magnetic accretion
mechanisms in protoplanetary disks. We employ a kinematic mean-field model for
poloidal field transport and focus on steady states where inward advection of a
field balances with outward diffusion due to effective resistivities. We
analytically derive the steady-state radial distribution of poloidal fields in
highly conducting accretion disks. The analytic solution reveals an upper limit
on the strength of large-scale vertical fields attainable in steady states. Any
excess poloidal field will be diffused away within a finite time, and we
demonstrate this with time-dependent numerical calculations of the mean-field
equations. We apply this upper limit to large-scale vertical fields threading
protoplanetary disks. We find that the maximum attainable strength is about 0.1
G at 1 AU, and about 1 mG at 10 AU from the central star. When combined with
recent magnetic accretion models, the maximum field strength translates into
the maximum steady-state accretion rate of $\sim 10^{-7} M_\odot {\rm
yr}^{-1}$, in agreement with observations. We also find that the maximum field
strength is ~ 1 kG at the surface of the central star provided that the disk
extends down to the stellar surface. This implies that any excess stellar
poloidal field of strength >~ kG can be transported to the surrounding disk.
This might in part resolve the magnetic flux problem in star formation. | astro-ph_EP |
Composition of Massive Giant Planets: The two current models for giant planet formation are core accretion and disk
instability. We discuss the core masses and overall planetary enrichment in
heavy elements predicted by the two formation models, and show that both models
could lead to a large range of final compositions. For example, both can form
giant planets with nearly stellar compositions. However, low-mass giant
planets, enriched in heavy elements compared to their host stars, are more
easily explained by the core accretion model. The final structure of the
planets, i.e., the distribution of heavy elements, is not firmly constrained in
either formation model. | astro-ph_EP |
3D shape of asteroid (6)~Hebe from VLT/SPHERE imaging: Implications for
the origin of ordinary H chondrites: Context. The high-angular-resolution capability of the new-generation
ground-based adaptive-optics camera SPHERE at ESO VLT allows us to assess, for
the very first time, the cratering record of medium-sized (D~100-200 km)
asteroids from the ground, opening the prospect of a new era of investigation
of the asteroid belt's collisional history. Aims. We investigate here the
collisional history of asteroid (6) Hebe and challenge the idea that Hebe may
be the parent body of ordinary H chondrites, the most common type of meteorites
found on Earth (~34% of the falls). Methods. We observed Hebe with SPHERE as
part of the science verification of the instrument. Combined with earlier
adaptive-optics images and optical light curves, we model the spin and
three-dimensional (3D) shape of Hebe and check the consistency of the derived
model against available stellar occultations and thermal measurements. Results.
Our 3D shape model fits the images with sub-pixel residuals and the light
curves to 0.02 mag. The rotation period (7.274 47 h), spin (343 deg,+47 deg),
and volume-equivalent diameter (193 +/- 6km) are consistent with previous
determinations and thermophysical modeling. Hebe's inferred density is 3.48 +/-
0.64 g.cm-3 , in agreement with an intact interior based on its H-chondrite
composition. Using the 3D shape model to derive the volume of the largest
depression (likely impact crater), it appears that the latter is significantly
smaller than the total volume of close-by S-type H-chondrite-like asteroid
families. Conclusions. Our results imply that (6) Hebe is not the most likely
source of H chondrites. Over the coming years, our team will collect similar
high-precision shape measurements with VLT/SPHERE for ~40 asteroids covering
the main compositional classes, thus providing an unprecedented dataset to
investigate the origin and collisional evolution of the asteroid belt. | astro-ph_EP |
Binary planetesimals and their role in planet formation: One of the main evolutionary stages of planet formation is the dynamical
evolution of planetesimal disks. These disks are thought to evolve through
gravitational encounters and physical collisions between single planetesimals.
In recent years, many binary planetesimals have been observed in the Solar
system, indicating that the binarity of planetesimals is high. However, current
studies of planetesimal disks formation and evolution do not account for the
role of binaries. Here we point out that gravitational encounters of binary
planetesimals can have an important role in the evolution of planetesimal
disks. Binary planetesimals catalyze close encounters between planetesimals,
and can strongly enhance their collision rate. Binaries may also serve as
additional heating source of the planetesimal disk, through the exchange of the
binaries gravitational potential energy into the kinetic energy of
planetesimals in the disk. | astro-ph_EP |
Growth and Evolution of Secondary Volcanic Atmospheres: II. The
Importance of Kinetics: Volcanism is a major and long-term source of volatile elements such as C and
H to Earth's atmosphere, likely has been to Venus's atmosphere, and may be for
exoplanets. Models simulating volcanic growth of atmospheres often make one of
two assumptions: either that atmospheric speciation is set by the
high-temperature equilibrium of volcanism; or, that volcanic gases
thermochemically re-equilibrate to the new, lower, temperature of the surface
environment. In the latter case it has been suggested that volcanic atmospheres
may create biosignature false positives. Here, we test the assumptions
underlying such inferences by performing chemical kinetic calculations to
estimate the relaxation timescale of volcanically-derived atmospheres to
thermochemical equilibrium, in a simple 0D atmosphere neglecting photochemistry
and reaction catalysis. We demonstrate that for planets with volcanic
atmospheres, thermochemical equilibrium over geological timescales can only be
assumed if the atmospheric temperature is above ~700K. Slow chemical kinetics
at lower temperatures inhibit the relaxation of redox-sensitive species to
low-temperature thermochemical equilibrium, precluding the production of two
independent biosignatures through thermochemistry alone: 1. ammonia, and 2. the
co-occurrence of CO$_2$ and CH$_4$ in an atmosphere in the absence of CO. This
supports the use of both biosignatures for detecting life. Quenched at the high
temperature of their degassing, volcanic gases also have speciations
characteristic of those produced from a more oxidized mantle, if interpreted as
being at thermochemical equilibrium. This therefore complicates linking
atmospheres to the interiors of rocky exoplanets, even when their atmospheres
are purely volcanic in origin. | astro-ph_EP |
The DEdicated MONitor of EXotransits (DEMONEX): Seven Transits of XO-4b: The DEdicated MONitor of EXotransits (DEMONEX) was a 20 inch robotic and
automated telescope to monitor bright stars hosting transiting exoplanets to
discover new planets and improve constraints on the properties of known
transiting planetary systems. We present results for the misaligned hot Jupiter
XO-4b containing 7 new transits from the DEMONEX telescope, including 3 full
and 4 partial transits. We combine these data with archival light curves and
archival radial velocity measurements to derive the host star mass
$M_{*}=1.293_{-0.029}^{+0.030} M_\odot$ and radius
$R_{*}=1.554_{-0.030}^{+0.042} R_\odot$ as well as the planet mass
$M_{P}=1.615_{-0.099}^{+0.10} M_{\rm J}$ and radius
$R_{P}=1.317_{-0.029}^{+0.040} R_{\rm J}$ and a refined ephemeris of
$P=4.1250687\pm0.0000024$ days and $T_{0}=2454758.18978\pm0.00024 \rm
{BJD_{TDB}}$. We include archival Rossiter-McLaughlin measurements of XO-4 to
infer the stellar spin-planetary orbit alignment $\lambda=-40.0_{-7.5}^{+8.8}$
degrees.
We test the effects of including various detrend parameters, theoretical and
empirical mass-radius relations, and Rossiter-McLaughlin models. We infer that
detrending against CCD position and time or airmass can improve data quality,
but can have significant effects on the inferred values of many parameters ---
most significantly $R_{P}/R_{*}$ and the observed central transit times
$T_{C}$. In the case of $R_{P}/R_{*}$ we find that the systematic uncertainty
due to detrending can be three times that of the quoted statistical
uncertainties. The choice of mass-radius relation has little effect on our
inferred values of the system parameters. The choice of Rossiter-McLaughlin
models can have significant effects of the inferred values of $v\sin{I_{*}}$
and the stellar spin-planet orbit angle $\lambda$. | astro-ph_EP |
Collisional Charging in the Low Pressure Range of Protoplanetary Disks: In recent years, collisional charging has been proposed to promote the growth
of pebbles in early phases of planet formation. Ambient pressure in
protoplanetary disks spans a wide range from below $10^{-9}$ mbar up to way
beyond mbar. Yet, experiments on collisional charging of same material surfaces
have only been conducted under Earth atmospheric pressure, Martian pressure and
more generally down to $10^{-2}$ mbar thus far. This work presents first
pressure dependent charge measurements of same material collisions between
$10^{-8}$ and $10^3$ mbar. Strong charging occurs down to the lowest pressure.
In detail, our observations show a strong similarity to the pressure dependence
of the breakdown voltage between two electrodes and we suggest that breakdown
also determines the maximum charge on colliding grains in protoplanetary disks.
We conclude that collisional charging can occur in all parts of protoplanetary
disks relevant for planet formation. | astro-ph_EP |
Near-Ultraviolet Absorption Distribution of Primitive Asteroids from
Spectrophotometric Surveys: Aims: Our objectives were first to evaluate the possibility for using the NUV
absorption as diagnostics of hydrated minerals based on the recent datasets of
primitive asteroids and hydrated carbonaceous chondrites, and second to
investigate the reflectance spectrophotometry of the primitive asteroids in the
NUV as functions of heliocentric distance and size. Methods: The NUV and
visible reflectance spectrophotometry of more than 9,000 primitive asteroids
was investigated using two spectrophotometric surveys, the Eight Color Asteroid
Survey (ECAS) and the Sloan Digital Sky Survey (SDSS), which cover wavelengths
down to 0.32 um and 0.36 um, respectively. We classified asteroids from the
main asteroid belt, the Cybele and Hilda zones, and Jupiter Trojans based on
Tholen's taxonomy and described the statistical distribution of primitive
asteroid types. We also examined the relationship of the NUV, 0.7 um, and 2.7
um absorptions among primitive asteroids and hydrous carbonaceous chondrites CI
and CM. Results: We found strong correlations between the NUV and the OH-band
(2.7 um) absorptions for primitive asteroids and hydrated meteorites,
suggesting the NUV absorption can be indicative of hydrated silicates.
Moreover, there is a great difference in the NUV absorption between the large
asteroids (diameter d > 50 km) and small asteroids (d < 10 km) in the taxonomic
distribution. The taxonomic distribution of asteroids differs between the inner
main belt and middle-outer main belt. Notably, the C types are dominating large
members through the main belt and the F types are dominating small asteroids of
the inner main belt. The asteroids beyond the main belt consist mostly of P and
D types, although P types are common everywhere in the main belt. The peculiar
distribution of F types might indicate a different formation reservoir or
displacement process of F types in the early Solar System. | astro-ph_EP |
Coplanar circumbinary planets can be unstable to large tilt oscillations
in the presence of an inner polar planet: Mutually misaligned circumbinary planets may form in a warped or broken gas
disc or from later planet-planet interactions. With numerical simulations and
analytic estimates we explore the dynamics of two circumbinary planets with a
large mutual inclination. A coplanar inner planet causes prograde apsidal
precession of the binary and the stationary inclination for the outer planet is
higher for larger outer planet orbital radius. In this case a coplanar outer
planet always remains coplanar. On the other hand, a polar inner planet causes
retrograde apsidal precession of the binary orbit and the stationary
inclination is smaller for larger outer planet orbital radius. For a range of
outer planet semi-major axes, an initially coplanar orbit is librating meaning
that the outer planet undergoes large tilt oscillations. Circumbinary planets
that are highly inclined to the binary are difficult to detect -- it is
unlikely for a planet to have an inclination below the transit detection limit
in the presence of a polar inner planet. These results suggest that there could
be a population of circumbinary planets that are undergoing large tilt
oscillations. | astro-ph_EP |
Transmission spectroscopy of the inflated exoplanet WASP-52b, and
evidence for a bright region on the stellar surface: We have measured the transmission spectrum of the extremely inflated hot
Jupiter WASP-52b using simultaneous photometric observations in SDSS u', g' and
a filter centred on the sodium doublet (NaI) with the ULTRACAM instrument
mounted on the 4.2m William Herschel Telescope. We find that Rayleigh
scattering is not the dominant source of opacity within the planetary
atmosphere and find a transmission spectrum more consistent with
wavelength-independent opacity such as from clouds. We detect an in-transit
anomaly that we attribute to the presence of stellar activity and find that
this feature can be more simply modelled as a bright region on the stellar
surface akin to Solar faculae rather than spots. A spot model requires a
significantly larger planet/star radius ratio than that found in previous
studies. Our results highlight the precision that can be achieved by
ground-based photometry with errors in the scaled planetary radii of less than
one atmospheric scale height, comparable to HST observations. | astro-ph_EP |
HATS-4b: A Dense Hot-Jupiter Transiting a Super Metal-Rich G Star: We report the discovery by the HATSouth survey of HATS-4b, an extrasolar
planet transiting a V=13.46 mag G star. HATS-4b has a period of P = 2.5167 d,
mass of Mp = 1.32 Mj, radius of Rp = 1.02 Rj and density of rho_p = 1.55 +-
0.16 g/cm^3 ~ 1.24 rhoj. The host star has a mass of 1.00 Msun, a radius of
0.92 Rsun and a very high metallicity [Fe/H]= 0.43 +- 0.08. HATS-4b is among
the densest known planets with masses between 1-2 Mj and is thus likely to have
a significant content of heavy elements of the order of 75 Mearth. In this
paper we present the data reduction, radial velocity measurement and stellar
classification techniques adopted by the HATSouth survey for the CORALIE
spectrograph. We also detail a technique to estimate simultaneously vsini and
macroturbulence using high resolution spectra. | astro-ph_EP |
WASP-180Ab: Doppler tomography of an hot Jupiter orbiting the primary
star in a visual binary: We report the discovery and characterisation of WASP-180Ab, a hot Jupiter
confirmed by the detection of its Doppler shadow and by measuring its mass
using radial velocities. We find the 0.9 $\pm$ 0.1 $M_{\rm Jup}$, 1.24 $\pm$
0.04 $R_{\rm Jup}$ planet to be in a misaligned, retrograde orbit around an F7
star with $T_{\rm eff}$ = 6500K and a moderate rotation speed of vsini = 19.9
km s$^{-1}$. The host star is the primary of a $V$ = 10.7 binary, where a
secondary separated by 5$''$ ($\sim$1200 AU) contributes $\sim$30% of the
light. WASP-180Ab therefore adds to a small sample of transiting hot Jupiters
known in binary systems. A 4.6-day modulation seen in the WASP data is likely
to be the rotational modulation of the companion star, WASP-180B. | astro-ph_EP |
Kepler's Earth-like Planets Should Not Be Confirmed Without Independent
Detection: The Case of Kepler-452b: We show that the claimed confirmed planet Kepler-452b (a.k.a. K07016.01, KIC
8311864) can not be confirmed using a purely statistical validation approach.
