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The Origin of Chondrules: Constraints from Matrix-Chondrule
Complementarity: One of the major unresolved problems in cosmochemistry is the origin of
chondrules, once molten, spherical silicate droplets with diameters of 0.2 to 2
mm. Chondrules are an essential component of primitive meteorites and perhaps
of all early solar system materials including the terrestrial planets. Numerous
hypotheses have been proposed for their origin. Many carbonaceous chondrites
are composed of about equal amounts of chondrules and fine-grained matrix.
Recent data confirm that matrix in carbonaceous chondrites has high Si/Mg and
Fe/Mg ratios when compared to bulk carbonaceous chondrites with solar abundance
ratios. Chondrules have the opposite signature, low Si/Mg and Fe/Mg ratios. In
some carbonaceous chondrites chondrules have low Al/Ti ratios, matrix has the
opposite signature and the bulk is chondritic. It is shown in detail that these
complementary relationships cannot have evolved on the parent asteroid(s) of
carbonaceous chondrites. They reflect preaccretionary processes. Both
chondrules and matrix must have formed from a single, solar-like reservoir.
Consequences of complementarity for chondrule formation models are discussed.
An independent origin and/or random mixing of chondrules and matrix can be
excluded. Hence, complementarity is a strong constraint for all
astrophysical-cosmochemical models of chondrule formation. Although chondrules
and matrix formed from a single reservoir, the chondrule-matrix system was open
to the addition of oxygen and other gaseous components. | 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 |
AKARI/IRC Near-Infrared Asteroid Spectroscopic Survey: AcuA-spec: Knowledge of water in the solar system is important for understanding of a
wide range of evolutionary processes and the thermal history of the solar
system. To explore the existence of water in the solar system, it is
indispensable to investigate hydrated minerals and/or water ice on asteroids.
These water-related materials show absorption features in the 3-$\micron$ band
(wavelengths from 2.7 to 3.1 $\micron$). We conducted a spectroscopic survey of
asteroids in the 3-$\micron$ band using the Infrared Camera (IRC) on board the
Japanese infrared satellite AKARI. In the warm mission period of AKARI, 147
pointed observations were performed for 66 asteroids in the grism mode for
wavelengths from 2.5 to 5 $\micron$. According to these observations, most
C-complex asteroids have clear absorption features ($> 10\%$ with respect to
the continuum) related to hydrated minerals at a peak wavelength of
approximately 2.75 $\micron$, while S-complex asteroids have no significant
feature in this wavelength range. The present data are released to the public
as the Asteroid Catalog using AKARI Spectroscopic Observations (AcuA-spec). | astro-ph_EP |
Transit of Exomoon Plasma Tori: New Diagnosis: In the solar system, moons largely exceed planets in number. The Kepler
database has been shown to be sensitive to exomoon detection down to the mass
of Mars, but the first search has been unsuccessful. Here, we use a
particles-in-cell code to predict the transit of the plasma torus produced by a
satellite. Despite the small size of a moon, the spatial extent of its plasma
torus can be large enough to produce substantial transit absorptions. The model
is used for the interpretation of Hubble Space Telescope early ingress
absorptions apparently observed during WASP-12b and HD 189733b UV transits for
which no consistent explanation exists. For HD 189733b an exomoon transiting
$\sim 16$ $R_p$ ahead of the planet and loading $\sim 10^{29}$ C II ions/s into
space is required to explain the tentative early ingress absorption observed
for C II. For WASP-12b, a moon transiting $\sim 6$ $R_p$ ahead from the planet
and ejecting $\sim 10^{28}$ Mg II ions per second is required to explain the
NUV early ingress absorption feature. Interestingly, both HD 189733b and
WASP-12b predicted satellites are outside the Hill sphere of their planets, an
indication that the moons, if present, were not formed in situ but probably
captured later. Finally, our simulations show a strong electromagnetic coupling
between the polar regions of planets and the orbital position of the moons, an
expected outcome of the unipolar induction DC circuit model. Future
observations should test our predictions with a potential opportunity to
unambiguously detect the first exomoon plasma torus. | astro-ph_EP |
Tidal spin down rates of homogeneous triaxial viscoelastic bodies: We use numerical simulations to measure the sensitivity of the tidal spin
down rate of a homogeneous triaxial ellipsoid to its axis ratios by comparing
the drift rate in orbital semi-major axis to that of a spherical body with the
same mass, volume and simulated rheology. We use a mass-spring model
approximating a viscoelastic body spinning around its shortest body axis, with
spin aligned with orbital spin axis, and in circular orbit about a point mass.
The torque or drift rate can be estimated from that predicted for a sphere with
equivalent volume if multiplied by $0.5 (1 + b^4/a^4)(b/a)^{-4/3}
(c/a)^{-\alpha_c}$ where $b/a$ and $c/a$ are the body axis ratios and index
$\alpha_c \approx 1.05$ is consistent with the random lattice mass spring model
simulations but $\alpha_c = 4/3$ suggested by scaling estimates.
A homogeneous body with axis ratios 0.5 and and 0.8, like Haumea, has orbital
semi-major axis drift rate about twice as fast as a spherical body with the
same mass, volume and material properties. A simulation approximating a mostly
rocky body but with 20\% of its mass as ice concentrated at its ends has a
drift rate 10 times faster than the equivalent homogeneous rocky sphere.
However, this increase in drift rate is not enough to allow Haumea's satellite,
Hi'iaka, to have tidally drifted away from Haumea to its current orbital
semi-major axis. | astro-ph_EP |
The secondary eclipse of the transiting exoplanet CoRoT-2b: We present a study of the light curve of the transiting exoplanet CoRoT-2b,
aimed at detecting the secondary eclipse and measuring its depth. The data were
obtained with the CoRoT satellite during its first run of more than 140 days.
After filtering the low frequencies with a pre-whitening technique, we detect a
0.0060$\pm$0.0020% secondary eclipse centered on the orbital phase
0.494$\pm$0.006. Assuming a black-body emission of the planet, we estimate a
surface brightness temperature of T$_{\rm p,CoRoT}$=1910$^{+90}_{-100}$ K. We
provide the planet's equilibrium temperature and re-distribution factors as a
function of the unknown amount of reflected light. The upper limit for the
geometric albedo is 0.12. The detected secondary is the shallowest ever found. | astro-ph_EP |
Extreme asteroids in the Pan-STARRS 1 Survey: Using the first 18 months of the Pan-STARRS 1 survey we have identified 33
candidate high-amplitude objects for follow-up observations and carried out
observations of 22 asteroids. 4 of the observed objects were found to have
observed amplitude $A_{obs}\geq 1.0$ mag. We find that these high amplitude
objects are most simply explained by single rubble pile objects with some
density-dependent internal strength, allowing them to resist mass shedding even
at their highly elongated shapes. 3 further objects although below the cut-off
for 'high-amplitude' had a combination of elongation and rotation period which
also may require internal cohesive strength, depending on the density of the
body. We find that none of the 'high-amplitude asteroids' identified here
require any unusual cohesive strengths to resist rotational fission. 3
asteroids were sufficiently observed to allow for shape and spin pole models to
be determined through light curve inversion. 45864 was determined to have
retrograde rotation with spin pole axes $\lambda=218\pm 10^{\circ},
\beta=-82\pm 5^{\circ}$ and asteroid 206167 was found to have best fit spin
pole axes $\lambda= 57 \pm 5^{\circ}$, $\beta=-67 \pm 5^{\circ}$. An additional
object not initially measured with $A_{obs}>1.0$ mag, 49257, was determined to
have a shape model which does suggest a high-amplitude object. Its spin pole
axes were best fit for values $\lambda=112\pm 6^{\circ}, \beta=6\pm 5^{\circ}$.
In the course of this project to date no large super-fast rotators ($P_{rot} <
2.2$ h) have been identified. | astro-ph_EP |
The late accretion and erosion of Vesta's crust recorded by eucrites and
diogenites as an astrochemical window into the formation of Jupiter and the
early evolution of the Solar System: For decades the limited thickness of Vesta's basaltic crust, revealed by the
link between the asteroid and the howardite-eucrite-diogenite family of
meteorites, and its survival to collisional erosion offered an important
constraint for the study of the early evolution of the Solar System. Some
results of the Dawn mission, however, cast doubts on our understanding of
Vesta's interior composition and of the characteristics of its basaltic crust,
weakening this classical constraint. In this work we investigate the late
accretion and erosion experienced by Vesta's crust after its differentiation
and recorded in the composition of eucrites and diogenites and show that it
offers an astrochemical window into the earliest evolution of the Solar System.
In our proof-of-concept case study focusing on the late accretion and erosion
of Vesta's crust during the growth and migration of Jupiter, the water
enrichment of eucrites appears to be a sensitive function of Jupiter's
migration while the enrichment in highly-siderophile elements of diogenites
appears to be particularly sensitive to the size-frequency distribution of the
planetesimals. The picture depicted by the enrichments created by late
accretion in eucrites and diogenites is not qualitatively affected by the
uncertainty on the primordial mass of Vesta. Crustal erosion, instead, is more
significantly affected by said uncertainty and Vesta's crust survival appears
to be mainly useful to study violent collisional scenarios where highly
energetic impacts can strip significant amounts of vestan material while
limitedly contributing to Vesta's late accretion. Our results suggest that the
astrochemical record of the late accretion and erosion of Vesta's crust
provided by eucrites and diogenites can be used as a tool to investigate any
process or scenario associated to the evolution of primordial Vesta and of the
early Solar System. | astro-ph_EP |
Turbulence in the TW Hya Disk: Turbulence is a fundamental parameter in models of grain growth during the
early stages of planet formation. As such, observational constraints on its
magnitude are crucial. Here we self-consistently analyze ALMA CO(2-1), SMA
CO(3-2), and SMA CO(6-5) observations of the disk around TW Hya and find an
upper limit on the turbulent broadening of $<$0.08c$_s$ ($\alpha<$0.007 for
$\alpha$ defined only within 2-3 pressure scale heights above the midplane),
lower than the tentative detection previously found from an analysis of the
CO(2-1) data. We examine in detail the challenges of image plane fitting vs
directly fitting the visibilities, while also considering the role of the
vertical temperature gradient, systematic uncertainty in the amplitude
calibration, and assumptions about the CO abundance, as potential sources of
the discrepancy in the turbulence measurements. These tests result in
variations of the turbulence limit between $<$0.04c$_s$ and $<$0.13c$_s$,
consistently lower than the 0.2-0.4c$_s$ found previously. Having ruled out
numerous factors, we restrict the source of the discrepancy to our assumed
coupling between temperature and density through hydrostatic equilibrium in the
presence of a vertical temperature gradient and/or the confinement of CO to a
thin molecular layer above the midplane, although further work is needed to
quantify the influence of these prescriptions. Assumptions about hydrostatic
equilibrium and the CO distribution are physically motivated, and may have a
small influence on measuring the kinematics of the gas, but they become
important when constraining small effects such as the strength of the
turbulence within a protoplanetary disk. | astro-ph_EP |
A Substellar Companion in a 1.3 yr Nearly-circular Orbit of HD 16760: We report the detection of a substellar companion orbiting the G5 dwarf HD
16760 from the N2K sample. Precise Doppler measurements of the star from Subaru
and Keck revealed a Keplerian velocity variation with a period of 466.47+-0.35
d, a semiamplitude of 407.71+-0.84 m/s, and an eccentricity of 0.084+-0.003.
Adopting a stellar mass of 0.78+-0.05 M_Sun, we obtain a minimum mass for the
companion of 13.13+-0.56 M_JUP, which is close to the planet/brown-dwarf
transition, and the semimajor axis of 1.084+-0.023 AU. The nearly circular
orbit despite the large mass and intermediate orbital period makes this
companion unique among known substellar companions. | astro-ph_EP |
Performance of near-infrared high-contrast imaging methods with JWST
from commissioning: The James Webb Space Telescope (JWST) will revolutionize the field of
high-contrast imaging and enable both the direct detection of Saturn-mass
planets and the characterization of substellar companions in the mid-infrared.
While JWST will feature unprecedented sensitivity, angular resolution will be
the key factor when competing with ground-based telescopes. Here, we aim to
characterize the performance of several extreme angular resolution imaging
techniques available with JWST in the 3-5 micron regime based on data taken
during commissioning. Firstly, we introduce custom tools to simulate, reduce,
and analyze NIRCam and MIRI coronagraphy data and use these tools to extract
companion detection limits from on-sky NIRCam round and bar mask coronagraphy
observations. Secondly, we present on-sky NIRISS aperture masking
interferometry (AMI) and kernel phase imaging (KPI) observations from which we
extract companion detection limits using the publicly available fouriever tool.
Scaled to a total integration time of one hour and a target of the brightness
of AB Dor, we find that NIRISS AMI and KPI reach contrasts of $\sim$7-8 mag at
$\sim$70 mas and $\sim$9 mag at $\sim$200 mas. Beyond $\sim$250 mas, NIRCam
coronagraphy reaches deeper contrasts of $\sim$13 mag at $\sim$500 mas and
$\sim$15 mag at $\sim$2 arcsec. While the bar mask performs $\sim$1 mag better
than the round mask at small angular separations $\lesssim$0.75 arcsec, it is
the other way around at large angular separations $\gtrsim$1.5 arcsec.