Kepler detects many more periodic signals from instrumental effects than it
does from transits, and it is likely impossible to confidently distinguish the
two types of event at low signal-to-noise. As a result, the scenario that the
observed signal is due to an instrumental artifact can't be ruled out with 99\%
confidence, and the system must still be considered a candidate planet. We
discuss the implications for other confirmed planets in or near the habitable
zone. | astro-ph_EP |
Interior Models of Saturn: Including the Uncertainties in Shape and
Rotation: The accurate determination of Saturn's gravitational coefficients by Cassini
could provide tighter constrains on Saturn's internal structure. Also,
occultation measurements provide important information on the planetary shape
which is often not considered in structure models. In this paper we explore how
wind velocities and internal rotation affect the planetary shape and the
constraints on Saturn's interior. We show that within the geodetic approach
(Lindal et al., 1985, ApJ, 90, 1136) the derived physical shape is insensitive
to the assumed deep rotation. Saturn's re-derived equatorial and polar radii at
100 mbar are found to be 54,445 $\pm$10 km and 60,365$\pm$10 km, respectively.
To determine Saturn's interior we use {\it 1 D} three-layer hydrostatic
structure models, and present two approaches to include the constraints on the
shape. These approaches, however, result in only small differences in Saturn's
derived composition. The uncertainty in Saturn's rotation period is more
significant: with Voyager's 10h39mns period, the derived mass of heavy elements
in the envelope is 0-7 M$_{\oplus}$. With a rotation period of 10h32mns, this
value becomes $<4$ $M_{\oplus}$, below the minimum mass inferred from
spectroscopic measurements. Saturn's core mass is found to depend strongly on
the pressure at which helium phase separation occurs, and is estimated to be
5-20 M$_{\oplus}$. Lower core masses are possible if the separation occurs
deeper than 4 Mbars. We suggest that the analysis of Cassini's radio
occultation measurements is crucial to test shape models and could lead to
constraints on Saturn's rotation profile and departures from hydrostatic
equilibrium. | astro-ph_EP |
The Exomoon Corridor: Half of all exomoons exhibit TTV frequencies
within a narrow window due to aliasing: Exomoons are expected to produce potentially detectable transit timing
variations (TTVs) upon their parent planet. Unfortunately, distinguishing
moon-induced TTVs from other sources, in particular planet-planet interactions,
has severely impeded its usefulness as a tool for identifying exomoon
candidates. A key feature of exomoon TTVs is that they will always be
undersampled, due to the simple fact that we can only observe the TTVs once per
transit/planetary period. We show that it is possible to analytically express
the aliased TTV periodicity as a function of planet and moon period. Further,
we show that inverting an aliased TTV period back to a true moon period is
fraught with hundreds of harmonic modes. However, a unique aspect of these TTV
aliases is that they are predicted to occur at consistently short periods,
irrespective of what model one assumes for the underlying moon population.
Specifically, 50% of all exomoons are expected to induce TTVs with a period
between 2 to 4 cycles, a range that planet-planet interactions rarely manifest
at. This provides an exciting and simple tool for quickly identifying exomoons
candidates and brings the TTV method back to the fore as an exomoon hunting
strategy. Applying this method to the candidate, Kepler-1625b i, reveals that
its TTV periodicity centers around the median period expected for exomoons. | astro-ph_EP |
Can Ground-based Telescopes Detect The Oxygen 1.27 Micron Absorption
Feature as a Biomarker in Exoplanets ?: The oxygen absorption line imprinted in the scattered light from the
Earth-like planets has been considered the most promising metabolic biomarker
of the exo-life. We examine the feasibility of the detection of the 1.27 micron
oxygen band from habitable exoplanets, in particular, around late- type stars
observed with a future instrument on a 30 m class ground-based telescope. We
analyzed the night airglow around 1.27 micron with IRCS/echelle spectrometer on
Subaru and found that the strong telluric emission from atmospheric oxygen
molecules declines by an order of magnitude by midnight. By compiling nearby
star catalogs combined with the sky background model, we estimate the
detectability of the oxygen absorption band from an Earth twin, if it exists,
around nearby stars. We find that the most dominant source of photon noise for
the oxygen 1.27 micron band detection comes from the night airglow if the
contribution of the stellar PSF halo is suppressed enough to detect the planet.
We conclude that the future detectors for which the detection contrast is
limited by photon noise can detect the oxygen 1.27 micron absorption band of
the Earth twins for ~50 candidates of the late type star. This paper
demonstrates the importance of deploying small inner working angle efficient
coronagraph and extreme adaptive optics on extremely large telescopes, and
clearly shows that doing so will enable study of potentially habitable planets. | astro-ph_EP |
Dusty tails of evaporating exoplanets. II. Physical modelling of the KIC
12557548b light curve: Evaporating rocky exoplanets, such as KIC 12557548b, eject large amounts of
dust grains, which can trail the planet in a comet-like tail. When such objects
occult their host star, the resulting transit signal contains information about
the dust in the tail. We aim to use the detailed shape of the Kepler light
curve of KIC 12557548b to constrain the size and composition of the dust grains
that make up the tail, as well as the mass loss rate of the planet. Using a
self-consistent numerical model of the dust dynamics and sublimation, we
calculate the shape of the tail by following dust grains from their ejection
from the planet to their destruction due to sublimation. From this dust cloud
shape, we generate synthetic light curves (incorporating the effects of
extinction and angle-dependent scattering), which are then compared with the
phase-folded Kepler light curve. We explore the free-parameter space thoroughly
using a Markov chain Monte Carlo method. Our physics-based model is capable of
reproducing the observed light curve in detail. Good fits are found for initial
grain sizes between 0.2 and 5.6 micron and dust mass loss rates of 0.6 to 15.6
M_earth/Gyr (2-sigma ranges). We find that only certain combinations of
material parameters yield the correct tail length. These constraints are
consistent with dust made of corundum (Al2O3), but do not agree with a range of
carbonaceous, silicate, or iron compositions. Using a detailed, physically
motivated model, it is possible to constrain the composition of the dust in the
tails of evaporating rocky exoplanets. This provides a unique opportunity to
probe to interior composition of the smallest known exoplanets. | astro-ph_EP |
Gamma Ray Bursts: Not so Much Deadlier than We Thought: We analyze the additional effect on planetary atmospheres of recently
detected gamma-ray burst afterglow photons in the range up to 1 TeV. For an
Earth-like atmosphere we find that there is a small additional depletion in
ozone versus that modeled for only prompt emission. We also find a small
enhancement of muon flux at the planet surface. Overall, we conclude that the
additional afterglow emission, even with TeV photons, does not result in a
significantly larger impact over that found in past studies. | astro-ph_EP |
Accretion onto a binary from a polar circumbinary disc: We present hydrodynamical simulations to model the accretion flow from a
polar circumbinary disc onto a high eccentricity ($e=0.78$) binary star system
with near unity mass ratio ($q=0.83$), as a model for binary HD 98800 BaBb. We
compare the polar circumbinary disc accretion flow with the previously studied
coplanar case. In the coplanar case, the circumbinary disc becomes eccentric
and the accretion alternates from being dominant onto one binary member to the
other. For the polar disc case involving a highly eccentric binary, we find
that the circumbinary disc retains its initially low eccentricity and that the
primary star accretion rate is always about the same as the secondary star
accretion rate. Recent observations of the binary HD 98800 BaBb, which has a
polar circumbinary disc, have been used to determine the value of the $\rm
H\alpha$ flux from the brighter component. From this value, we infer that the
accretion rate is much lower than for typical T Tauri stars. The eccentric
orbit of the outer companion HD 98800 A increases the accretion rate onto HD
98800 B by $\sim 20$ per cent after each periastron passage. Our hydrodynamical
simulations are unable to explain such a low accretion rate unless the disc
viscosity parameter is very small, $\alpha < 10^{-5}$. Additional observations
of this system would be useful to check on this low accretion rate. | astro-ph_EP |
Analytical determination of orbital elements using Fourier analysis. II.
Gaia astrometry and its combination with radial velocities: The ESA global astrometry space mission Gaia has been monitoring the position
of a billion stars since 2014. The analysis of such a massive dataset is
challenging in terms of the data processing involved. In particular, the blind
detection and characterization of single or multiple companions to stars
(planets, brown dwarfs, or stars) using Gaia astrometry requires highly
efficient algorithms. In this article, we present a set of analytical methods
to detect and characterize companions in scanning space astrometric time series
as well as via a combination of astrometric and radial velocity time series. We
propose a general linear periodogram framework and we derive analytical
formulas for the false alarm probability (FAP) of periodogram peaks. Once a
significant peak has been identified, we provide analytical estimates of all
the orbital elements of the companion based on the Fourier decomposition of the
signal. The periodogram, FAP, and orbital elements estimates can be computed
for the astrometric and radial velocity time series separately or in tandem.
These methods are complementary with more accurate and more computationally
intensive numerical algorithms (e.g., least-squares minimization, Markov chain
Monte Carlo, genetic algorithms). In particular, our analytical approximations
can be used as an initial condition to accelerate the convergence of numerical
algorithms. Our formalism has been partially implemented in the Gaia exoplanet
pipeline for the third Gaia data release. Since the Gaia astrometric time
series are not yet publicly available, we illustrate our methods on the basis
of Hipparcos data, together with on-ground CORALIE radial velocities, for three
targets known to host a companion: HD 223636 (HIP 117622), HD 17289 (HIP
12726), and HD 3277 (HIP 2790). | astro-ph_EP |
The Atmosphere: These notes contain everything necessary to run a flipped course on "The
Atmosphere" at an introductory undergraduate level. There are notes for the
students to read before each course meeting and problems for them to work on in
small groups during course meetings. Topics include (1) atmospheric
composition, structure, and thermodynamics; (2) solar and terrestrial radiation
in the atmospheric energy balance; (3) atmospheric dynamics and circulation. I
include 10 problem sets, six practice midterms, and three practice finals.
Problems are drawn from the atmospheres of modern and past Earth, solar system
planets, and extrasolar planets. I can provide solutions to the in-class
problems and problem sets to teachers upon request. You are free to use these
notes in your classes, and to expand them as you please. If you catch any typos
or errors, please send them to me. Enjoy! | astro-ph_EP |
A Near-coplanar Stellar Flyby of the Planet Host Star HD 106906: We present an investigation into the kinematics of HD 106906 using the newly
released Gaia DR2 catalog to search for close encounters with other members of
the Scorpius-Centaurus (Sco-Cen) association. HD 106906 is an eccentric
spectroscopic binary that hosts both a large asymmetric debris disk extending
out to at least 500 au and a directly imaged planetary-mass companion at a
projected separation of 738 au. The cause of the asymmetry in the debris disk
and the unusually wide separation of the planet is not currently known. Using a
combination of Gaia DR2 astrometry and ground-based radial velocities, we
explore the hypothesis that a close encounter with another cluster member
within the last 15 Myr is responsible for the present configuration of the
system. Out of 461 stars analyzed, we identified two candidate perturbers that
had a median closest approach (CA) distance within 1 pc of HD 106906: HIP 59716
at $D_{\rm CA}=0.65_{-0.40}^{+0.93}$ pc ($t_{\rm CA}=-3.49_{-1.76}^{+0.90}$
Myr) and HIP 59721 at $D_{\rm CA}=0.71_{-0.11}^{+0.18}$ pc ($t_{\rm
CA}=-2.18_{-1.04}^{+0.54}$ Myr), with the two stars likely forming a wide
physical binary. The trajectories of both stars relative to HD 106906 are
almost coplanar with the inner disk ($\Delta\theta = 5.4\pm1.7$ deg and
$4.2_{-1.1}^{+0.9}$ deg). These two stars are the best candidates of the
currently known members of Sco-Cen for having a dynamically important close
encounter with HD 106906, which may have stabilized the orbit of HD 106906 b in
the scenario where the planet formed in the inner system and attained high
eccentricity by interaction with the central binary. | astro-ph_EP |
Formation of Regular Satellites from Ancient Massive Rings in the Solar
System: When a planetary tidal disk -like Saturn's rings- spreads beyond the Roche
radius (inside which planetary tides prevent aggregation), satellites form and
migrate away. Here, we show that most regular satellites in the solar system
probably formed in this way. According to our analytical model, when the
spreading is slow, a retinue of satellites appear with masses increasing with
distance to the Roche radius, in excellent agreement with Saturn's, Uranus',
and Neptune's satellite systems. This suggests that Uranus and Neptune used to
have massive rings that disappeared to give birth to most of their regular
satellites. When the spreading is fast, only one large satellite forms, as was
the case for Pluto and Earth. This conceptually bridges the gap between
terrestrial and giant planet systems. | astro-ph_EP |
Planetesimals Around Stars with TESS (PAST): I. Transient Dimming of a
Binary Solar Analog at the End of the Planet Accretion Era: We report detection of quasi-periodic (1.5 day) dimming of HD 240779, the
solar-mass primary in a 5" visual binary (also TIC 284730577), by the
Transiting Exoplanet Survey Satellite. This dimming, as has been shown for
other "dipper" stars, is likely due to occultation by circumstellar dust. The
barycentric space motion, lithium abundance, rotation, and chromospheric
emission of the stars in this system point to an age of ~125 Myr, and possible
membership in the AB Doradus moving group. As such it occupies an important but
poorly explored intermediate regime of stars with transient dimming between
young stellar objects in star forming regions and main sequence stars, and
between UX Orionis-type Ae/Be stars and M-type "dippers". HD 240779, but not
its companion BD+10714B, has WISE-detected excess infrared emission at 12 and
22 microns indicative of circumstellar dust. We propose that infrared emission
is produced by collisions of planetesimals during clearing of a residual disk
at the end of rocky planet formation, and that quasi-periodic dimming is
produced by the rapid disintegration of a 100 km planetesimal near the silicate
evaporation radius. Further studies of this and similar systems will illuminate
a poorly understood final phase of rocky planet formation like that which
produced the inner Solar System. | astro-ph_EP |
What are little worlds made of? Stellar abundances and the building
blocks of planets: If the photospheres of solar-type stars represent the composition of
circumstellar disks from which any planets formed, spectroscopic determinations
of stellar elemental abundances offer information on the composition of those
planets, including smaller, rocky planets. In particular, the C/O ratio is
proposed to be a key determinant of the composition of solids that condense
from disk gas and are incorporated into planets. Also, planets may leave
chemical signatures on the photospheres of their host stars by sequestering
heavy elements, or by being accreted by the stars. The presence, absence, and
composition of planets could be revealed by small differences in the relative
abundances between stars. I critically examine these scenarios and show that
(i) a model of Galactic chemical evolution predicts that the C/O ratio is
expected to be close to the solar value and vary little between dwarf stars in
the solar neighborhood; (ii) spectroscopic surveys of M dwarf stars limit the
occurrence of stars with C/O $\gtrsim 1$ to $<10^{-3}$; and (iii) planetesimal
chemistry will be controlled by the composition of oxygen-rich dust inherited
from the molecular cloud and processed in a dust-rich environment, not a gas
with the stellar composition. A second generation of more reduced planetesimals
could be produced by re-equilibration of some material with dust-depleted gas.