Moreover, the round mask gives access to the full 360 deg field-of-view which
is beneficial for the search of new companions. We conclude that already during
the instrument commissioning, JWST high-contrast imaging in the L- and M-bands
performs close to its predicted limits. | astro-ph_EP |
The secondary eclipse of CoRoT-1b: The transiting planet CoRoT-1b is thought to belong to the pM-class of
planets, in which the thermal emission dominates in the optical wavelengths. We
present a detection of its secondary eclipse in the CoRoT white channel data,
whose response function goes from ~400 to ~1000 nm. We used two different
filtering approaches, and several methods to evaluate the significance of a
detection of the secondary eclipse. We detect a secondary eclipse centered
within 20 min at the expected times for a circular orbit, with a depth of
0.016+/-0.006%. The center of the eclipse is translated in a 1-sigma upper
limit to the planet's eccentricity of ecosomega<0.014. Under the assumption of
a zero Bond Albedo and blackbody emission from the planet, it corresponds to a
T_{CoRoT}=2330 +120-140 K. We provide the equilibrium temperatures of the
planet as a function of the amount of reflected light. If the planet is in
thermal equilibrium with the incident flux from the star, our results imply an
inefficient transport mechanism of the flux from the day to the night sides. | astro-ph_EP |
A Lyman-alpha transit left undetected: the environment and atmospheric
behavior of K2-25b: K2-25b is a Neptune-sized exoplanet (3.45 Earth radii) that orbits its M4.5
host with a period of 3.48 days. Due to its membership in the Hyades Cluster,
the system has a known age (727 +/- 75 Myr). K2-25b's youth and its
similarities with Gl 436b suggested that K2-25b could be undergoing strong
atmospheric escape. We observed two transits of K2-25b at Lyman-alpha using
HST/STIS in order to search for escaping neutral hydrogen. We were unable to
detect an exospheric signature, but placed an upper limit of (R_p/R_s) < 0.56
at 95% confidence by fitting the light curve of the Lyman-alpha red-wing, or <
1.20 in the blue-wing. We reconstructed the intrinsic Lyman-alpha profile of
K2-25 to determine its Lyman-alpha flux, and analyzed XMM-Newton observations
to determined its X-ray flux. Based on the total X-ray and extreme ultraviolet
irradiation of the planet (8763 +/- 1049 erg/s/cm^2), we estimated the maximum
energy-limited mass loss rate of K2-25b to be 10.6 x 10^10 g/s (0.56 Earth
masses per 1 Gyr), five times larger than the similarly estimated mass loss
rate of Gl 436b (2.2 x 10^10 g/s). The photoionization time is about 3 hours,
significantly shorter than Gl 436b's 14 hours. A non-detection of a Lyman-alpha
transit could suggest K2-25b is not significantly losing its atmosphere, or
factors of the system are resulting in the mass loss being unobservable (e.g.,
atmosphere composition or the system's large high energy flux). Further
observations could provide more stringent constraints. | astro-ph_EP |
Vital Signs: Seismology of ocean worlds: Ice-covered ocean worlds possess diverse energy sources and associated
mechanisms that are capable of driving significant seismic activity, but to
date no measurements of their seismic activity have been obtained. Such
investigations could probe their transport properties and radial structures,
with possibilities for locating and characterizing trapped liquids that may
host life and yielding critical constraints on redox fluxes, and thus on
habitability. Modeling efforts have examined seismic sources from tectonic
fracturing and impacts. Here, we describe other possible seismic sources, their
associations with science questions constraining habitability, and the
feasibility of implementing such investigations. We argue, by analogy with the
Moon, that detectable seismic activity on tidally flexed ocean worlds should
occur frequently. Their ices fracture more easily than rocks, and dissipate
more tidal energy than the <1 GW of the Moon and Mars. Icy ocean worlds also
should create less thermal noise for a due to their greater distance and
consequently smaller diurnal temperature variations. They also lack substantial
atmospheres (except in the case of Titan) that would create additional noise.
Thus, seismic experiments could be less complex and less susceptible to noise
than prior or planned planetary seismology investigations of the Moon or Mars. | astro-ph_EP |
Seismometer Detection of Dust Devil Vortices by Ground Tilt: We report seismic signals on a desert playa caused by convective vortices and
dust devils. The long-period (10-100s) signatures, with tilts of ~10$^{-7}$
radians, are correlated with the presence of vortices, detected with nearby
sensors as sharp temporary pressure drops (0.2-1 mbar) and solar obscuration by
dust. We show that the shape and amplitude of the signals, manifesting
primarily as horizontal accelerations, can be modeled approximately with a
simple quasi-static point-load model of the negative pressure field associated
with the vortices acting on the ground as an elastic half space. We suggest the
load imposed by a dust devil of diameter D and core pressure {\Delta}Po is
~({\pi}/2){\Delta}PoD$^2$, or for a typical terrestrial devil of 5 m diameter
and 2 mbar, about the weight of a small car. The tilt depends on the inverse
square of distance, and on the elastic properties of the ground, and the large
signals we observe are in part due to the relatively soft playa sediment and
the shallow installation of the instrument. Ground tilt may be a particularly
sensitive means of detecting dust devils. The simple point-load model fails for
large dust devils at short ranges, but more elaborate models incorporating the
work of Sorrells (1971) may explain some of the more complex features in such
cases, taking the vortex winds and ground velocity into account. We discuss
some implications for the InSight mission to Mars. | astro-ph_EP |
Asteroid 4 Vesta: dynamical and collisional evolution during the Late
Heavy Bombardment: Vesta is the only currently identified asteroid for which we possess samples,
which revealed us that the asteroid is differentiated and possesses a
relatively thin basaltic crust that survived to the evolution of the asteroid
belt and the Solar System. However, little is know about the effects of past
events like the Late Heavy Bombardment on this crust. We address this gap in
our knowledge by simulating the LHB in the different dynamical scenarios
proposed for the migration of the giant planets in the broad framework of the
Nice Model. The results of simulations generate information about produced
crater population, surface saturation, mass loss and mass gain of Vesta and
number of energetic or catastrophic impacts during LHB. Our results reveal that
planet-planet scattering is a dynamically favourable migration mechanism for
the survival of Vesta and its crust. The number of impacts on Vesta estimated
as due to the LHB is $31\pm5$, i.e. about 5 times larger than the number of
impacts that would have occurred in an unperturbed main belt in the same time
interval. The contribution of a possible extended belt, instead, is quite
limited and can be quantified in $2\pm1$ impacts. The chance of energetic and
catastrophic impacts is less than 10\% and is compatible with the absence of
giant craters dated back to 4 Ga ago and with the survival of the asteroid
during the LHB. The mass loss translates in the erosion of $3-5$ meters of the
crust, consistently with the global survival of the basaltic crust of Vesta
confirmed by the Dawn mission. Our analysis revealed that the contribution of
the LHB to the cratering of Vesta' surface is not significant and is actually
erased by the crater population produced by the following 4 Ga of collisional
evolution of the asteroid, in agreement with the data provided by the Dawn
mission. | astro-ph_EP |
Resonance locking in giant planets indicated by the rapid orbital
expansion of Titan: Tidal effects in planetary systems are the main driver in the orbital
migration of natural satellites. They result from physical processes occurring
deep inside celestial bodies, whose effects are rarely observable from surface
imaging. For giant planet systems, the tidal migration rate is determined by
poorly understood dissipative processes in the planet, and standard theories
suggest an orbital expansion rate inversely proportional to the power 11/2 in
distance, implying little migration for outer moons such as Saturn's largest
moon, Titan. Here, we use two independent measurements obtained with the
Cassini spacecraft to measure Titan's orbital expansion rate. We find Titan
migrates away from Saturn at 11.3 $\pm$ 2.0 cm/year, corresponding to a tidal