Finally, I discuss how minor differences in relative abundances between stars
that correlate with condensation temperature can be explained by dust-gas
segregation, perhaps in circumstellar disks, rather than planet formation. | astro-ph_EP |
Condensation of Rocky Material in Astrophysical Environments: Volatility-dependent fractionation of the rock-forming elements at high
temperatures is an early, widespread process during formation of the earliest
solids in protoplanetary disks. Equilibrium condensation calculations allow
prediction of the identities and compositions of mineral and liquid phases
coexisting with gas under presumed bulk chemical, pressure and temperature
conditions. A graphical survey of such results is presented for systems of
solar and non-solar bulk composition. Chemical equilibrium was approached to
varying degrees in the local regions where meteoritic chondrules, Ca-Al-rich
inclusions, matrix and other components formed. Early, repeated vapor-solid
cycling and homogenization, followed by hierarchical accretion in dust-rich
regions, is hypothesized for meteoritic inclusions. Disequilibrium chemical
effects appear to have been common at all temperatures, but increasingly so in
less refractory meteoritic components. Work is needed to better model
high-temperature solid solutions, indicators of these processes. | astro-ph_EP |
The effect of Poynting-Robertson drag on the triangular Lagrangian
points: We investigate the stability of motion close to the Lagrangian equilibrium
points L4 and L5 in the framework of the spatial, elliptic, restricted three-
body problem, subject to the radial component of Poynting-Robertson drag. For
this reason we develop a simplified resonant model, that is based on averaging
theory, i.e. averaged over the mean anomaly of the perturbing planet. We find
temporary stability of particles displaying a tadpole motion in the 1:1
resonance. From the linear stability study of the averaged simplified resonant
model, we find that the time of temporary stability is proportional to beta a1
n1 , where beta is the ratio of the solar radiation over the gravitational
force, and a1, n1 are the semi-major axis and the mean motion of the perturbing
planet, respectively. We extend previous results (Murray (1994)) on the
asymmetry of the stability indices of L4 and L5 to a more realistic force
model. Our analytical results are supported by means of numerical simulations.
We implement our study to Jupiter-like perturbing planets, that are also found
in extra-solar planetary systems. | astro-ph_EP |
Neptune's resonances in the Scattered Disk: The Scattered Disk Objects (SDOs) are thought to be a small fraction of the
ancient population of leftover planetesimals in the outer solar system that
were gravitationally scattered by the giant planets and have managed to survive
primarily by capture and sticking in Neptune's exterior mean motion resonances
(MMRs). In order to advance understanding of the role of MMRs in the dynamics
of the SDOs, we investigate the phase space structure of a large number of
Neptune's MMRs in the semi-major axis range 33--140~au by use of Poincar\'e
sections of the circular planar restricted three body model for the full range
of particle eccentricity pertinent to SDOs. We find that, for eccentricities
corresponding to perihelion distances near Neptune's orbit, distant MMRs have
stable regions with widths that are surprisingly large and of similar size to
those of the closer-in MMRs. We identify a phase-shifted second resonance zone
that exists in the phase space at planet-crossing eccentricities but not at
lower eccentricities; this second resonance zone plays an important role in the
dynamics of SDOs in lengthening their dynamical lifetimes. Our non-perturbative
measurements of the sizes of the stable resonance zones confirm previous
results and provide an additional explanation for the prominence of the $N$:1
sequence of MMRs over the $N$:2, $N$:3 sequences and other MMRs in the
population statistics of SDOs; our results also provide a tool to more easily
identify resonant objects. | astro-ph_EP |
Cooling Requirements for the Vertical Shear Instability in
Protoplanetary Disks: The vertical shear instability (VSI) offers a potential hydrodynamic
mechanism for angular momentum transport in protoplanetary disks (PPDs). The
VSI is driven by a weak vertical gradient in the disk's orbital motion, but
must overcome vertical buoyancy, a strongly stabilizing influence in cold
disks, where heating is dominated by external irradiation. Rapid radiative
cooling reduces the effective buoyancy and allows the VSI to operate. We
quantify the cooling timescale $t_c$ needed for efficient VSI growth, through a
linear analysis of the VSI with cooling in vertically global, radially local
disk models. We find the VSI is most vigorous for rapid cooling with
$t_c<\Omega_\mathrm{K}^{-1}h|q|/(\gamma -1)$ in terms of the Keplerian orbital
frequency, $\Omega_\mathrm{K}$; the disk's aspect-ratio, $h\ll1$; the radial
power-law temperature gradient, $q$; and the adiabatic index, $\gamma$. For
longer $t_c$, the VSI is much less effective because growth slows and shifts to
smaller length scales, which are more prone to viscous or turbulent decay. We
apply our results to PPD models where $t_c$ is determined by the opacity of
dust grains. We find that the VSI is most effective at intermediate radii, from
$\sim5$AU to $\sim50$AU with a characteristic growth time of $\sim30$ local
orbital periods. Growth is suppressed by long cooling times both in the opaque
inner disk and the optically thin outer disk. Reducing the dust opacity by a
factor of 10 increases cooling times enough to quench the VSI at all disk
radii. Thus the formation of solid protoplanets, a sink for dust grains, can
impede the VSI. | astro-ph_EP |
Predicting Exoplanets Mass and Radius: A Nonparametric Approach: A fundamental endeavor in exoplanetary research is to characterize the bulk
compositions of planets via measurements of their masses and radii. With future
sample sizes of hundreds of planets to come from TESS and PLATO, we develop a
statistical method that can flexibly yet robustly characterize these
compositions empirically, via the exoplanet M-R relation. Although the M-R
relation has been explored in many prior works, they mostly use a power-law
model, with assumptions that are not flexible enough to capture important
features in current and future M-R diagrams. To address these shortcomings, a
nonparametric approach is developed using a sequence of Bernstein polynomials.
We demonstrate the benefit of taking the nonparametric approach by benchmarking
our findings with previous work and showing that a power-law can only
reasonably describe the M-R relation of the smallest planets and that the
intrinsic scatter can change non-monotonically with different values of a
radius. We then apply this method to a larger dataset, consisting of all the
Kepler observations in the NASA Exoplanet Archive. Our nonparametric approach
provides a tool to estimate the M-R relation by incorporating heteroskedastic
measurement errors into the model. As more observations will be obtained in the
near future, this approach can be used with the provided R code to analyze a
larger dataset for a better understanding of the M-R relation. | astro-ph_EP |
Normal forms for the Laplace resonance: We describe a comprehensive model for systems locked in the Laplace
resonance. The framework is based on the simplest possible dynamical structure
provided by the Keplerian problem perturbed by the resonant coupling truncated
at second order in the eccentricities. The reduced Hamiltonian, constructed by
a transformation to resonant coordinates, is then submitted to a suitable
ordering of the terms and to the study of its equilibria. Henceforth, resonant
normal forms are computed. The main result is the identification of two
different classes of equilibria. In the first class, only one kind of stable
equilibrium is present: the paradigmatic case is that of the Galilean system.
In the second class, three kinds of stable equilibria are possible and at least
one of them is characterised by a high value of the forced eccentricity for the
`first planet': here the paradigmatic case is the exo-planetary system GJ-876,
in which the combination of libration centers admits triple conjunctions
otherwise not possible in the Galilean case. The normal form obtained by
averaging with respect to the free eccentricity oscillations, describes the
libration of the Laplace argument for arbitrary amplitudes and allows us to
determine the libration width of the resonance. The agreement of the analytic
predictions with the numerical integration of the toy models is very good. | astro-ph_EP |
Spin Axes and Shape Models of Asteroid Pairs: Fingerprints of YORP and a
Path to the Density of Rubble Piles: An asteroid pair consists of two unbound objects with almost identical
heliocentric orbital elements that were formed when a single "rubble pile"
asteroid failed to remain bound against an increasing rotation rate. Models
suggest that the pairs' progenitors gained the fast rotation due to the YORP
effect. Since it was shown that the spin axis vector can be aligned by the YORP
effect, such a behavior should be seen on asteroid pairs, if they were indeed
formed by the described mechanism. Alternatively, if the pairs were formed by a
collision, the spin axes should have a random direction and small or young
bodies might have a tumbling rotation.
Here I apply the lightcurve inversion method on self-obtained photometric
data, in order to derive the rotation axis vectors and shape models of the
asteroid pairs 2110, 3749, 5026, 6070, 7343 and 44612. Three asteroids resulted
with polar-directed spin axes and three objects with ambiguous results. In
addition, the secondary member 44612 presents the same sense of rotation as its
primary member 2110, and its spin is not tumbling. Finally, I use a rotational
fission model, based on the assumption of an angular momentum conservation, and
match it to the measured spin, shape, and mass ratio parameters in order to
constrain the density of the primary members in the pairs. Using this method,
low density values that are expected from a "rubble pile" are derived. All
these results lead to the conclusion that the disruption of these asteroid
pairs was most likely the outcome of the YORP effect that spun-up "rubble pile"
asteroids. | astro-ph_EP |
Extreme Climate Variations from Milankovitch-like Eccentricity
Oscillations in Extrasolar Planetary Systems: Although our solar system features predominantly circular orbits, the
exoplanets discovered so far indicate that this is the exception rather than
the rule. This could have crucial consequences for exoplanet climates, both
because eccentric terrestrial exoplanets could have extreme seasonal variation,
and because giant planets on eccentric orbits could excite Milankovitch-like
variations of a potentially habitable terrestrial planet,\"A\^os eccentricity,
on timescales of thousands-to-millions of years. A particularly interesting
implication concerns the fact that the Earth is thought to have gone through at
least one globally frozen, "snowball" state in the last billion years that it
presumably exited after several million years of buildup of greenhouse gases
when the ice-cover shut off the carbonate-silicate cycle. Water-rich extrasolar
terrestrial planets with the capacity to host life might be at risk of falling
into similar snowball states. Here we show that if a terrestrial planet has a
giant companion on a sufficiently eccentric orbit, it can undergo
Milankovitch-like oscillations of eccentricity of great enough magnitude to
melt out of a snowball state. | astro-ph_EP |
Modeling Self-Subtraction in Angular Differential Imaging: Application
to the HD 32297 Debris Disk: We present a new technique for forward-modeling self-subtraction of spatially
extended emission in observations processed with angular differential imaging
(ADI) algorithms. High-contrast direct imaging of circumstellar disks is
limited by quasi-static speckle noise and ADI is commonly used to suppress
those speckles. However, the application of ADI can result in self-subtraction
of the disk signal due to the disk's finite spatial extent. This signal
attenuation varies with radial separation and biases measurements of the disk's
surface brightness, thereby compromising inferences regarding the physical
processes responsible for the dust distribution. To compensate for this
attenuation, we forward-model the disk structure and compute the form of the
self-subtraction function at each separation. As a proof of concept, we apply
our method to 1.6 and 2.2 micron Keck AO NIRC2 scattered-light observations of
the HD 32297 debris disk reduced using a variant of the "locally optimized
combination of images" (LOCI) algorithm. We are able to recover disk surface
brightness that was otherwise lost to self-subtraction and produce simplified
models of the brightness distribution as it appears with and without
self-subtraction. From the latter models, we extract radial profiles for the
disk's brightness, width, midplane position, and color that are unbiased by
self-subtraction. Our analysis of these measurements indicates a break in the
brightness profile power law at r~110 AU and a disk width that increases with
separation from the star. We also verify disk curvature that displaces the
midplane by up to 30 AU towards the northwest relative to a straight fiducial
midplane. | astro-ph_EP |
Using HCO$^+$ isotopologues as tracers of gas depletion in
protoplanetary disk gaps: The widespread rings and gaps seen in the dust continuum in protoplanetary
disks are sometimes accompanied by similar substructures seen in molecular line
emission. One example is the outer gap at 100 au in AS 209, which shows that
the H$_{13}$CO$^+$ and C$_{18}$O emission intensities decrease along with the
continuum in the gap, while the DCO$^+$ emission increases inside the gap.
We aim to study the behavior of DCO$^+$/H$_{13}$CO$^+$ and DCO$^+$/HCO$^+$
ratios in protoplanetary disk gaps assuming the two scenarios: the gas
depletion follows the dust depletion and only the dust is depleted.
We first modeled the physical disk structure using the thermo-chemical model
ANDES. This 1+1D steady-state disk model calculates the thermal balance of gas
and dust and includes the FUV, X-rays, cosmic rays, and other ionization
sources together with the reduced chemical network for molecular coolants.
Afterward, this physical structure was adopted for calculations of molecular
abundances with the extended gas-grain chemical network with deuterium
fractionation. Ideal synthetic spectra and 0th-moment maps were produced with
LIME.
We are able to qualitatively reproduce the increase in the DCO$^+$ intensity
and the decrease in the H$_{13}$CO$^+$ and C$_{18}$O intensities inside the
disk gap, which is qualitatively similar to what is observed in the outer AS
209 gap. The corresponding disk model assumes that both the gas and dust are
depleted in the gap. The model with the gas-rich gap, where only the dust is
depleted, produces emission that is too bright in all HCO$^+$ isotopologues and
C$_{18}$O.