quality factor of Saturn of Q $\simeq$ 100, and a migration timescale of
roughly 10 Gyr. This rapid orbital expansion suggests Titan formed
significantly closer to Saturn and has migrated outward to its current
position. Our results for Titan and five other moons agree with the predictions
of a resonance locking tidal theory, sustained by excitation of inertial waves
inside the planet. The associated tidal expansion is only weakly sensitive to
orbital distance, motivating a revision of the evolutionary history of Saturn's
moon system. The resonance locking mechanism could operate in other systems
such as stellar binaries and exoplanet systems, and it may allow for tidal
dissipation to occur at larger orbital separations than previously believed. | astro-ph_EP |
Four Small Planets Buried in K2 Systems: What Can We Learn for TESS?: The Kepler, K2, and Transiting Exoplanet Survey Satellite (TESS) missions
have provided a wealth of confirmed exoplanets, benefiting from a huge effort
from the planet-hunting and follow-up community. With careful systematics
mitigation, these missions provide precise photometric time series, which
enable detection of transiting exoplanet signals. However, exoplanet hunting
can be confounded by several factors, including instrumental noise, search
biases, and host star variability. In this Letter, we discuss strategies to
overcome these challenges using newly emerging techniques and tools. We
demonstrate the power of new, fast open-source community tools (e.g.,
lightkurve, starry, celerite, exoplanet), and discuss four high signal-to-noise
ratio (S/N) exoplanets that showcase specific challenges present in planet
detection: K2-43c, K2-168c, K2-198c, and K2-198d. These planets have been
undetected in several large K2 planet searches, despite having transit signals
with S/N > 10. Two of the planets discussed here are new discoveries. In this
work we confirm all four as true planets. Alongside these planet systems, we
discuss three key challenges in finding small transiting exoplanets. The aim of
this Letter is to help new researchers understand where planet detection
efficiency gains can be made, and to encourage the continued use of K2 archive
data. The considerations presented in this Letter are equally applicable to
Kepler, K2, and TESS, and the tools discussed here are available for the
community to apply to improve exoplanet discovery and fitting. | astro-ph_EP |
NGTS-13b: A hot 4.8 Jupiter-mass planet transiting a subgiant star: We report the discovery of the massive hot Jupiter NGTS-13b by the Next
Generation Transit Survey (NGTS). The V = 12.7 host star is likely in the
subgiant evolutionary phase with log g$_{*}$ = 4.04 $\pm$ 0.05, T$_{eff}$ =
5819 $\pm$ 73 K, M$_{*}$ = 1.30$^{+0.11}_{-0.18}$ M$_{\odot}$, and R$_{*}$ =
1.79 $\pm$ 0.06 R$_{\odot}$. NGTS detected a transiting planet with a period of
P = 4.12 days around the star, which was later validated with the Transiting
Exoplanet Survey Satellite (TESS; TIC 454069765). We confirm the planet using
radial velocities from the CORALIE spectrograph. Using NGTS and TESS full-frame
image photometry combined with CORALIE radial velocities we determine NGTS-13b
to have a radius of R$_{P}$ = 1.142 $\pm$ 0.046 R$_{Jup}$, mass of M$_{P}$ =
4.84 $\pm$ 0.44 M$_{Jup}$ and eccentricity e = 0.086 $\pm$ 0.034. Some previous
studies suggest that $\sim$4 M$_{Jup}$ may be a border between two separate
formation scenarios (e.g., core accretion and disk instability) and that
massive giant planets share similar formation mechanisms as lower-mass brown
dwarfs. NGTS-13b is just above 4 M$_{Jup}$ making it an important addition to
the statistical sample needed to understand the differences between various
classes of substellar companions. The high metallicity, [Fe/H] = 0.25 $\pm$
0.17, of NGTS-13 does not support previous suggestions that massive giants are
found preferentially around lower metallicity host stars, but NGTS-13b does
support findings that more massive and evolved hosts may have a higher
occurrence of close-in massive planets than lower-mass unevolved stars. | astro-ph_EP |
The Complex History of Trojan Asteroids: The Trojan asteroids provide a unique perspective on the history of Solar
System. As a large population of small bodies, they record important
gravitational interactions and dynamical evolution of the Solar System. In the
past decade, significant advances have been made in understanding physical
properties, and there has been a revolution in thinking about the origin of
Trojans. The ice and organics generally presumed to be a significant part of
Trojan compositions have yet to be detected directly, though low density of the
binary system Patroclus (and possibly low density of the binary/moonlet system
Hektor) is consistent with an interior ice component. By contrast, fine-grained
silicates that appear to be similar to cometary silicates in composition have
been detected, and a color bimodality may indicate distinct compositional
groups among the Trojans. Whereas Trojans had traditionally been thought to
have formed near 5 AU, a new paradigm has developed in which the Trojans formed
in the proto-Kuiper Belt, and they were scattered inward and captured in the
Trojan swarms as a result of resonant interactions of the giant planets.
Whereas the orbital and population distributions of current Trojans are
consistent with this origin scenario, there are significant differences between
current physical properties of Trojans and those of Kuiper Belt objects. These
differences may be indicative of surface modification due to the inward
migration of objects that became the Trojans, but understanding of appropriate
modification mechanisms is poor and would benefit from additional laboratory
studies. Many open questions remain, and the future promises significant
strides in our understanding of Trojans. The time is ripe for a spacecraft
mission to the Trojans, to turn these objects into geologic worlds that can be
studied in detail to unravel their complex history. | astro-ph_EP |
Radiation hydrodynamical models of the inner rim in protoplanetary disks: Many stars host planets orbiting within a few astronomical units (AU). The
occurrence rate and distributions of masses and orbits vary greatly with the
host stars mass. These close planets origins are a mystery that motivates
investigating protoplanetary disks central regions. A key factor governing the
conditions near the star is the silicate sublimation front, which largely
determines where the starlight is absorbed, and which is often called the inner
rim. We present the first radiation hydrodynamical modeling of the sublimation
front in the disks around the young intermediate-mass stars called Herbig Ae
stars. The models are axisymmetric, and include starlight heating, silicate
grains sublimating and condensing to equilibrium at the local, time-dependent
temperature and density, and accretion stresses parametrizing the results of
MHD magneto-rotational turbulence models. The results compare well with
radiation hydrostatic solutions, and prove to be dynamically stable. Passing
the model disks into Monte Carlo radiative transfer calculations, we show that
the models satisfy observational constraints on the inner rims location. A
small optically-thin halo of hot dust naturally arises between the inner rim
and the star. The inner rim has a substantial radial extent, corresponding to
several disk scale heights. While the fronts overall position varies with the
stellar luminosity, its radial extent depends on the mass accretion rate. A
pressure maximum develops near the location of thermal ionization at
temperatures about 1000 K. The pressure maximum is capable of halting solid
pebbles radial drift and concentrating them in a zone where temperatures are
sufficiently high for annealing to form crystalline silicates. | astro-ph_EP |
The GAPS Programme at TNG XXXVIII. Five molecules in the atmosphere of
the warm giant planet WASP-69b detected at high spectral resolution: The field of exo-atmospheric characterisation is progressing at an
extraordinary pace. Atmospheric observations are now available for tens of
exoplanets, mainly hot and warm inflated gas giants, and new molecular species
continue to be detected revealing a richer atmospheric composition than
previously expected. Thanks to its warm equilibrium temperature (963$\pm$18~K)
and low-density (0.219$\pm$0.031~g cm$^{-3}$), the close-in gas giant WASP-69b
represents a golden target for atmospheric characterization. With the aim of
searching for molecules in the atmosphere of WASP-69b and investigating its
properties, we performed high-resolution transmission spectroscopy with the
GIANO-B near-infrared spectrograph at the Telescopio Nazionale Galileo. We
observed three transit events of WASP-69b. During a transit, the planetary
lines are Doppler-shifted due to the large change in the planet's radial
velocity, allowing us to separate the planetary signal from the
quasi-stationary telluric and stellar spectrum. Considering the three nights
together, we report the detection of CH$_4$, NH$_3$, CO, C$_2$H$_2$, and
H$_2$O, at more than $3.3\sigma$ level. We did not identify the presence of HCN
and CO$_2$ with confidence level higher than 3$\sigma$. This is the first time
that five molecules are simultaneously detected in the atmosphere of a warm
giant planet. These results suggest that the atmosphere of WASP-69b is possibly
carbon-rich and characterised by the presence of disequilibrium chemistry. | astro-ph_EP |
Preparing an unsupervised massive analysis of SPHERE high contrast data
with the PACO algorithm: We aim at searching for exoplanets on the whole ESO/VLT-SPHERE archive with
improved and unsupervised data analysis algorithm that could allow to detect
massive giant planets at 5 au. To prepare, test and optimize our approach, we
gathered a sample of twenty four solar-type stars observed with SPHERE using
angular and spectral differential imaging modes. We use PACO, a new generation
algorithm recently developed, that has been shown to outperform classical
methods. We also improve the SPHERE pre-reduction pipeline, and optimize the
outputs of PACO to enhance the detection performance. We develop custom built
spectral prior libraries to optimize the detection capability of the ASDI mode
for both IRDIS and IFS. Compared to previous works conducted with more
classical algorithms than PACO, the contrast limits we derived are more
reliable and significantly better, especially at short angular separations
where a gain by a factor ten is obtained between 0.2 and 0.5 arcsec. Under good
observing conditions, planets down to 5 MJup, orbiting at 5 au could be
detected around stars within 60 parsec. We identified two exoplanet candidates
that require follow-up to test for common proper motion. In this work, we
demonstrated on a small sample the benefits of PACO in terms of achievable
contrast and of control of the confidence levels. Besides, we have developed
custom tools to take full benefits of this algorithm and to quantity the total
error budget on the estimated astrometry and photometry. This work paves the
way towards an end-to-end, homogeneous, and unsupervised massive re-reduction
of archival direct imaging surveys in the quest of new exoJupiters. | astro-ph_EP |
Super-Eccentric Migrating Jupiters: An important class of formation theories for hot Jupiters involves the
excitation of extreme orbital eccentricity (e=0.99 or even larger) followed by
tidal dissipation at periastron passage that eventually circularizes the
planetary orbit at a period less than 10 days. In a steady state, this
mechanism requires the existence of a significant population of super-eccentric
(e>0.9) migrating Jupiters with long orbital periods and periastron distances
of only a few stellar radii. For these super-eccentric planets, the periastron
is fixed due to conservation of orbital angular momentum and the energy
dissipated per orbit is constant, implying that the rate of change in
semi-major axis a is \dot a \propto a^0.5 and consequently the number
distribution satisfies dN/dlog a\propto a^0.5. If this formation process
produces most hot Jupiters, Kepler should detect several super-eccentric
migrating progenitors of hot Jupiters, allowing for a test of high-eccentricity
migration scenarios. | astro-ph_EP |
Stellar wind effects on the atmospheres of close-in giants: a possible
reduction in escape instead of increased erosion: The atmospheres of highly irradiated exoplanets are observed to undergo
hydrodynamic escape. However, due to strong pressures, stellar winds can
confine planetary atmospheres, reducing their escape. Here, we investigate
under which conditions atmospheric escape of close-in giants could be confined
by the large pressure of their host star's winds. For that, we simulate escape
in planets at a range of orbital distances ([0.04, 0.14] au), planetary
gravities ([36%, 87%] of Jupiter's gravity), and ages ([1, 6.9] Gyr). For each
of these simulations, we calculate the ram pressure of these escaping
atmospheres and compare them to the expected stellar wind external pressure to
determine whether a given atmosphere is confined or not. We show that, although
younger close-in giants should experience higher levels of atmospheric escape,
due to higher stellar irradiation, stellar winds are also stronger at young
ages, potentially reducing escape of young exoplanets. Regardless of the age,
we also find that there is always a region in our parameter space where
atmospheric escape is confined, preferably occurring at higher planetary
gravities and orbital distances. We investigate confinement of some known
exoplanets and find that the atmosphere of several of them, including pi Men c,
should be confined by the winds of their host stars, thus potentially
preventing escape in highly irradiated planets. Thus, the lack of hydrogen
escape recently reported for pi Men c could be caused by the stellar wind. | astro-ph_EP |
Meta-modelling the climate of dry tide locked rocky planets: Rocky planets hosted by close-in extrasolar systems are likely to be tidally
locked in 1:1 spin-orbit resonance, a configuration where they exhibit
permanent dayside and nightside. Because of the resulting day-night temperature
gradient, the climate and large-scale circulation of these planets are strongly
determined by their atmospheric stability against collapse, which designates
the runaway condensation of greenhouse gases. To better constrain the surface
conditions of rocky planets located in the habitable zone of their host star,
it is therefore crucial to elucidate the mechanisms that govern the day-night
heat redistribution. As a first attempt to bridge the gap between multiple
modelling approaches ranging from idealised models to 3-D General Circulation
Models (GCM), we developed a General Circulation Meta-Model (GCMM) able to
reproduce both the closed-form solutions provided by analytical models and the
numerical solutions obtained from GCM simulations. We used this approach to
characterise the atmospheric stability of Earth-sized rocky planets with dry
atmospheres containing CO2, and we benchmarked it against 3-D GCM simulations
using THOR GCM. We observe that the collapse pressure below which collapse
occurs can vary by ~40% around the value predicted by analytical scaling laws
depending on the mechanisms taken into account among radiative transfer,
atmospheric dynamics, and turbulent diffusion. Particularly, we find (i) that
the turbulent diffusion taking place in the dayside planetary boundary layer
(PBL) globally tends to warm up the nightside surface hemisphere except in the
transition zone between optically thin and optically thick regimes, (ii) that
the PBL also significantly affects the day-night advection timescale, and (iii)
that the slow rotator approximation holds from the moment that the normalised
equatorial Rossby deformation radius is greater than 2. | astro-ph_EP |
Polarized microwave emission from space particles in the upper
atmosphere of the Earth: Tons of space particles enter the Earth atmosphere every year, being detected
when they produce fireballs, meteor showers, or when they impact the Earth
surface. Particle detection in the showers could also be attempted from space
using satellites in low Earth orbit. Measuring the polarization would provide
extra crucial information on the dominant alignment mechanisms and the
properties of the meteor families. In this article, we evaluate the expected
signal to aid in the design of space probes for this purpose. We have used the
RADMC-3D code to simulate the polarized microwave emission of aligned dust
particles with different compositions: silicates, carbonates and irons. We have
assumed a constant spatial particle density distribution of 0.22 cm$^{-3}$,
based on particle density measurements carried during meteor showers. Four
different grain size distributions with power indices ranging from $-3.5$ to
$-2.0$ and dust particles with radius ranging from 0.01 $\mathrm{\mu}$m to 1 cm
have been considered for the simulations. Silicates and carbonates align their
minor axis with the direction of the solar radiation field; during the flight
time into the Earth atmosphere, iron grains get oriented with the Earth's
magnetic field depending on their size. Alignment direction is reflected in the
$Q$-Stokes parameter and in the polarization variation along the orbit.