The DCO$^+$/H$_{13}$CO$^+$ line ratio can be used to probe gas depletion in
dust continuum gaps outside of the CO snow line. The DCO$^+$/C$_{18}$O line
ratio shows a similar, albeit weaker, effect; however, these species can be
observed simultaneously with a single ALMA or NOEMA setup. | astro-ph_EP |
A Methane Extension to the Classical Habitable Zone: The habitable zone (HZ) is the circumstellar region where standing bodies of
liquid water could exist on the surface of a rocky planet. Conventional
definitions assume that CO2 and H2O are the only greenhouse gases. The outer
edge of this classical N2-CO2-H2O HZ extends out to nearly 1.7 AU in our solar
system, beyond which condensation and scattering by CO2 outstrip its greenhouse
capacity. We use a single column radiative-convective climate model to assess
the greenhouse effect of CH4 (10 to about 100,000 ppm) on the classical
habitable zone (N2-CO2-H2O) for main-sequence stars with stellar temperatures
between 2,600 to 10,000 K (about A3 to M8). Assuming N2-CO2-H2O atmospheres,
previous studies have shown that cooler stars more effectively heat terrestrial
planets. However, we find that the addition of CH4 produces net greenhouse
warming (tens of degrees) in planets orbiting stars hotter than a mid-K (about
4500K), whereas a prominent anti-greenhouse effect is noted for planets around
cooler stars. We show that 10% CH4 can increase the width of the classical HZ
of the hottest stars (TEFF = 10,000 K) by over 20%. In contrast, the CH4
anti-greenhouse can shrink the HZ for the coolest stars (TEFF = 2,600 K) by a
similar percentage. We find that dense CO2-CH4 atmospheres near the outer edge
of hotter stars may suggest inhabitance, highlighting the importance of
including secondary greenhouse gases in alternative definitions of the HZ. We
parameterize the limits of this N2-CO2-H2O-CH4 habitable zone and discuss
implications in the search for extraterrestrial life. | astro-ph_EP |
Observational evidence for a metal rich atmosphere on the super-Earth
GJ1214b: We report observations of two consecutive transits of the warm super-Earth
exoplanet GJ1214b at 3.6 and 4.5 microns with the Infrared Array Camera
instrument on-board the Spitzer Space Telescope. The two transit light curves
allow for the determination of the transit parameters for this system. We find
these paremeters to be consistent with the previously determined values and no
evidence for transit timing variations. The main investigation consists of
measuring the transit depths in each bandpass to constrain the planet's
transmission spectrum. Fixing the system scale and impact parameters, we
measure R_p/R_star=0.1176 (+0.0008/-0.0009) and 0.1163 (+0.0010/-0.0008) at 3.6
and 4.5 microns, respectively. Combining these data with the previously
reported MEarth Observatory measurements in the red optical yields constraints
on the GJ1214b's transmission spectrum and allows us to rule-out a cloud-free,
solar composition (i.e., hydrogen-dominated) atmosphere at 4.5 sigma
confidence. This independently confirms a recent finding that was based on a
measurement of the planet's transmission spectrum using the VLT. The Spitzer,
MEarth, and VLT observations together yield a remarkably flat transmission
spectrum over the large wavelength domain spanned by the data. Consequently,
cloud-free atmospheric models require more than 30% metals (assumed to be in
the form of H2O by volume to be consistent with all the observations. | astro-ph_EP |
The Effect of Composition on the Evolution of Giant and
Intermediate-Mass Planets: We model the evolution of planets with various masses and compositions. We
investigate the effects of the composition and its depth dependence on the
long-term evolution of the planets. The effects of opacity and stellar
irradiation are also considered. It is shown that the change in radius due to
various compositions can be significantly smaller than the change in radius
caused by the opacity. Irradiation also affects the planetary contraction but
is found to be less important than the opacity effects. We suggest that the
mass-radius relationship used for characterization of observed extrasolar
planets should be taken with great caution since different physical conditions
can result in very different mass-radius relationships. | astro-ph_EP |
Model atmospheres of irradiated exoplanets: The influence of stellar
parameters, metallicity, and the C/O ratio: Many parameters constraining the spectral appearance of exoplanets are still
poorly understood. We therefore study the properties of irradiated exoplanet
atmospheres over a wide parameter range including metallicity, C/O ratio and
host spectral type. We calculate a grid of 1-d radiative-convective atmospheres
and emission spectra. We perform the calculations with our new
Pressure-Temperature Iterator and Spectral Emission Calculator for Planetary
Atmospheres (PETIT) code, assuming chemical equilibrium. The atmospheric
structures and spectra are made available online. We find that atmospheres of
planets with C/O ratios $\sim$ 1 and $T_{\rm eff}$ $\gtrsim$ 1500 K can exhibit
inversions due to heating by the alkalis because the main coolants CH$_4$,
H$_2$O and HCN are depleted. Therefore, temperature inversions possibly occur
without the presence of additional absorbers like TiO and VO. At low
temperatures we find that the pressure level of the photosphere strongly
influences whether the atmospheric opacity is dominated by either water (for
low C/O) or methane (for high C/O), or both (regardless of the C/O). For hot,
carbon-rich objects this pressure level governs whether the atmosphere is
dominated by methane or HCN. Further we find that host stars of late spectral
type lead to planetary atmospheres which have shallower, more isothermal
temperature profiles. In agreement with prior work we find that for planets
with $T_{\rm eff}$ $<$ 1750 K the transition between water or methane dominated
spectra occurs at C/O $\sim$ 0.7, instead of $\sim$ 1, because condensation
preferentially removes oxygen. | astro-ph_EP |
Detection of Earth-mass and Super-Earth Trojan Planets Using Transit
Timing Variation Method: We have carried out an extensive study of the possibility of the detection of
Earth-mass and super-Earth Trojan planets using transit timing variation method
with the Kepler space telescope. We have considered a system consisting of a
transiting Jovian-type planet in a short period orbit, and determined the
induced variations in its transit timing due to an Earth-mass/super-Earth
Trojan planet. We mapped a large section of the phase space around the 1:1
mean-motion resonance and identified regions corresponding to several other
mean-motion resonances where the orbit of the planet would be stable. We
calculated TTVs for different values of the mass and orbital elements of the
transiting and perturbing bodies as well as the mass of central star, and
identified orbital configurations of these objects (ranges of their orbital
elements and masses) for which the resulted TTVs would be within the range of
the variations of the transit timing of Kepler's planetary candidates. Results
of our study indicate that in general, the amplitudes of the TTVs fall within
the detectable range of timing precision obtained from the Kepler's
long-cadence data, and depending on the parameters of the system, their
magnitudes may become as large as a few hours. The probability of detection is
higher for super-Earth Trojans with slightly eccentric orbits around
short-period Jovian-type planets with masses slightly smaller than Jupiter. We
present the details of our study and discuss the implications of its results. | astro-ph_EP |
New Nomenclature Rules for Meteor Showers Adopted: The Shower Database (SD) of the Meteor Data Center (MDC) had been operating
on the basis of stream-naming rules which were too complex and insufficiently
precise for 15 years. With a gradual increase in the number of discovered
meteor showers, the procedure for submitting new showers to the database and
naming them lead to situations that were inconsistent with the fundamental role
of the SD - the disambiguation of stream names in the scientific literature.
Our aim is to simplify the meteor shower nomenclature rules. We propose a much
simpler set of meteor shower nomenclature rules, based on a two-stage approach,
similar to those used in the case of asteroids. The first stage applies to a
new shower just after its discovery. The second stage concerns the repeatedly
observed shower, the existence of which no longer raises any doubts. Our
proposed new procedure was approved by a vote of the commission F1 of the IAU
in July 2022. | astro-ph_EP |
The weird and the wonderful in our Solar System: Searching for
serendipity in the Legacy Survey of Space and Time: We present a novel method for anomaly detection in Solar System object data,
in preparation for the Legacy Survey of Space and Time. We train a deep
autoencoder for anomaly detection and use the learned latent space to search
for other interesting objects. We demonstrate the efficacy of the autoencoder
approach by finding interesting examples, such as interstellar objects, and
show that using the autoencoder, further examples of interesting classes can be
found. We also investigate the limits of classic unsupervised approaches to
anomaly detection through the generation of synthetic anomalies and evaluate
the feasibility of using a supervised learning approach. Future work should
consider expanding the feature space to increase the variety of anomalies that
can be uncovered during the survey using an autoencoder. | astro-ph_EP |
A Comprehensive Dust Model Applied to the Resolved Beta Pictoris Debris
Disk from Optical to Radio Wavelengths: We investigate whether varying the dust composition (described by the optical
constants) can solve a persistent problem in debris disk modeling--the
inability to fit the thermal emission without over-predicting the scattered
light. We model five images of the beta Pictoris disk: two in scattered light
from HST/STIS at 0.58 microns and HST/WFC3 at 1.16 microns, and three in
thermal emission from Spitzer/MIPS at 24 microns, Herschel/PACS at 70 microns,
and ALMA at 870 microns. The WFC3 and MIPS data are published here for the
first time. We focus our modeling on the outer part of this disk, consisting of
a parent body ring and a halo of small grains. First, we confirm that a model
using astronomical silicates cannot simultaneously fit the thermal and
scattered light data. Next, we use a simple, generic function for the optical
constants to show that varying the dust composition can improve the fit
substantially. Finally, we model the dust as a mixture of the most plausible
debris constituents: astronomical silicates, water ice, organic refractory
material, and vacuum. We achieve a good fit to all datasets with grains
composed predominantly of silicates and organics, while ice and vacuum are, at
most, present in small amounts. This composition is similar to one derived from
previous work on the HR 4796A disk. Our model also fits the thermal SED,
scattered light colors, and high-resolution mid-IR data from T-ReCS for this
disk. Additionally, we show that sub-blowout grains are a necessary component
of the halo. | astro-ph_EP |
TEM analyses of in situ presolar grains from unequilibrated ordinary
chondrite LL3.0 Semarkona: We investigated six presolar grains from very primitive regions of the matrix
in the unequilibrated ordinary chondrite Semarkona with TEM. These grains
include one SiC, one oxide (Mg-Al spinel), and four silicates. Structural and
elemental compositional studies of presolar grains located within their
meteorite hosts have the potential to provide information on conditions and
processes throughout the grains' histories. Our analyses show that the SiC and
spinel grains are stoichiometric and well crystallized. In contrast, the
majority of the silicate grains are non-stoichiometric and poorly crystallized.
These findings are consistent with previous TEM studies of presolar grains from
interplanetary dust particles and chondritic meteorites. We interpret the
poorly crystalline nature, non-stoichiometry, more Fe- rather than Mg-rich
compositions, and/or compositional heterogeneities as features of the formation
by condensation under non-equilibrium conditions. Evidence for parent body
alteration includes rims with mobile elements (S or Fe) on the SiC grain and
one silicate grain. Other features characteristic of secondary processing in
the interstellar medium, the solar nebula, and/or on parent bodies, were not
observed or are better explained by processes operating in circumstellar
envelopes. In general, there was very little overprinting of primary features
of the presolar grains by secondary processes (e.g., ion irradiation,
grain-grain collisions, thermal metamorphism, aqueous alteration). This finding
underlines the need for additional TEM studies of presolar grains located in
the primitive matrix regions of Semarkona, to address gaps in our knowledge of
presolar grain populations accreted to ordinary chondrites. | astro-ph_EP |
Synergies between Asteroseismology and Exoplanetary Science: Over the past decade asteroseismology has become a powerful method to
systematically characterize host stars and dynamical architectures of exoplanet
systems. In this contribution I review current key synergies between
asteroseismology and exoplanetary science such as the precise determination of
planet radii and ages, the measurement of orbital eccentricities, stellar
obliquities and their impact on hot Jupiter formation theories, and the
importance of asteroseismology on spectroscopic analyses of exoplanet hosts. I
also give an outlook on future synergies such as the characterization of
sub-Neptune-size planets orbiting solar-type stars, the study of planet
populations orbiting evolved stars, and the determination of ages of
intermediate-mass stars hosting directly imaged planets. | astro-ph_EP |
Studying wave optics in the light curve of exoplanet microlensing: We study the wave optics features of gravitational microlensing by a binary
lens composed of a planet and a parent star. In this system, the source star
near the caustic line produces a pair of images in which they can play the role
of secondary sources for the observer. This optical system is similar to the
Young double-slit experiment. The coherent wave fronts from a source on the
lens plane can form diffraction pattern on the observer plane. This diffraction
pattern has two modes from the close- and wide-pair images. From the
observational point of view, we study the possibility of detecting this effect
through the Square Kilometer Array (SKA) project in the resonance and high
magnification channels of binary lensing. While the red giant sources do not
seem satisfy the spatial coherency condition, during the caustic crossing, a
small part of source traversing the caustic line can produce coherent pair
images. Observations of wave optics effect in the longer wavelengths
accompanied by optical observations of a microlensing event provide extra
information from the parameter space of the planet. These observations can
provide a new basis for study of exoplanets. | astro-ph_EP |
Finding Long Lost Lexell's Comet: The Fate of the First Discovered
Near-Earth Object: Jupiter-family Comet D/1770 L1 (Lexell) was the first discovered Near-Earth
Object (NEO), and passed the Earth on 1770 Jul 1 at a recorded distance of
0.015 au. The comet was subsequently lost due to unfavorable observing
circumstances during its next apparition followed by a close encounter with
Jupiter in 1779. Since then, the fate of D/Lexell has attracted interest from
the scientific community, and now we revisit this long-standing question. We
investigate the dynamical evolution of D/Lexell based on a set of orbits
recalculated using the observations made by Charles Messier, the comet's
discoverer, and find that there is a $98\%$ chance that D/Lexell remains in the
Solar System by the year of 2000. This finding remains valid even if a moderate
non-gravitational effect is imposed. Messier's observations also suggest that
the comet is one of the largest known near-Earth comets, with a nucleus of
$\gtrsim 10$ km in diameter. This implies that the comet should have been
detected by contemporary NEO surveys regardless of its activity level if it has
remained in the inner Solar System. We identify asteroid 2010 JL$_{33}$ as a
possible descendant of D/Lexell, with a $0.8\%$ probability of chance
alignment, but a direct orbital linkage of the two bodies has not been
successfully accomplished. We also use the recalculated orbit to investigate
the meteors potentially originating from D/Lexell. While no associated meteors
have been unambiguously detected, we show that meteor observations can be used
to better constrain the orbit of D/Lexell despite the comet being long lost. | astro-ph_EP |
A Wideband Self-Consistent Disk-Averaged Spectrum of Jupiter Near 30 GHz
and Its Implications for NH$_{3}$ Saturation in the Upper Troposphere: We present a new set of measurements obtained with the Combined Array for
Research in Millimeter-wave Astronomy (CARMA) of Jupiter's microwave thermal
emission near the 1.3 cm ammonia (NH$_{3}$) absorption band. We use these
observations to investigate the ammonia mole fraction in the upper troposphere,
near $0.3 < P < 2$ bar, based on radiative transfer modeling. We find that the
ammonia mole fraction must be $\sim2.4\times 10^{-4}$ below the NH$_{3}$ ice
cloud, i.e., at $0.8 < P < 8$ bar, in agreement with results by de Pater et al.
(2001, 2016a). We find the NH$_{3}$ cloud-forming region between $0.3 < P <
0.8$ bar to be globally sub-saturated by 55% on average, in accordance with the
result in Gibson et al. (2005). Although our data are not very sensitive to the
region above the cloud layer, we are able to set an upper limit of $2.4\times
10^{-7}$ on the mole fraction here, a factor of $\sim$10 above the saturated
vapor curve. | astro-ph_EP |
Astrometry of the main satellites of Uranus: 18 years of observations: We determine accurate positions of the main satellites of Uranus: Miranda,
Ariel, Umbriel, Titania, and Oberon. Positions of Uranus, as derived from those
of these satellites, are also determined. The observational period spans from
1992 to 2011. All runs were made at the Pico dos Dias Observatory, Brazil.
We used the software called Platform for Reduction of Astronomical Images
Automatically (PRAIA) to minimise (digital coronography) the influence of the
scattered light of Uranus on the astrometric measurements and to determine
accurate positions of the main satellites. The positions of Uranus were then
indirectly determined by computing the mean differences between the observed
and ephemeris positions of these satellites. A series of numerical filters was
applied to filter out spurious data. These filters are mostly based on the
comparison between the positions of Oberon with those of the other satellites
and on the offsets as given by the differences between the observed and
ephemeris positions of all satellites.