Polarization depends on the composition and on the size distribution of the
particles. The simulations show that some specific particle populations might
be detectable even with a small probe equipped with high sensitivity,
photon-counting microwave detectors operating in low Earth orbit. | astro-ph_EP |
Complete Tidal Evolution of Pluto-Charon: Both Pluto and its satellite Charon have rotation rates synchronous with
their orbital mean motion. This is the theoretical end point of tidal evolution
where transfer of angular momentum has ceased. Here we follow Pluto's tidal
evolution from an initial state having the current total angular momentum of
the system but with Charon in an eccentric orbit with semimajor axis $a \approx
4R_P$ (where $R_P$ is the radius of Pluto), consistent with its impact origin.
Two tidal models are used, where the tidal dissipation function $Q \propto$
1/frequency and $Q=$ constant, where details of the evolution are strongly
model dependent. The inclusion of the gravitational harmonic coefficient
$C_{22}$ of both bodies in the analysis allows smooth, self consistent
evolution to the dual synchronous state, whereas its omission frustrates
successful evolution in some cases. The zonal harmonic $J_2$ can also be
included, but does not cause a significant effect on the overall evolution. The
ratio of dissipation in Charon to that in Pluto controls the behavior of the
orbital eccentricity, where a judicious choice leads to a nearly constant
eccentricity until the final approach to dual synchronous rotation. The tidal
models are complete in the sense that every nuance of tidal evolution is
realized while conserving total angular momentum - including temporary capture
into spin-orbit resonances as Charon's spin decreases and damped librations
about the same. | astro-ph_EP |
Statistical-likelihood Exo-Planetary Habitability Index (SEPHI): A new Statistical-likelihood Exo-Planetary Habitability Index (SEPHI) is
presented. It has been developed to cover the current and future features
required for a classification scheme disentangling whether any discovered
exoplanet is potentially habitable compared with life on Earth. The SEPHI uses
likelihood functions to estimate the habitability potential. It is defined as
the geometric mean of four sub-indexes related with four comparison criteria:
Is the planet telluric?; Does it have an atmosphere dense enough and a gravity
compatible with life?; Does it have liquid water on its surface?; Does it have
a magnetic field shielding its surface from harmful radiation and stellar
winds?. Only with seven physical characteristics, can the SEPHI be estimated:
Planetary mass, radius, and orbital period; stellar mass, radius, and effective
temperature; planetary system age. We have applied the SEPHI to all the planets
in the Exoplanet Encyclopaedia using a Monte Carlo Method. Kepler-1229 b,
Kepler-186 f, and Kepler-442 b have the largest SEPHI values assuming certain
physical descriptions. Kepler-1229 b is the most unexpected planet in this
privileged position since no previous study pointed to this planet as a
potentially interesting and habitable one. In addition, most of the tidally
locked Earth-like planets present a weak magnetic field, incompatible with
habitability potential. We must stress that our results are linked to the
physics used in this study. Any change in the physics used only implies an
updating of the likelihood functions. We have developed a web application
allowing the on-line estimation of the SEPHI: http://sephi.azurewebsites.net/ | astro-ph_EP |
XX. CoRoT-20b: A very high density, high eccentricity transiting giant
planet: We report the discovery by the CoRoT space mission of a new giant planet,
CoRoT-20b. The planet has a mass of 4.24 +/- 0.23 MJ and a radius of 0.84 +/-
0.04 RJ. With a mean density of 8.87 +/- 1.10 g/cm^3, it is among the most
compact planets known so far. Evolution models for the planet suggest a mass of
heavy elements of the order of 800 ME if embedded in a central core, requiring
a revision either of the planet formation models or of planet evolution and
structure models. We note however that smaller amounts of heavy elements are
expected from more realistic models in which they are mixed throughout the
envelope. The planet orbits a G-type star with an orbital period of 9.24 days
and an eccentricity of 0.56. The star's projected rotational velocity is vsini
= 4.5 +/- 1.0 km/s, corresponding to a spin period of 11.5 +/- 3.1 days if its
axis of rotation is perpendicular to the orbital plane. In the framework of
Darwinian theories and neglecting stellar magnetic breaking, we calculate the
tidal evolution of the system and show that CoRoT-20b is presently one of the
very few Darwin-stable planets that is evolving towards a triple synchronous
state with equality of the orbital, planetary and stellar spin periods. | astro-ph_EP |
Impact Erosion Model for Gravity-Dominated Planetesimals: Disruptive collisions have been regarded as an important process for planet
formation, while non-disruptive, small-scale collisions (hereafter called
erosive collisions) have been underestimated or neglected by many studies.
However, recent studies have suggested that erosive collisions are also
important to the growth of planets, because they are much more frequent than
disruptive collisions. Although the thresholds of the specific impact energy
for disruptive collisions (Q_RD^*) have been investigated well, there is no
reliable model for erosive collisions. In this study, we systematically carried
out impact simulations of gravity-dominated planetesimals for a wide range of
specific impact energy (Q_R) from disruptive collisions (Q_R ~ Q_RD^*) to
erosive ones (Q_R << Q_RD^*) using the smoothed particle hydrodynamics method.
We found that the ejected mass normalized by the total mass (M_ej/M_tot)
depends on the numerical resolution, the target radius (R_tar) and the impact
velocity (v_imp), as well as on Q_R, but that it can be nicely scaled by Q_RD^*
for the parameter ranges investigated (R_tar = 30-300 km, v_imp = 2-5 km/s).