We have, for the overall offsets of the five satellites, -29 (+/-63) mas in
right ascension and -27 (+/-46) mas in declination. For the overall difference
between the offsets of Oberon and those of the other satellites, we have +3
(+/-30) mas in right ascension and -2 (+/-28) mas in declination. Ephemeris
positions for the satellites were determined from DE432+ura111. Comparisons
using other modern ephemerides for the solar system -INPOP13c- and for the
motion of the satellites -NOE-7-2013- were also made. They confirm that the
largest contribution to the offsets we find comes from the motion of the
barycenter of the Uranus system around the barycenter of the solar system, as
given by the planetary ephemerides. Catalogues with the observed positions are
provided. | astro-ph_EP |
HADES RV program with HARPS-N at TNG. IX. A super-Earth around the M
dwarf Gl686: The HArps-n red Dwarf Exoplanet Survey is providing a major contribution to
the widening of the current statistics of low-mass planets, through the
in-depth analysis of precise radial velocity measurements in a narrow range of
spectral sub-types. As part of that program, we obtained radial velocity
measurements of Gl 686, an M1 dwarf at d = 8.2 pc. The analysis of data
obtained within an intensive observing campaign demonstrates that the excess
dispersion is due to a coherent signal, with a period of 15.53 d. Almost
simultaneous photometric observations were carried out within the APACHE and
EXORAP programs to characterize the stellar activity and to distinguish
periodic variations related to activity from signals due to the presence of
planetary companions, complemented also with ASAS photometric data. We took
advantage of the available radial velocity measurements for this target from
other observing campaigns. The analysis of the radial velocity composite time
series from the HIRES, HARPS and HARPS-N spectrographs, consisting of 198
measurements taken over 20 years, enabled us to address the nature of periodic
signals and also to characterize stellar physical parameters (mass,
temperature, and rotation). We report the discovery of a super-Earth orbiting
at a distance of 0.092 AU from the host star Gl 686. Gl 686 b has a minimum
mass of 7.1 +/- 0.9 MEarth and an orbital period of 15.532 +/- 0.002 d. The
analysis of the activity indexes, correlated noise through a Gaussian process
framework and photometry, provides an estimate of the stellar rotation period
at 37 d, and highlights the variability of the spot configuration during the
long timespan covering 20 yrs. The observed periodicities around 2000 d likely
point to the existence of an activity cycle. | astro-ph_EP |
Imaging polarimetry of Comet C/2012 L2 (LINEAR): We present the polarimetric results and analysis of comet C/2012 L2 (LINEAR)
observed at 31$^\circ$.1 phase angle before perihelion passage. The
observations of the comet were carried out using ARIES Imaging Polarimeter
(AIMPOL) mounted on the 1.04-m Sampurnanand telescope of ARIES, Nainital, India
on 11 and 12 March, 2013 using R photometric band ($\lambda$ = 630 nm,
$\Delta$$\lambda$ =120nm). The extended coma of the comet ($\sim65000$ km)
shows a significant variation in the intensity as well as polarization profile
in all considered directions which suggest that the dust particles originate
from the active areas of the nucleus. The elongation of the coma is prominent
along the Sun-comet position angle. The polarization of Comet C/2012 L2
(LINEAR) does not show steep radial dependence on the aperture size during both
the nights of observation. A jet extended in the antisolar direction is well
observed in both intensity and polarization map. | astro-ph_EP |
Bouncing on Titan: Motion of the Huygens Probe in the Seconds After
Landing: While landing on Titan, several instruments onboard Huygens acquired
measurements that indicate the probe did not immediately come to rest. Detailed
knowledge of the probe's motion can provide insight into the nature of Titan's
surface. Combining accelerometer data from the Huygens Atmospheric Structure
Instrument (HASI) and the Surface Science Package (SSP) with photometry data
from the Descent Imager/Spectral Radiometer (DISR) we develop a quantitative
model to describe motion of the probe, and its interaction with the surface.
The most likely scenario is the following. Upon impact, Huygens created a 12 cm
deep hole in the surface of Titan. It bounced back, out of the hole onto the
flat surface, after which it commenced a 30-40 cm long slide in the southward
direction. The slide ended with the probe out of balance, tilted in the
direction of DISR by around 10 degrees. The probe then wobbled back and forth
five times in the north-south direction, during which it probably encountered a
1-2 cm sized pebble. The SSP provides evidence for movement up to 10 s after
impact. This scenario puts the following constraints on the physical properties
of the surface. For the slide over the surface we determine a friction
coefficient of 0.4. While this value is not necessarily representative for the
surface itself due to the presence of protruding structures on the bottom of
the probe, the dynamics appear to be consistent with a surface consistency of
damp sand. Additionally, we find that spectral changes observed in the first
four seconds after landing are consistent with a transient dust cloud, created
by the impact of the turbulent wake behind the probe on the surface. The
optical properties of the dust particles are consistent with those of Titan
aerosols from Tomasko et al. (P&SS 56, 669). We suggest that the surface at the
landing site was covered by a dust layer, possibly the 7 mm layer of... | astro-ph_EP |
Detection of acoustic-gravity waves in lower ionosphere by VLF radio
waves: We present a new method to study harmonic waves in the low ionosphere (60 -
90 km) by detecting their effects on reflection of very low frequency (VLF)
radio waves. Our procedure is based on amplitude analysis of reflected VLF
radio waves recorded in real time, which yields an insight into the dynamics of
the ionosphere at heights where VLF radio waves are being reflected. The method
was applied to perturbations induced by the solar terminator motions at
sunrises and sunsets. The obtained results show that typical perturbation
frequencies found to exist in higher regions of the atmosphere are also present
in the lower ionosphere, which indicates a global nature of the considered
oscillations. In our model atmosphere, they turn out to be the acoustic and
gravity waves with comparatively short and long periods, respectively. | astro-ph_EP |
Proton and antiproton modulation in the heliosphere for different solar
conditions and AMS-02 measurements prediction: Galactic Cosmic Rays (GCRs) are mainly protons confined in the galactic
magnetic field to form an isotropic flux inside the galaxy. Before reaching the
Earth orbit they enter the Heliosphere and undergo diffusion, convection,
magnetic drift and adiabatic energy loss. The result is a reduction of
particles flux at low energy (below 10 GeV), called solar modulation. We
realized a quasi time-dependent 2D Stochastic Simulation of Solar Modulation
that is able to reproduce CR spectra once known the Local Interstellar Spectrum
(LIS). We were able to estimate the different behaviors associated to the
polarity dependence of the Heliospheric modulation for particles as well as for
antiparticles. We show a good agreement with the antiproton/proton ratio
measured by AMS-01, Pamela, BESS, Heat and Caprice and we performed a
prediction for the AMS-02 Experiment. | astro-ph_EP |
Particle-Particle Particle-Tree: A Direct-Tree Hybrid Scheme for
Collisional N-Body Simulations: In this paper, we present a new hybrid algorithm for the time integration of
collisional N-body systems. In this algorithm, gravitational force between two
particles is divided into short-range and long-range terms, using a
distance-dependent cutoff function. The long-range interaction is calculated
using the tree algorithm and integrated with the constant-timestep leapfrog
integrator. The short-range term is calculated directly and integrated with the
high-order Hermite scheme. We can reduce the calculation cost per orbital
period from O(N^2) to O(N log N), without significantly increasing the
long-term integration error. The results of our test simulations show that
close encounters are integrated accurately. Long-term errors of the total
energy shows random-walk behaviour, because it is dominated by the error caused
by tree approximation. | astro-ph_EP |
Hydrodynamic outcomes of planet scattering in transitional discs: A significant fraction of unstable multiple planet systems likely scatter
during the transitional disc phase as gas damping becomes ineffectual. Using an
ensemble of FARGO hydrodynamic simulations and MERCURY n-body integrations, we
directly follow planet-disc and planet-planet interactions through the clearing
phase and on through 50 Myr of dynamical evolution. Disc clearing occurs via
X-ray driven photoevaporation. The hydrodynamic evolution of individual
scattering systems is complex, and involves phases in which massive planets
orbit within eccentric gaps, or accrete directly from the disc without a gap.
Comparing the results to a gas-free model, we find that the n-body dynamics and
hydrodynamics of scattering into one- and two-planet final states are almost
identical. The eccentricity distributions in these channels are almost
unaltered by the presence of gas. The hydrodynamic simulations, however, also
form low eccentricity three-planet systems in long-term stable configurations,
and the admixture of these systems results in modestly lower eccentricities in
hydrodynamic as opposed to gas-free simulations. The incidence of these
three-planet systems is likely a function of the initial conditions; different
planet setups (number or spacing) may change the character of this result. We
analyze the properties of surviving multiple planet systems, and show that only
a small fraction (a few percent) enter mean-motion resonances after scattering,
while a larger fraction form stable resonant chains and avoid scattering
entirely. Our results remain consistent with the hypothesis that exoplanet
eccentricity results from scattering, though the detailed agreement between
observations and gas-free simulation results is likely coincidental. We discuss
the prospects for testing scattering models by observing planets or
non-axisymmetric gas structure in transitional discs. | astro-ph_EP |
Apsidal asymmetric-alignment of Jupiter Trojans: The most distant Kuiper belt objects exhibit the clustering in their orbits,
and this anomalous architecture could be caused by Planet 9 with large
eccentricity and high inclination. We then suppose that the orbital clustering
of minor planets may be observed somewhere else in the solar system. In this
paper, we consider the over 7000 Jupiter Trojans from the Minor Planet Center,
and find that they are clustered in the longitude of perihelion $\varpi$,
around the locations $\varpi_{\mbox{{J}}}+60^{\circ}$ and
$\varpi_{\mbox{{J}}}-60^{\circ}$ ($\varpi_{\mbox{{J}}}$ is the longitude of
perihelion of Jupiter) for the L4 and L5 swarms, respectively. Then we build a
Hamiltonian system to describe the associated dynamical aspects for the
co-orbital motion. The phase space displays the existence of the apsidally
aligned islands of libration centered on
$\Delta\varpi=\varpi-\varpi_{\mbox{{J}}}\approx\pm60^{\circ}$, for the
Trojan-like orbits with eccentricities $e<0.1$. Through a detailed analysis, we
have shown that the observed Jupiter Trojans with proper eccentricities
$e_p<0.1$ spend most of their time in the range of
$|\Delta\varpi|=0-120^{\circ}$, while the more eccentric ones with $e_p>0.1$
are too few to affect the orbital clustering within this $\Delta\varpi$ range
for the entire Trojan population. Our numerical results further prove that,
even starting from a uniform $\Delta\varpi$ distribution, the apsidal alignment
of simulated Trojans similar to the observation can appear on the order of the
age of the solar system. We conclude that the apsidal asymmetric-alignment of
Jupiter Trojans is robust, and this new finding can be helpful to design the
survey strategy in the future. | astro-ph_EP |
K2-106, a system containing a metal-rich planet and a planet of lower
density: Planets in the mass range from 2 to 15 MEarth are very diverse. Some of them
have low densities, while others are very dense. By measuring the masses and
radii, the mean densities, structure, and composition of the planets are
constrained. These parameters also give us important information about their
formation and evolution, and about possible processes for atmospheric loss.We
determined the masses, radii, and mean densities for the two transiting planets
orbiting K2-106. The inner planet has an ultra-short period of 0.57 days. The
period of the outer planet is 13.3 days.Although the two planets have similar
masses, their densities are very different. For K2-106b we derive
Mb=8.36-0.94+0.96 MEarh, Rb=1.52+/-0.16 REarth, and a high density of
13.1-3.6+5.4 gcm-3. For K2-106c, we find Mc=5.8-3.0+3.3 MEarth,
Rc=2.50-0.26+0.27 REarth and a relatively low density of 2.0-1.1+1.6
gcm-3.Since the system contains two planets of almost the same mass, but
different distances from the host star, it is an excellent laboratory to study
atmospheric escape. In agreement with the theory of atmospheric-loss processes,
it is likely that the outer planet has a hydrogen-dominated atmosphere. The
mass and radius of the inner planet is in agreement with theoretical models
predicting an iron core containing 80+20-30% of its mass. Such a high metal
content is surprising, particularly given that the star has an ordinary (solar)
metal abundance. We discuss various possible formation scenarios for this
unusual planet. | astro-ph_EP |
Molecular abundances and C/O ratios in chemically evolving
planet-forming disk midplanes: (Abridged) Exoplanet atmospheres are thought be built up from accretion of
gas as well as pebbles and planetesimals in the midplanes of planet-forming
disks. The chemical composition of this material is usually assumed to be
unchanged during the disk lifetime. However, chemistry can alter the relative
abundances of molecules in this planet-building material. To assess the impact
of disk chemistry during the era of planet formation, an extensive kinetic
chemistry gas-grain reaction network is utilised to evolve the abundances of
chemical species over time. Given a high level of ionisation, chemical
evolution in protoplanetary disk midplanes becomes significant after a few
times $10^{5}$ yrs, and is still ongoing by 7 Myr between the H$_{2}$O and the
O$_{2}$ icelines. Importantly, the changes in the abundances of the major
elemental carbon and oxygen-bearing molecules imply that the traditional
"stepfunction" for the C/O ratios in gas and ice in the disk midplane (as
defined by sharp changes at icelines of H$_{2}$O, CO$_{2}$ and CO) evolves over
time, and cannot be assumed fixed. In addition, at lower temperatures (< 29 K),
gaseous CO colliding with the grains gets converted into CO$_{2}$ and other
more complex ices, lowering the CO gas abundance between the O$_{2}$ and CO
thermal icelines. This effect can mimic a CO iceline at a higher temperature
than suggested by its binding energy. Chemistry in the disk midplane is
ionisation-driven, and evolves over time. In order to reliably predict the
atmospheric compositions of forming planets, as well as to relate observed
atmospheric C/O ratios of exoplanets to where and how the atmospheres have
formed in a disk midplane, chemical evolution needs to be considered and
implemented into planet formation models. | astro-ph_EP |
Ephemeris Updates for Seven Selected HATNet Survey Transiting Exoplanets: We refined the ephemeris of seven transiting exoplanets HAT-P-6b, HAT-P-12b,
HAT-P-18b, HAT-P-22b, HAT-P-32b, HAT-P-33b, and HAT-P-52b. We observed 11
transits from eight observatories in different filters for HAT-P-6b and
HAT-P-32b. Also, the Exoplanet Transit Database (ETD) observations for each of
the seven exoplanets were analyzed, and the light curves of five systems were
studied using Transiting light Exoplanet Survey Satellite (TESS) data. We used
Exofast-v1 to simulate these ground- and space-based light curves and estimate
mid-transit times. We obtained a total of 11, 175 and 67 mid-transit times for
these seven exoplanets from our observations, ETD and TESS data, respectively,
along with 155 mid-transit times from the literature. Then, we generated
transit timing variation (TTV) diagrams for each using derived mid-transit
times as well as those found in the literature. The systems' linear ephemeris
was then refined and improved using the Markov Chain Monte Carlo (MCMC) method.
All of the studied exoplanets, with the exception of the HAT-P-12b system,
displayed an increasing trend in the orbital period in the TTV diagrams. | astro-ph_EP |
Turbulence-Induced Relative Velocity of Dust particles IV: the Collision
Kernel: Motivated by its importance for modeling dust particle growth in
protoplanetary disks, we study turbulence-induced collision statistics of
inertial particles as a function of the particle friction time, tau_p. We show
that turbulent clustering significantly enhances the collision rate for
particles of similar sizes with tau_p corresponding to the inertial range of
the flow. If the friction time, tau_p,h, of the larger particle is in the
inertial range, the collision kernel per unit cross section increases with
increasing friction time, tau_p,l, of the smaller particle, and reaches the
maximum at tau_p,l = tau_p,h, where the clustering effect peaks. This feature
is not captured by the commonly-used kernel formula, which neglects the effect
of clustering. We argue that turbulent clustering helps alleviate the bouncing
barrier problem for planetesimal formation. We also investigate the collision
velocity statistics using a collision-rate weighting factor to account for
higher collision frequency for particle pairs with larger relative velocity.
For tau_p,h in the inertial range, the rms relative velocity with
collision-rate weighting is found to be invariant with tau_p,l and scales with
tau_p,h roughly as ~ tau_p,h^(1/2). The weighting factor favors collisions with
larger relative velocity, and including it leads to more destructive and less
sticking collisions. We compare two collision kernel formulations based on
spherical and cylindrical geometries. The two formulations give consistent
results for the collision rate and the collision-rate weighted statistics,
except that the spherical formulation predicts more head-on collisions than the
cylindrical formulation. | astro-ph_EP |
Formation of the warped debris disc around $β$ Pictoris: In light of the recent confirmation of an eccentric orbit giant planet,
$\beta$ Pic c, I revisit the formation and evolution of the warped debris disc
in the system. $\beta$ Pic c is interior to $\beta$ Pic b, and the debris disc
is exterior to both planets. Previous $N$-body simulations have shown that
$\beta$ Pic b is responsible for exciting the inclination of the debris disc.
With hydrodynamical simulations, I model a protoplanetary gas disc misaligned
with the planets. I find that the gas disc does not exhibit significant long
lasting inclination excitation from the planets even for the observed disc
size. The warp that is excited by the planets propagates through the entire
disc with a timescale much less than the gas disc lifetime. Therefore, the
observed warp in the debris disc must be produced after the gas disc has
dispersed. With analytical secular theory calculations, I show that two secular
resonances are exterior to $\beta$ Pic b, located at $\sim 20\, \rm au$ and
$\sim 25\, \rm au$. This agrees with my $N$-body simulations that show that
these secular resonances shape the inner edge of the $\beta$ Pic debris disc at
a radius that agrees with observations. | astro-ph_EP |
Corotation torques experienced by planets embedded in weakly magnetized
turbulent discs: The migration of low-mass planets is driven by the differential Lindblad
torque and the corotation torque in non-magnetic viscous models of
protoplanetary discs. The corotation torque has recently received detailed
attention as it may slow down, stall, or reverse migration. In laminar viscous
disc models, the long-term evolution of the corotation torque is intimately
related to viscous and thermal diffusion processes in the planet's horseshoe
region. This paper examines the properties of the corotation torque in discs
where MHD turbulence develops as a result of the magnetorotational instability,
considering a weak initial toroidal magnetic field. We present results of 3D
MHD simulations carried out with two different codes. Non-ideal MHD effects and
the disc's vertical stratification are neglected, and locally isothermal disc
models are considered. The running time-averaged torque exerted by the disc on
a fixed planet is evaluated in three disc models. We first present results with
an inner disc cavity (planet trap). As in viscous disc models, the planet is
found to experience a positive running time-averaged torque over several
hundred orbits, which highlights the existence of an unsaturated corotation
torque maintained in the long term in MHD turbulent discs. Two disc models with
initial power-law density and temperature profiles are also adopted, in which
the time-averaged torque is found to be in decent agreement with its
counterpart in laminar viscous disc models with similar viscosity at the planet
location. Detailed analysis of the averaged torque density distributions
indicates that the differential Lindblad torque takes very similar values in
MHD turbulent and laminar viscous discs, and there exists an unsaturated
corotation torque in MHD turbulent discs. This analysis also reveals the
existence of an additional corotation torque in weakly magnetized discs. | astro-ph_EP |
Formation and dynamics of water clouds on temperate sub-Neptunes: The
example of K2-18b: Hubble (HST) spectroscopic transit observations of the temperate sub-Neptune
K2-18b were interpreted as the presence of water vapour with potential water
clouds. 1D modelling studies also predict the formation of water clouds at some
conditions. However, such models cannot predict the cloud cover, driven by
atmospheric dynamics and thermal contrasts, and thus their real impact on
spectra. The main goal of this study is to understand the formation,
distribution and observational consequences of water clouds on K2-18b and other
temperate sub-Neptunes. We simulated the atmospheric dynamics, water cloud
formation and spectra of K2-18b for H2-dominated atmosphere using a 3D GCM. We
analysed the impact of atmospheric composition (with metallicity from 1*solar
to 1000*solar), concentration of cloud condensation nuclei and planetary
rotation rate. Assuming that K2-18b has a synchronous rotation, we show that
the atmospheric circulation in the upper atmosphere essentially corresponds to
a symmetric day-to-night circulation. This regime preferentially leads to cloud
formation at the substellar point or at the terminator. Clouds form for
metallicity >100*solar with relatively large particles. For 100-300*solar
metallicity, the cloud fraction at the terminators is small with a limited
impact on transit spectra. For 1000*solar metallicity, very thick clouds form
at the terminator. The cloud distribution appears very sensitive to the
concentration of CCN and to the planetary rotation rate. Fitting HST transit
data with our simulated spectra suggests a metallicity of ~100-300*solar. In
addition, we found that the cloud fraction at the terminator can be highly
variable, leading to a potential variability in transit spectra. This effect
could be common on cloudy exoplanets and could be detectable with multiple
transit observations. | astro-ph_EP |
Precise Dynamical Masses and Orbital Fits for $β$ Pic b and $β$
Pic c: We present a comprehensive orbital analysis to the exoplanets $\beta$
Pictoris b and c that resolves previously reported tensions between the
dynamical and evolutionary mass constraints on $\beta$ Pic b. We use the MCMC
orbit code orvara to fit fifteen years of radial velocities and relative
astrometry (including recent GRAVITY measurements), absolute astrometry from
Hipparcos and Gaia, and a single relative radial velocity measurement between
$\beta$ Pic A and b. We measure model-independent masses of
$9.3^{+2.6}_{-2.5}\, M_{\rm Jup}$ for $\beta$ Pic b and $8.3\pm 1.0\,M_{\rm
Jup}$ for $\beta$ Pic c. These masses are robust to modest changes to the input
data selection. We find a well-constrained eccentricity of $0.119 \pm 0.008$
for $\beta$ Pic b, and an eccentricity of $0.21^{+0.16}_{-0.09}$ for $\beta$
Pic c, with the two orbital planes aligned to within $\sim$0.5$^\circ$. Both
planets' masses are within $\sim$1$\sigma$ of the predictions of hot-start
evolutionary models and exclude cold starts. We validate our approach on
$N$-body synthetic data integrated using REBOUND. We show that orvara can
account for three-body effects in the $\beta$ Pic system down to a level
$\sim$5 times smaller than the GRAVITY uncertainties. Systematics in the masses
and orbital parameters from orvara's approximate treatment of multiplanet
orbits are a factor of $\sim$5 smaller than the uncertainties we derive here.
Future GRAVITY observations will improve the constraints on $\beta$ Pic c's
mass and (especially) eccentricity, but improved constraints on the mass of
$\beta$ Pic b will likely require years of additional RV monitoring and
improved precision from future Gaia data releases. | astro-ph_EP |
Molecules with ALMA at Planet-forming Scales (MAPS) V: CO gas
distributions: Here we present high resolution (15-24 au) observations of CO isotopologue
lines from the Molecules with ALMA on Planet-forming Scales (MAPS) ALMA Large
Program. Our analysis employs $^{13}$CO and C$^{18}$O ($J$=2-1), (1-0), and
C$^{17}$O (1-0) line observations of five protoplanetary disks. We retrieve CO
gas density distributions, using three independent methods: (1) a
thermo-chemical modeling framework based on the CO data, the broadband spectral
energy distribution, and the mm-continuum emission; (2) an empirical
temperature distribution based on optically thick CO lines; and (3) a direct
fit to the C$^{17}$O hyperfine lines. Results from these methods generally show
excellent agreement. The CO gas column density profiles of the five disks show
significant variations in the absolute value and the radial shape. Assuming a
gas-to-dust mass ratio of 100, all five disks have a global CO-to-H$_2$
abundance of 10-100 times lower than the ISM ratio. The CO gas distributions
between 150-400 au match well with models of viscous disks, supporting the
long-standing theory. CO gas gaps appear to be correlated with continuum gap
locations, but some deep continuum gaps do not have corresponding CO gaps. The
relative depths of CO and dust gaps are generally consistent with predictions
of planet-disk interactions, but some CO gaps are 5-10 times shallower than
predictions based on dust gaps. This paper is part of the MAPS special issue of
the Astrophysical Journal Supplement. | astro-ph_EP |
The Formation of the Cold Classical Kuiper Belt by a Short Range
Transport Mechanism: The Classical Kuiper Belt is populated by a group of objects with low
inclination orbits, reddish colors and usually belonging to a binary system.
This so called Cold Classical Kuiper Belt is considered to have been formed in
situ from primordial ice pebbles that coagulated into planetesimals hundreds of
kilometers in diameter. According to this scenario, the accretion of pebbles
into large planetesimals would have occurred through the streaming instability
mechanism that would be effective in the primordial Solar System disk of gas
and solids. Nevertheless other objects with the same color characteristics as
those found in the Cold Classical Kuiper Belt can be encountered also past the
2:1 mean motion resonance with Neptune as scattered or detached objects. Here I
propose a mechanism that can account for both the cold Classical Kuiper Belt
objects and other reddish objects outside the Classical Kuiper Belt. According
to the proposed scenario, reddish objects were primordially in the outer
portion of the planetesimal disk which was however truncated somewhere below 42
au. In this manner the cold Classical Kuiper Belt and its scattered / detached
counterpart were respectively transported outwards by a short range or slightly
scattered to their present locations. Resonant objects were also formed by the
same process. This mechanism is aimed at explaining the distribution of all
objects that share the same color characteristics as coming from a common
origin in the outer borders of the primordial planetesimal disk. According to
the scenario here proposed the Cold Classical Kuiper Belt would have been
formed around 4 au inside its present location with a total mass 20 to 100
times as large as its present value. | astro-ph_EP |
Star Hoppers: Planet Instability and Capture in Evolving Binary Systems: Many planets are observed in stellar binary systems, and their frequency may
be comparable to that of planetary systems around single stars. Binary stellar
evolution in such systems influences the dynamical evolution of the resident
planets. Here we study the evolution of a single planet orbiting one star in an
evolving binary system. We find that stellar evolution can trigger dynamical
instabilities that drive planets into chaotic orbits. This instability leads to
planet-star collisions, exchange of the planet between the binary stars
("star-hoppers"), and ejection of the planet from the system. The means by
which planets can be recaptured is similar to the pull-down capture mechanism
for irregular solar system satellites. Because planets often suffer close
encounters with the primary on the asymptotic giant branch, captures during a
collision with the stellar envelope are also possible. Such capture could
populate the habitable zone around white dwarfs. | astro-ph_EP |
The future lifespan of Earth's oxygenated Atmosphere: Earth's modern atmosphere is highly oxygenated and is a remotely detectable
signal of its surface biosphere. However, the lifespan of oxygen-based
biosignatures in Earth's atmosphere remains uncertain, particularly for the
distant future. Here we use a combined biogeochemistry and climate model to
examine the likely timescale of oxygen-rich atmospheric conditions on Earth.
Using a stochastic approach, we find that the mean future lifespan of Earth's
atmosphere with oxygen levels more than 1% of the present atmospheric level is
1.08+-0.14 billion years. The model projects that a deoxygenation of the
atmosphere, with atmospheric oxygen dropping sharply to levels reminiscnet of
the Archaean Earth, will most probably be triggered before the inception of
moist greenhouse conditions in Earth's climate system and before the extensive
loss of surface water from the atmosphere. We find that future deoxygenation is
an inevitable consequence of increasing solar fluxes, whereas its precise
timing is modulated by the exchange flux of reducing power between the mantle
and the ocean-atmosphere-crust system. Our results suggest that the planetary
carbonate-silicate cycle will tend to lead to terminally CO2-limited biospheres
and rapid atmospheric deoxygenation, emphasizing the need for robust
atmospheric biosignatures applicable to weakly oxygenated and anoxic exoplanet
atmospheres and highlighting the potential importance of atmospheric organic
haze during the terminal stages of planetary habitability. | astro-ph_EP |
Effects of primitive photosynthesis on Earth's early climate system: The evolution of different forms of photosynthetic life has profoundly
altered the activity level of the biosphere, radically reshaping the
composition of Earth's oceans and atmosphere over time. However, the
mechanistic impacts of a primitive photosynthetic biosphere on Earth's early
atmospheric chemistry and climate are poorly understood. Here, we use a global
redox balance model to explore the biogeochemical and climatological effects of
different forms of primitive photosynthesis. We find that a hybrid ecosystem of
H2-based and Fe-based anoxygenic photoautotrophs - organisms that perform
photosynthesis without producing oxygen - gives rise to a strong nonlinear
amplification of Earth's methane (CH4) cycle, and would thus have represented a
critical component of Earth's early climate system before the advent of
oxygenic photosynthesis. Using a Monte Carlo approach, we find that a hybrid
photosynthetic biosphere widens the range of geochemical conditions that allow
for warm climate states well beyond either of these metabolic processes acting
in isolation. Our results imply that Earth's early climate was governed by a
novel and poorly explored set of regulatory feedbacks linking the anoxic
biosphere and the coupled H, C and Fe cycles. We suggest that similar processes
should be considered when assessing the potential for sustained habitability on
Earth-like planets with reducing atmospheres. | astro-ph_EP |
Observability of molecular species in a nitrogen dominated atmosphere
for 55 Cancri e: One of the key goals of exoplanet science is the atmospheric characterisation
of super-Earths. Atmospheric abundances provide insight on the formation and
evolution of those planets and help to put our own rocky planets in context.
Observations on 55 Cancri e point towards a N-dominated atmosphere. In this
paper we explore this possibility, showing which will be the most abundant
gases and observable species in emission and transmission spectroscopy of such
an atmosphere. We use analytical arguments and observed parameters to estimate
the possible thermal profile of the atmosphere and test three different extreme
possibilities. The chemistry is calculated using equilibrium calculations and
adopting Titan's elemental abundances as a potential N-dominated atmospheric
composition. We also test the effect of different N/O ratios in the atmosphere.
Emission and transmission spectra are computed and showed with a resolution
relevant to future missions suitable to observe super-Earths (e.g. JWST,
ARIEL). We find that even though N$_2$ is the most abundant molecule in the
atmosphere followed by H$_2$ and CO, the transmission spectra shows strong
features of NH3 and HCN, and CO and HCN dominate emission spectra. We also show
that a decrease in the N/O ratio leads to stronger H2O, CO and CO2 and weaker
NH3 and HCN features. A larger N/O is also more consistent with observations.
Our exploration of a N-atmosphere for 55 Cancri e serve as a guide to
understand such atmospheres and provide a reference for future observations. | astro-ph_EP |
Detectability of biosignatures in anoxic atmospheres with the James Webb
Space Telescope: A TRAPPIST-1e case study: The James Webb Space Telescope (JWST) may be capable of finding biogenic
gases in the atmospheres of habitable exoplanets around low mass stars.
Considerable attention has been given to the detectability of biogenic oxygen,
which could be found using an ozone proxy, but ozone detection with JWST will
be extremely challenging, even for the most favorable targets. Here, we
investigate the detectability of biosignatures in anoxic atmospheres analogous
to those that likely existed on the early Earth. Arguably, such anoxic
biosignatures could be more prevalent than oxygen biosignatures if life exists
elsewhere. Specifically, we simulate JWST retrievals of TRAPPIST-1e to
determine whether the methane plus carbon dioxide disequilibrium biosignature
pair is detectable in transit transmission. We find that ~10 transits using the
Near InfraRed Spectrograph (NIRSpec) prism instrument may be sufficient to
detect carbon dioxide and constrain methane abundances sufficiently well to
rule out known, non-biological CH$_{4}$ production scenarios to ~90%
confidence. Furthermore, it might be possible to put an upper limit on carbon
monoxide abundances that would help rule out non-biological methane-production
scenarios, assuming the surface biosphere would efficiently drawdown
atmospheric CO. Our results are relatively insensitive to high altitude clouds
and instrument noise floor assumptions, although stellar heterogeneity and
variability may present challenges. | astro-ph_EP |
Can Moons Have Moons?: Each of the giant planets within the Solar System has large moons but none of
these moons have their own moons (which we call ${\it submoons}$). By analogy
with studies of moons around short-period exoplanets, we investigate the
tidal-dynamical stability of submoons. We find that 10 km-scale submoons can
only survive around large (1000 km-scale) moons on wide-separation orbits.