This means that M_ej/M_tot depends only on Q_R/Q_RD^* in these parameter
ranges. We confirmed that the collision outcomes for much less erosive
collisions (Q_R < 0.01 Q_RD^*) converge to the results of an impact onto a
planar target for various impact angles and that M_ej/M_tot = C * QR/QRD*
holds. For disruptive collisions (Q_R ~ Q_RD^*), the curvature of the target
has a significant effect on Mej/Mtot. We also examined the angle-averaged value
of M_ej/M_tot and found that the numerically obtained relation between
angle-averaged M_ej/M_tot and Q_R/Q_RD^* is very similar to the cases for
45-degree impacts. We proposed a new erosion model based on our numerical
simulations for future research on planet formation with collisional erosion. | astro-ph_EP |
Listening to the gravitational wave sound of circumbinary exoplanets: To date more than 3500 exoplanets have been discovered orbiting a large
variety of stars. Due to the sensitivity limits of the currently used detection
techniques, these planets populate zones restricted either to the solar
neighbourhood or towards the Galactic bulge. This selection problem prevents us
from unveiling the true Galactic planetary population and is not set to change
for the next two decades. Here we present a new detection method that overcomes
this issue and that will allow us to detect gas giant exoplanets using
gravitational wave astronomy. We show that the Laser Interferometer Space
Antenna (LISA) mission can characterise hundreds of new circumbinary exoplanets
orbiting white dwarf binaries everywhere in our Galaxy - a population of
exoplanets so far completely unprobed - as well as detecting extragalactic
bound exoplanets in the Magellanic Clouds. Such a method is not limited by
stellar activity and, in extremely favourable cases, will allow LISA to detect
super-Earths down to 10 Earth masses. | astro-ph_EP |
Modeling Indications of Technology in Planetary Transit Light Curves --
Dark-side illumination: We analyze potential effects of an extraterrestrial civilization's use of
orbiting mirrors to illuminate the dark side of a synchronously rotating planet
on planetary transit light curves. Previous efforts to detect civilizations
based on side effects of planetary-scale engineering have focused on structures
affecting the host star output (e.g. Dyson spheres). However, younger
civilizations are likely to be less advanced in their engineering efforts, yet
still capable of sending small spacecraft into orbit. Since M dwarfs are the
most common type of star in the solar neighborhood, it seems plausible that
many of the nearest habitable planets orbit dim, low-mass M stars, and will be
in synchronous rotation. Logically, a civilization evolving on such a planet
may be inspired to illuminate their planet's dark side by placing a single
large mirror at the L2 Lagrangian point, or launching a fleet of small thin
mirrors into planetary orbit. We briefly examine the requirements and
engineering challenges of such a collection of orbiting mirrors, then explore
their impact on transit light curves. We incorporate stellar limb darkening and
model a simplistic mirror fleet's effects for transits of Earth-like (R = 0.5
to 2 R_Earth) planets which would be synchronously rotating for orbits within
the habitable zone of their host star. Although such an installation is
undetectable in Kepler data, JWST will provide the sensitivity necessary to
detect a fleet of mirrors orbiting Earth-like habitable planets around nearby
stars. | astro-ph_EP |
Constraining High Speed Winds in Exoplanet Atmospheres Through
Observations of Anomalous Doppler Shifts During Transit: Three-dimensional (3-D) dynamical models of hot Jupiter atmospheres predict
very strong wind speeds. For tidally locked hot Jupiters, winds at high
altitude in the planet's atmosphere advect heat from the day side to the cooler
night side of the planet. Net wind speeds on the order of 1-10 km/s directed
towards the night side of the planet are predicted at mbar pressures, which is
the approximate pressure level probed by transmission spectroscopy. These winds
should result in an observed blue shift of spectral lines in transmission on
the order of the wind speed. Indeed, Snellen et al. (2010) recently observed a
2 +/- 1 km/s blue shift of CO transmission features for HD 209458b, which has
been interpreted as a detection of the day-to-night winds that have been
predicted by 3-D atmospheric dynamics modeling. Here we present the results of
a coupled 3-D atmospheric dynamics and transmission spectrum model, which
predicts the Doppler-shifted spectrum of a hot Jupiter during transit resulting
from winds in the planet's atmosphere. We explore four different models for the
hot Jupiter atmosphere using different prescriptions for atmospheric drag via
interaction with planetary magnetic fields. We find that models with no
magnetic drag produce net Doppler blue shifts in the transmission spectrum of
~2 km/s and that lower Doppler shifts of ~1 km/s are found for the higher drag
cases, results consistent with -- but not yet strongly constrained by -- the
Snellen et al. (2010) measurement. We additionally explore the possibility of
recovering the average terminator wind speed as a function of altitude by
measuring Doppler shifts of individual spectral lines and spatially resolving
wind speeds across the leading and trailing terminators during ingress and
egress. | astro-ph_EP |
An enhanced slope in the transmission spectrum of the hot Jupiter
WASP-104b: We present the optical transmission spectrum of the hot Jupiter WASP-104b
based on one transit observed by the blue and red channels of the DBSP
spectrograph at the Palomar 200-inch telescope and 14 transits observed by the
MuSCAT2 four-channel imager at the 1.52 m Telescopio Carlos Sanchez. We also
analyse 45 additional K2 transits, after correcting for the flux contamination
from a companion star. Together with the transit light curves acquired by DBSP
and MuSCAT2, we are able to revise the system parameters and orbital ephemeris,
confirming that no transit timing variations exist. Our DBSP and MuSCAT2
combined transmission spectrum reveals an enhanced slope at wavelengths shorter
than 630 nm and suggests the presence of a cloud deck at longer wavelengths.
While the Bayesian spectral retrieval analyses favour a hazy atmosphere,
stellar spot contamination cannot be completely ruled out. Further evidence,
from transmission spectroscopy and detailed characterisation of the host star's
activity, is required to distinguish the physical origin of the enhanced slope. | astro-ph_EP |
Where does Titan Sand Come From: Insight from Mechanical Properties of
Titan Sand Candidates: Extensive equatorial linear dunes exist on Titan, but the origin of the sand,
which appears to be organic, is unknown. We used nanoindentation to study the
mechanical properties of a few Titan sand candidates, several natural sands on
Earth, and common materials used in the Titan Wind Tunnel, to understand the
mobility of Titan sand. We measured the elastic modulus (E), hardness (H), and
fracture toughness (Kc) of these materials. Tholin's elastic modulus
(10.4+/-0.5 GPa) and hardness (0.53+/-0.03 GPa) are both an order of magnitude
smaller than silicate sand, and is also smaller than the mechanically weak
white gypsum sand. With a magnitude smaller fracture toughness
(Kc=0.036+/-0.007 MPa-m^(1/2)), tholin is also much more brittle than silicate
sand. This indicates that Titan sand should be derived close to the equatorial
regions where the current dunes are located, because tholin is too soft and
brittle to be transported for long distances. | astro-ph_EP |
Computing Apparent Planetary Magnitudes for The Astronomical Almanac: Improved equations for computing planetary magnitudes are reported. These
formulas model V-band observations acquired from the time of the earliest
filter photometry in the 1950s up to the present era. The new equations
incorporate several terms that have not previously been used for generating
physical ephemerides. These include the rotation and revolution angles of Mars,
the sub-solar and sub-Earth latitudes of Uranus, and the secular time
dependence of Neptune. Formulas for use in The Astronomical Almanac cover the
planetary phase angles visible from Earth. Supplementary equations cover those
phase angles beyond the geocentric limits. Geocentric magnitudes were computed
over a span of at least 50 years and the results were statistically analyzed.