Tidal dissipation destabilizes the orbits of submoons around moons that are
small or too close to their host planet; this is the case for most of the Solar
System's moons. A handful of known moons are, however, capable of hosting
long-lived submoons: Saturn's moons Titan and Iapetus, Jupiter's moon Callisto,
and Earth's Moon. Based on its inferred mass and orbital separation, the
newly-discovered exomoon candidate Kepler-1625b-I can in principle host a large
submoon, although its stability depends on a number of unknown parameters. We
discuss the possible habitability of submoons and the potential for
subsubmoons. The existence, or lack thereof, of submoons, may yield important
constraints on satellite formation and evolution in planetary systems. | astro-ph_EP |
On the observed clustering of major bodies in solar and extrasolar
subsystems: Major (exo)planetary and satellite bodies seem to concentrate at intermediate
areas of the radial distributions of all the objects present in each
(sub)system. We prove rigorously that the secular evolution of (exo)planets and
satellites necessarily results in the observed intermediate accumulation of the
massive objects in all such subsystems. We quantify a "middle" as the mean of
mean motions (orbital angular velocities) of three or more massive objects
involved. Orbital evolution is expected to be halted or severely diminished
when the survivors settle near mean-motion resonances and substantial
angular-momentum transfer between bodies ceases to occur (gravitational Landau
damping). The dynamics is opposite in direction to what has been theorized for
viscous and magnetized accretion disks in which gas spreads out and away from
either side of any conceivable intermediate area. The results are bound to
change the way we think about planet and moon formation and evolution. | astro-ph_EP |
Bouncing on Titan: Motion of the Huygens Probe in the Seconds After
Landing: While landing on Titan, several instruments onboard Huygens acquired
measurements that indicate the probe did not immediately come to rest. Detailed
knowledge of the probe's motion can provide insight into the nature of Titan's
surface. Combining accelerometer data from the Huygens Atmospheric Structure
Instrument (HASI) and the Surface Science Package (SSP) with photometry data
from the Descent Imager/Spectral Radiometer (DISR) we develop a quantitative
model to describe motion of the probe, and its interaction with the surface.
The most likely scenario is the following. Upon impact, Huygens created a 12 cm
deep hole in the surface of Titan. It bounced back, out of the hole onto the
flat surface, after which it commenced a 30-40 cm long slide in the southward
direction. The slide ended with the probe out of balance, tilted in the
direction of DISR by around 10 degrees. The probe then wobbled back and forth
five times in the north-south direction, during which it probably encountered a
1-2 cm sized pebble. The SSP provides evidence for movement up to 10 s after
impact. This scenario puts the following constraints on the physical properties
of the surface. For the slide over the surface we determine a friction
coefficient of 0.4. While this value is not necessarily representative for the
surface itself due to the presence of protruding structures on the bottom of
the probe, the dynamics appear to be consistent with a surface consistency of
damp sand. Additionally, we find that spectral changes observed in the first
four seconds after landing are consistent with a transient dust cloud, created
by the impact of the turbulent wake behind the probe on the surface. The
optical properties of the dust particles are consistent with those of Titan
aerosols from Tomasko et al. (P&SS 56, 669). We suggest that the surface at the
landing site was covered by a dust layer, possibly the 7 mm layer of... | astro-ph_EP |
A statistical search for a population of Exo-Trojans in the Kepler
dataset: Trojans are small bodies in planetary Lagrangian points. In our solar system,
Jupiter has the largest number of such companions. Their existence is assumed
for exoplanetary systems as well, but none has been found so far. We present an
analysis by super-stacking $\sim4\times10^4$ Kepler planets with a total of
$\sim9\times10^5$ transits, searching for an average trojan transit dip. Our
result gives an upper limit to the average Trojan transiting area (per planet)
corresponding to one body of radius $<460$km at $2\sigma$ confidence. We find a
significant Trojan-like signal in a sub-sample for planets with more (or
larger) Trojans for periods $>$60 days. Our tentative results can and should be
checked with improved data from future missions like PLATO2.0, and can guide
planetary formation theories. | astro-ph_EP |
A Detailed Investigation of the Proposed NN Serpentis Planetary System: The post-main sequence eclipsing binary NN Serpentis was recently announced
as the potential host of at least two massive planetary companions. In that
work, the authors put forward two potential architectures that fit the
observations of the eclipsing binary with almost identical precision. In this
work, we present the results of a dynamical investigation of the orbital
stability of both proposed system architectures, finding that they are only
stable for scenarios in which the planets are locked in mutual mean motion
resonance. In the discovery work, the authors artificially fixed the orbital
eccentricity of the more massive planet, NN Ser(AB) c, at 0. Here, we reanalyse
the observational data on NN Serpentis without this artificial constraint, and
derive a new orbital solution for the two proposed planets. We detail the
results of further dynamical simulations investigating the stability of our new
orbital solution, and find that allowing a small non-zero eccentricity for the
outer planet renders the system unstable. We conclude that, although the
original orbits proposed for the NN Serpentis planetary system prove
dynamically feasible, further observations of the system are vital in order to
better constrain the system's true architecture. | astro-ph_EP |
Planets Around Low-Mass Stars (PALMS). IV. The Outer Architecture of M
Dwarf Planetary Systems: We present results from a high-contrast adaptive optics imaging search for
giant planets and brown dwarfs (>1 MJup) around 122 newly identified nearby
(<40 pc) young M dwarfs. Half of our targets are younger than 135 Myr and 90%
are younger than the Hyades (620 Myr). Our H- and K-band coronagraphic
observations with Keck/NIRC2 and Subaru/HiCIAO achieve typical contrasts of
12-14 mag and 9-13 mag at 1", respectively, which corresponds to limiting
planet masses of 0.5-10 MJup at 5-33 AU for 85% of our sample. We discovered
four young brown dwarf companions: 1RXS J235133.3+312720 B (32 $\pm$ 6 MJup;
L0$^{+2}_{-1}$; 120 $\pm$ 20 AU), GJ 3629 B (64$^{+30}_{-23}$ MJup; M7.5 $\pm$
0.5; 6.5 $\pm$ 0.5 AU), 1RXS J034231.8+121622 B (35 $\pm$ 8 MJup; L0 $\pm$ 1;
19.8 $\pm$ 0.9 AU), and 2MASS J15594729+4403595 B (43 $\pm$ 9 MJup; M8.0 $\pm$
0.5; 190 $\pm$ 20 AU). Over 150 candidate planets were identified; we obtained
follow-up imaging for 56% of these but all are consistent with background
stars. Our null detection of planets enables strong statistical constraints on
the occurrence rate of long-period giant planets around single M dwarfs. We
infer an upper limit (at the 95% confidence level) of 10.3% and 16.0% for 1-13
MJup planets between 10-100 AU for hot-start and cold-start (Fortney)
evolutionary models, respectively. Fewer than 6.0% (9.9%) of M dwarfs harbor
massive gas giants in the 5-13 MJup range like those orbiting HR 8799 and
$\beta$ Pictoris between 10-100 AU for a hot-start (cold-start) formation
scenario. Although the first directly imaged planets were found around massive
stars, there is currently no statistical evidence for a trend of giant planet
frequency with stellar host mass at large separations as predicted by the disk
instability model of giant planet formation. | astro-ph_EP |
Determination of uncertainty profiles in neutral atmospheric properties
measured by radio occultation experiments: Radio occultations are commonly used to assess remotely the thermodynamic
properties of planets or satellites' atmospheres within the solar system. The
data processing usually involves the so-called Abel inversion method or the
numerical ray-tracing technique. Both these approaches are now well
established, however, they do not allow to easily determine the uncertainty
profiles in the atmospheric properties, and this makes the results difficult to
interpret statistically. Recently, a purely analytical approach based on the
time transfer functions formalism was proposed for modeling radio occultation
data. Using this formulation, we derive uncertainty relationships between the
frequency shift and the thermodynamic properties of the neutral atmosphere such
as the temperature, pressure, and neutral number density. These expressions are
important for interpreting previous results from past radio occultation
experiments. They are especially relevant for deriving the system requirements
for future missions in a rigorous manner and consistently with the scientific
requirements about the atmospheric properties retrieval. | astro-ph_EP |
Hints on the origins of particle traps in protoplanetary disks given by
the $M_{\rm{dust}}-M_{\star}$ relation: Demographic surveys of protoplanetary disks, carried out mainly with ALMA,
have provided access to a large range of disk dust masses ($M_{\rm{dust}}$)
around stars with different stellar types and in different star-forming
regions. These surveys found a power-law relation between $M_{\rm{dust}}$ and
$M_{\star}$ that steepens in time, but which is also flatter for transition
disks (TDs). We performed dust evolution models, which included perturbations
to the gas surface density with different amplitudes to investigate the effect
of particle trapping on the $M_{\rm{dust}}-M_{\star}$ relation. These
perturbations were aimed at mimicking pressure bumps that originated from
planets. We focused on the effect caused by different stellar and disk masses
based on exoplanet statistics that demonstrate a dependence of planet mass on
stellar mass and metallicity. Models of dust evolution can reproduce the
observed $M_{\rm{dust}}-M_{\star}$ relation in different star-forming regions
when strong pressure bumps are included and when the disk mass scales with
stellar mass (case of $M_{\rm{disk}}=0.05\,M_\star$ in our models). This result
arises from dust trapping and dust growth beyond centimeter-sized grains inside
pressure bumps. However, the flatter relation of $M_{\rm{dust}}-M_{\star}$ for
TDs and disks with substructures cannot be reproduced by the models unless the
formation of boulders is inhibited inside pressure bumps. In the context of
pressure bumps originating from planets, our results agree with current
exoplanet statistics on giant planet occurrence increasing with stellar mass,
but we cannot draw a conclusion about the type of planets needed in the case of
low-mass stars. This is attributed to the fact that for $M_\star<1\,M_\odot$,
the observed $M_{\rm{dust}}$ obtained from models is very low due to the
efficient growth of dust particles beyond centimeter-sizes inside pressure
bumps. | astro-ph_EP |
The Atmospheres of Earth-like Planets after Giant Impact Events: It is now understood that the accretion of terrestrial planets naturally
involves giant collisions, the moon-forming impact being a well known example.
In the aftermath of such collisions the surface of the surviving planet is very
hot and potentially detectable. Here we explore the atmospheric chemistry,
photochemistry, and spectral signatures of post-giant-impact terrestrial
planets enveloped by thick atmospheres consisting predominantly of CO2, and
H2O. The atmospheric chemistry and structure are computed self-consistently for
atmospheres in equilibrium with hot surfaces with composition reflecting either
the bulk silicate Earth (which includes the crust, mantle, atmosphere and
oceans) or Earth's continental crust. We account for all major molecular and
atomic opacity sources including collision-induced absorption. We find that
these atmospheres are dominated by H2O and CO2, while the formation of CH4, and
NH3 is quenched due to short dynamical timescales. Other important constituents
are HF, HCl, NaCl, and SO2. These are apparent in the emerging spectra, and can
be indicative that an impact has occurred. The use of comprehensive opacities
results in spectra that are a factor of 2 lower in surface brightness in the
spectral windows than predicted by previous models. The estimated luminosities
show that the hottest post-giant-impact planets will be detectable with
near-infrared coronagraphs on the planned 30m-class telescopes. The 1-4um
region will be most favorable for such detections, offering bright features and
better contrast between the planet and a potential debris disk. We derive
cooling timescales on the order of 10^5-10^6 Myrs, based on the modeled
effective temperatures. This leads to the possibility of discovering tens of
such planets in future surveys. | astro-ph_EP |
Habitable Zones with Stable Orbits for Planets around Binary Systems: A general formulation to compute habitable zones (HZ) around binary stars is
presented. A HZ in this context must satisfy two separate conditions: a
radiative one and one of dynamical stability. For the case of single stars, the
usual concept of circumstellar habitable zone is based on the radiative
condition only, as the dynamical stability condition is taken for granted. For
the radiative condition, we extend the simple formulation of the circumstellar
habitable zone for single stars, to the case of eccentric stellar binary
systems, where two sources of luminosity at different orbital phases contribute
to the irradiance of their planetary circumstellar and circumbinary regions.
Our approach considers binaries with eccentric orbits and guarantees that
orbits in the computed habitable zone remain within it at all orbital phases.
For the dynamical stability condition, we use the approach of invariant loops
developed by Pichardo et al. 2005 to find regions of stable, non-intersecting
orbits, which is a robust method to find stable regions in binary stars, as it
is based in the existence of integrals of motion. We apply the combined
criteria to calculate HZ for 64 binary stars in the solar neighborhood with
known orbital parameters, including some with discovered planets. Formulae and
interpolating tables are provided, so the reader can compute the boundaries of
the HZ for an arbitrary binary system, using the stellar flux limits they
prefer. Together with the formulae provided for stable zones, these allow the
computation of both regions of stability and habitability around any binary
stellar system. We found 56% of the cases we consider can satisfy both
restrictions, this is a very important constriction to binary systems.
Nevertheless, we conclude that these systems where a dynamical and radiative
safe zone exists, must be considered strong candidates in the search for
habitable planets | astro-ph_EP |
The Dynamics of Dust Grains in the Outer Solar System: We study the dynamics of large dust grains >1 micron with orbits outside of
the heliosphere (beyond 250 AU). Motion of the Solar System through the
interstellar medium (ISM) at a velocity of 26 km/s subjects these particles to
gas and Coulomb drag (grains are expected to be photoelectrically charged) as
well as the Lorentz force and the electric force caused by the induction
electric field. We show that to zeroth order the combined effect of these
forces can be well described in the framework of the classical Stark problem:
particle motion in a Keplerian potential subject to an additional constant
force. Based on this analogy, we elucidate the circumstances in which the
motion becomes unbound, and show that under local ISM conditions dust grains
smaller than ~100 microns originating in the Oort Cloud (e.g. in collisions of
comets) beyond 10000 AU are ejected from the Solar System under the action of
the electric force. Orbital motion of larger, bound grains is described
analytically using the orbit-averaged Hamiltonian approach and consists of
orbital plane precession at a fixed semi-major axis, accompanied by the
periodic variations of the inclination and eccentricity (the latter may
approach unity in some cases). A more detailed analysis of the combined effect
of gas and Coulomb drag shows it is possible to reduce particle semi-major
axes, but that the degree of orbital decay is limited (a factor of several at
best) by passages through atomic and molecular clouds, which easily eject small
particles. | astro-ph_EP |
ALMA constraints on assembly of Core Accretion planets: Resolved dust continuum and CO line ALMA imaging, and in some cases detection
of H$\alpha$ emission, hint that young massive planets are abundant at wide
separations in protoplanetary discs. Here, we show how these observations can
probe the runaway phase of planetary growth in the Core Accretion theory.
Planets in this phase have the right range of masses to account for the
predominantly moderate contrast gaps and rings seen in ALMA observations.
However, we find that these planets gain mass and migrate inward very rapidly.
As a result, the phase when they could produce gaps with properties similar to
those observed is very short, i.e., $t_{\rm gap} \lesssim 0.1$~Myr,
independently of the disc viscosity parameter. This would require many tens to
hundreds of gas giant planets to be born per ALMA system, violating the
available mass budget of solids in realistic discs. This also predicts
preponderance of discs with very wide gaps or complete inner disc holes, which
is not observed. We show that suppression of both planet accretion and
migration by a factor of at least ten is a possible solution to these serious
problems. Future population synthesis models of planet formation should aim to
address both exoplanetary data of older discless planetary systems and ALMA
discs with embedded planets in one framework. | astro-ph_EP |
Evolution of the Dust Coma in Comet 67P/Churyumov-Gerasimenko Before
2009 Perihelion: Comet 67P/Churyumov-Gerasimenko is the main target of ESA's Rosetta mission
and will be encountered in May 2014. As the spacecraft shall be in orbit the
comet nucleus before and after release of the lander {\it Philae}, it is
necessary necessary to know the conditions in the coma. Study the dust
environment, including the dust production rate and its variations along its
preperihelion orbit. The comet was observed during its approach to the Sun on
four epochs between early-June 2008 and mid-January 2009, over a large range of
heliocentric distances that will be covered by the mission in 2014. An
anomalous enhancement of the coma dust density was measured towards the comet
nucleus. The scalelength of this enhancement increased with decreasing
heliocentric distance of the comet. This is interpreted as a result of an
unusually slow expansion of the dust coma. Assuming a spherical symmetric coma,
the average amount of dust as well as its ejection velocity have been derived.
The latter increases exponentially with decreasing heliocentric distance (\rh),
ranging from about 1 m/s at 3 AU to about 25-35 m/s at 1.4 AU. Based on these
results we describe the dust environment at those nucleocentric distances at
which the spacecraft will presumably be in orbit.
Astronomy and Astrophysics, in press | astro-ph_EP |
The periodic and chaotic regimes of motion in the exoplanet 2/1
mean-motion resonance: We present the dynamical structure of the phase space of the planar planetary
2/1 mean-motion resonance (MMR). Inside the resonant domain, there exist two
families of periodic orbits, one associated to the librational motion of the
critical angle ($\sigma$-family) and the other related to the circulatory
motion of the angle between the pericentres ($\Delta\varpi$-family). The
well-known apsidal corotation resonances (ACR) appear at the intersections of
these families. A complex web of secondary resonances exists also for low
eccentricities, whose strengths and positions are dependent on the individual
masses and spatial scale of the system.
Depending on initial conditions, a resonant system is found in one of the two
topologically different states, referred to as \textit{internal} and
\textit{external} resonances. The internal resonance is characterized by
symmetric ACR and its resonant angle is $2\,\lambda_2-\lambda_1-\varpi_1$,
where $\lambda_i$ and $\varpi_i$ stand for the planetary mean longitudes and
longitudes of pericentre, respectively. In contrast, the external resonance is
characterized by asymmetric ACR and the resonant angle is
$2\,\lambda_2-\lambda_1-\varpi_2$. We show that systems with more massive outer
planets always envolve inside internal resonances. The limit case is the
well-known asteroidal resonances with Jupiter. At variance, systems with more
massive inner planets may evolve in either internal or external resonances; the
internal resonances are typical for low-to-moderate eccentricity
configurations, whereas the external ones for high eccentricity configurations
of the systems. In the limit case, analogous to Kuiper belt objects in
resonances with Neptune, the systems are always in the external resonances
characterized by asymmetric equilibria. | astro-ph_EP |
The Rossiter-McLaughlin effect of CoRoT-3b & HD189733b: We present radial-velocity sequences acquired during three transits of the
exoplanet HD 189733b and one transit of the CoRoT-3b. We applied a combined
Markov-Chain Monte Carlo analysis of spectroscopic and photometric data on
these stars, to determine a full set of system parameters including the project
spin-orbit misalignement angle of HD 189733b to an unprecedented precision via
the Rossiter-McLaughlin effect: beta = 0.85 degrees (+0.32 -0.28) . This small
but non-zero inclination of the planetary orbit is important to understand the
origin of the system. On CoRoT-3b, results seem to point towards a non-zero
inclination as well with beta = 37.6 degrees (+10.0 -22.3), but this remains
marginal. Systematic effects due to non-gaussian cross-correlation functions
appear to be the main cause of significant residuals that prevent an accurate
determination of the projected stellar rotation velocity V sin(I) for both
stars. | astro-ph_EP |
Planetary Orbital Equations in Externally-Perturbed Systems: Position
and Velocity-Dependent Forces: The increasing number and variety of extrasolar planets illustrates the
importance of characterizing planetary perturbations. Planetary orbits are
typically described by physically intuitive orbital elements. Here, we
explicitly express the equations of motion of the unaveraged perturbed two-body
problem in terms of planetary orbital elements by using a generalized form of
Gauss' equations. We consider a varied set of position and velocity-dependent
perturbations, and also derive relevant specific cases of the equations: when
they are averaged over fast variables (the "adiabatic" approximation), and in
the prograde and retrograde planar cases. In each instance, we delineate the
properties of the equations. As brief demonstrations of potential applications,
we consider the effect of Galactic tides. We measure the effect on the
widest-known exoplanet orbit, Sedna-like objects, and distant scattered disk
objects, particularly with regard to where the adiabatic approximation breaks
down. The Mathematica code which can help derive the equations is freely
available upon request. | astro-ph_EP |
BEAST detection of a brown dwarf and a low-mass stellar companion around
the young bright B star HIP 81208: Recent observations from B-star Exoplanet Abundance Study (BEAST) have
illustrated the existence of sub-stellar companions around very massive stars.
In this paper, we present the detection of two lower mass companions to a
relatively nearby ($148.7^{+1.5}_{-1.3}$ pc), young ($17^{+3}_{-4}$ Myr),
bright (V=$6.632\pm0.006$ mag), $2.58\pm0.06~ M_{\odot}$ B9V star HIP 81208
residing in the Sco-Cen association, using the Spectro-Polarimetric
High-contrast Exoplanet REsearch (SPHERE) instrument at the Very Large
Telescope (VLT) in Chile. Analysis of the photometry obtained gives mass
estimates of $67^{+6}_{-7}~M_J$ for the inner companion and
$0.135^{+0.010}_{-0.013}~M_{\odot}$ for the outer companion, indicating the
former to be most likely a brown dwarf and the latter to be a low-mass star.
The system is compact but unusual, as the orbital planes of the two companions
are likely close to orthogonal. The preliminary orbital solutions we derived
for the system indicate that the star and the two companions are likely in a
Kozai resonance, rendering the system dynamically very interesting for future
studies. | astro-ph_EP |
Discovery of the first Earth-sized planets orbiting a star other than
our Sun in the Kepler-20 system: Discovering other worlds the size of our own has been a long-held dream of
astronomers. The transiting planets Kepler-20e and Kepler-20f, which belong to
a multi-planet system, hold a very special place among the many groundbreaking
discoveries of the Kepler mission because they finally realized that dream. The
radius of Kepler-20f is essentially identical to that of the Earth, while
Kepler-20e is even smaller (0.87 R[Earth]), and was the first exoplanet to earn
that distinction. Their masses, however, are too light to measure with current
instrumentation, and this has prevented their confirmation by the usual Doppler
technique that has been used so successfully to confirm many other larger
planets. To persuade themselves of the planetary nature of these tiny objects,
astronomers employed instead a statistical technique to "validate" them,
showing that the likelihood they are planets is orders of magnitude larger than
a false positive. Kepler-20e and 20f orbit their Sun-like star every 6.1 and
19.6 days, respectively, and are most likely of rocky composition. Here we
review the history of how they were found, and present an overview of the
methodology that was used to validate them. | astro-ph_EP |
Incomplete cooling down of Saturn's A ring at solar equinox: Implication
for seasonal thermal inertia and internal structure of ring particles: At the solar equinox in August 2009, the Composite Infrared Spectrometer
(CIRS) onboard Cassini showed the lowest Saturn's ring temperatures ever
observed. Detailed radiative transfer models show that the observed equinox
temperatures of Saturn's A ring are much higher than model predictions as long
as only the flux from Saturn is taken into account. This indicates that the A
ring was not completely cooled down at the equinox. We develop a simple
seasonal model for ring temperatures and first assume that the internal density
and the thermal inertia of a ring particle are uniform with depth. The particle
size is estimated to be 1-2 m. The seasonal thermal inertia is found to be
30-50 Jm$^{-2}$K$^{-1}$s$^{-1/2}$ in the middle A ring whereas it is $\sim$ 10
Jm$^{-2}$K$^{-1}$s$^{-1/2}$ or as low as the diurnal thermal inertia in the
inner and outermost regions of the A ring. An additional internal structure
model, in which a particle has a high density core surrounded by a fluffy
regolith mantle, shows that the core radius relative to the particle radius is
about 0.9 for the middle A ring and is much less for the inner and outer
regions of the A ring. This means that the radial variation of the internal
density of ring particles exists across the A ring. Some mechanisms may be
confining dense particles in the middle A ring against viscous diffusion.
Alternatively, the (middle) A ring might have recently formed ($<$ 10$^{8}$ yr)
by destruction of an icy satellite, so that dense particles have not yet
diffused over the A ring and regolith mantles of particles have not grown
thick. Our model results also indicate that the composition of the core is
predominantly water ice, not rock. | astro-ph_EP |
Extrasolar planets and brown dwarfs around A--F type stars. VIII. A
giant planet orbiting the young star HD113337: In the frame of the search for extrasolar planets and brown dwarfs around
early-type main-sequence stars, we present the detection of a giant planet
around the young F-type star HD113337. We estimated the age of the system to be
150 +100/-50 Myr. Interestingly, an IR excess attributed to a cold debris disk
was previously detected on this star. The SOPHIE spectrograph on the 1.93m
telescope at Observatoire de Haute-Provence was used to obtain ~300 spectra
over 6 years. We used our SAFIR tool, dedicated to the spectra analysis of A
and F stars, to derive the radial velocity variations. The data reveal a 324.0
+1.7/-3.3 days period that we attribute to a giant planet with a minimum mass
of 2.83 +- 0.24 Mjup in an eccentric orbit with e=0.46 +- 0.04. A long-term
quadratic drift, that we assign to be probably of stellar origin, is
superimposed to the Keplerian solution. | astro-ph_EP |
Transit least-squares survey IV. Earth-like transiting planets expected
from the PLATO mission: In its long-duration observation phase, the PLATO satellite will observe two
non-overlapping fields for a total of 4 yr. The exact duration of each pointing
will be determined 2 yr before launch. Previous estimates of PLATO's yield of
Earth-sized planets in the habitable zones (HZs) around solar-type stars ranged
between 6 and 280. We use the PLATO Solar-like Light curve Simulator (PSLS) to
simulate light curves with transiting planets around bright (m_V > 11) Sun-like
stars at a cadence of 25 s, roughly representative of the >15,000 targets in
PLATO's high-priority P1 sample (mostly F5-K7 dwarfs and sub-dwarfs). Our study
includes light curves generated from synchronous observations of 6, 12, 18, and
24 of PLATO's 12 cm aperture cameras over both 2 yr and 3 yr of continuous
observations. Automated detrending is done with the Wotan software and
post-detrending transit detection is performed with the Transit Least Squares
(TLS) algorithm. We scale the true positive rates (TPRs) with the expected
number of stars in the P1 sample and with modern estimates of the exoplanet
occurrence rates and predict the detection of planets with 0.5 R_E <= R_p <=
1.5 R_E in the HZs around F5-K7 dwarf stars. For the (2 yr + 2 yr)
long-duration observation phase strategy we predict 11-34 detections, for the
(3 yr + 1 yr) strategy we predict 8-25 discoveries. Our study of the effects of
stellar variability on shallow transits of Earth-like planets illustrates that
our estimates of PLATO's planet yield, which we derive using a photometrically
quiet star like the Sun, must be seen as upper limits. In conclusion, PLATO's
detection of about a dozen Earth-sized planets in the HZs around solar-type
stars will mean a major contribution to this yet poorly sampled part of the
exoplanet parameter space with Earth-like planets. | astro-ph_EP |
Hydrodynamics of Embedded Planets' First Atmospheres. II. A Rapid
Recycling of Atmospheric Gas: Following Paper I we investigate the properties of atmospheres that form
around small protoplanets embedded in a protoplanetary disc by conducting
hydrodynamical simulations. These are now extended to three dimensions,
employing a spherical grid centred on the planet. Compression of gas is shown
to reduce rotational motions. Contrasting the 2D case, no clear boundary
demarcates bound atmospheric gas from disc material; instead, we find an open
system where gas enters the Bondi sphere at high latitudes and leaves through
the midplane regions, or, vice versa, when the disc gas rotates sub-Keplerian.
The simulations do not converge to a time-independent solution; instead, the
atmosphere is characterized by a time-varying velocity field. Of particular
interest is the timescale to replenish the atmosphere by nebular gas,
$t_\mathrm{replenish}$. It is shown that the replenishment rate,
$M_\mathrm{atm}/t_\mathrm{replenish}$, can be understood in terms of a modified
Bondi accretion rate,
$\sim$$R_\mathrm{Bondi}^2\rho_\mathrm{gas}v_\mathrm{Bondi}$, where
$v_\mathrm{Bondi}$ is set by the Keplerian shear or the magnitude of the
sub-Keplerian motion of the gas, whichever is larger. In the inner disk, the
atmosphere of embedded protoplanets replenishes on a timescale that is shorter
than the Kelvin-Helmholtz contraction (or cooling) timescale. As a result,
atmospheric gas can no longer contract and the growth of these atmospheres
terminates. Future work must confirm whether these findings continue to apply
when the (thermodynamical) idealizations employed in this study are relaxed.
But if shown to be broadly applicable, replenishment of atmospheric gas
provides a natural explanation for the preponderance of gas-rich but
rock-dominant planets like super-Earths and mini-Neptunes. | astro-ph_EP |
The optical transmission spectrum of the hot Jupiter HAT-P-32b: clouds
explain the absence of broad spectral features?: We report Gemini-North GMOS observations of the inflated hot Jupiter
HAT-P-32b during two primary transits. We simultaneously observed two
comparison stars and used differential spectro-photometry to produce
multi-wavelength light curves. 'White' light curves and 29 'spectral' light
curves were extracted for each transit and analysed to refine the system
parameters and produce transmission spectra from 520-930nm in ~14nm bins. The
light curves contain time-varying white noise as well as time-correlated noise,
and we used a Gaussian process model to fit this complex noise model. Common
mode corrections derived from the white light curve fits were applied to the
spectral light curves which significantly improved our precision, reaching
typical uncertainties in the transit depth of ~2x10^-4, corresponding to about
half a pressure scale height. The low resolution transmission spectra are
consistent with a featureless model, and we can confidently rule out broad
features larger than about one scale height. The absence of Na/K wings or
prominent TiO/VO features is most easily explained by grey absorption from
clouds in the upper atmosphere, masking the spectral features. However, we
cannot confidently rule out clear atmosphere models with low abundances (~10^-3
solar) of TiO, VO or even metal hydrides masking the Na and K wings. A smaller
scale height or ionisation could also contribute to muted spectral features,
but alone are unable to to account for the absence of features reported here. | astro-ph_EP |
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