The mean, variation and extreme magnitudes for each planet are reported. Other
bands besides V on the Johnson-Cousins and Sloan photometric systems are
briefly discussed. The planetary magnitude data products available from the
U.S. Naval Observatory are also listed. An appendix describes source code and
test data sets that are available on-line for computing planetary magnitudes
according to the equations and circumstances given in this paper. The files are
posted as supplementary material for this paper. They are also available at
SourceForge under project
https://sourceforge.net/projects/planetary-magnitudes/ under the 'Files' tab in
the folder 'Ap_Mag_Current_Version'. | astro-ph_EP |
Q-type asteroids: Possibility of non-fresh weathered surfaces: Itokawa particles, which are the recovered samples from the S-complex
asteroid 25143 Itokawa by the Hayabusa spacecraft, demonstrate that S-complex
asteroids are parent bodies of ordinary chondrite meteorites. Furthermore, they
clarify that the space weathering age of the Itokawa surface is of the order of
several thousand years. Traditionally, Q-type asteroids have been considered
fresh-surfaced. However, as the space weathering timescale is approximately
three orders of magnitude lesser than the conventionally considered age, the
previously proposed formation mechanisms of Q-type asteroids cannot
sufficiently explain the surface refreshening. In this study, we propose a new
hypothesis on the surface state of Q-type asteroids: Q-type asteroids have a
non-fresh weathered surface with a paucity of fine particles. For verifying
this hypothesis, laboratory experiments on the space weathering of ordinary
chondrites are performed. Based on the results of these experiments, we found
that large (more than 100 {\mu}m) ordinary chondritic particles with space
weathering exhibit spectra consistent with Q-type asteroids. | astro-ph_EP |
An N-body Integrator for Gravitating Planetary Rings, and the Outer Edge
of Saturn's B Ring: A new symplectic N-body integrator is introduced, one designed to calculate
the global 360 degree evolution of a self-gravitating planetary ring that is in
orbit about an oblate planet. This freely-available code is called epi_int, and
it is distinct from other such codes in its use of streamlines to calculate the
effects of ring self-gravity. The great advantage of this approach is that the
perturbing forces arise from smooth wires of ring matter rather than discreet
particles, so there is very little gravitational scattering and so only a
modest number of particles are needed to simulate, say, the scalloped edge of a
resonantly confined ring or the propagation of spiral density waves.
The code is applied to the outer edge of Saturn's B ring, and a comparison of
Cassini measurements of the ring's forced response to simulations of Mimas'
resonant perturbations reveals that the B ring's surface density at its outer
edge is 195+-60 gm/cm^2 which, if the same everywhere across the ring would
mean that the B ring's mass is about 90% of Mimas' mass.
Cassini observations show that the B ring-edge has several free normal modes,
which are long-lived disturbances of the ring-edge that are not driven by any
known satellite resonances. Although the mechanism that excites or sustains
these normal modes is unknown, we can plant such a disturbance at a simulated
ring's edge, and find that these modes persist without any damping for more
than ~10^5 orbits or ~100 yrs despite the simulated ring's viscosity of 100
cm^2/sec. These simulations also indicate that impulsive disturbances at a ring
can excite long-lived normal modes, which suggests that an impact in the recent
past by perhaps a cloud of cometary debris might have excited these
disturbances which are quite common to many of Saturn's sharp-edged rings. | astro-ph_EP |
Gap carving by a migrating planet embedded in a massive debris disc: When considering gaps in debris discs, a typical approach is to invoke
clearing by an unseen planet within the gap, and derive the planet mass using
Wisdom overlap or Hill radius arguments. However, this approach can be invalid
if the disc is massive, because this clearing would also cause planet
migration. This could result in a calculated planet mass that is incompatible
with the inferred disc mass, because the predicted planet would in reality be
too small to carve the gap without significant migration. We investigate the
gap that a single embedded planet would carve in a massive debris disc. We show
that a degeneracy is introduced, whereby an observed gap could be carved by two
different planets: either a high-mass, barely-migrating planet, or a smaller
planet that clears debris as it migrates. We find that, depending on disc mass,
there is a minimum possible gap width that an embedded planet could carve
(because smaller planets, rather than carving a smaller gap, would actually
migrate through the disc and clear a wider region). We provide simple formulae
for the planet-to-debris disc mass ratio at which planet migration becomes
important, the gap width that an embedded planet would carve in a massive
debris disc, and the interaction timescale. We also apply our results to
various systems, and in particular show that the disc of HD 107146 can be
reasonably well-reproduced with a migrating, embedded planet. Finally, we
discuss the importance of planet-debris disc interactions as a tool for
constraining debris disc masses. | astro-ph_EP |
Investigating the Temperature Distribution of Diatomic Carbon in Comets
using the Swan Bands: We present high spectral-resolution observations of comets 122P/de Vico and
153P/Ikeya-Zhang obtained with the Tull Coud\'{e} spectrograph on the 2.7m
Harlan J. Smith telescope of McDonald Observatory. We used these data to study
the distribution of the lines of the $\mathrm{d} ^3\Pi_g - \mathrm{a} ^3\Pi_u$
C$_2$ (Swan) bands. We show that the data are best represented with two
rotational temperatures, with the lowest energy lines being at a relatively
cool temperature and the higher energy lines being at a higher temperature. We
discuss the implications of this two temperature distribution and suggest
future work. | astro-ph_EP |
Capture of interstellar objects II: by the Solar system: Capture of interstellar objects (ISOs) into the Solar system is dominated by
ISOs with asymptotic incoming speeds $v_\infty<4\,$km\,s$^{-1}$. The capture
rate is proportional to the ISO phase-space density in the Solar vicinity and
does not vary along the Sun's Galactic orbit, i.e.\ is not enhanced during a
passage through a cloud of ISOs (in contrast to previous suggestions). Most
bound orbits crossing those of Jupiter and Saturn are fully mixed with unbound
phase space, implying that they hold the same ISO phase-space density. Assuming
an interstellar number density $n_{iso}\sim0.1\,$au$^{-3}$, we estimate that in
1000 years the planets capture $\sim2$ ISOs (while $\sim17$ fall into the Sun),
resulting in a population of $\sim8$ captured ISOs within 5\,au of the Sun at
any time, less than the number of visiting ISOs passing through the same volume
on hyperbolic orbits. In terms of phase-space volume, capture onto and ejection
from the Solar system are equal, such that on average ISOs will not remain
captive at $a\lesssim2000\,$au for extensive periods. | astro-ph_EP |
Behaviour of electron content in the ionospheric D-region during solar
X-ray flares: One of the most important parameters in ionospheric plasma research also
having a wide practical application in wireless satellite telecommunications is
the total electron content (TEC) representing the columnal electron number
density. The F region with high electron density provides the biggest
contribution to TEC while the relatively weakly ionized plasma of the D region
(60 km - 90 km above Earths surface) is often considered as a negligible cause
of satellite signal disturbances. However, sudden intensive ionization
processes like those induced by solar X ray flares can cause relative increases
of electron density that are significantly larger in the D-region than in
regions at higher altitudes. Therefore, one cannot exclude a priori the D
region from investigations of ionospheric influences on propagation of
electromagnetic signals emitted by satellites. We discuss here this problem
which has not been sufficiently treated in literature so far. The obtained
results are based on data collected from the D region monitoring by very low
frequency radio waves and on vertical TEC calculations from the Global
Navigation Satellite System (GNSS) signal analyses, and they show noticeable
variations in the D region electron content (TECD) during activity of a solar X
ray flare (it rises by a factor of 136 in the considered case) when TECD
contribution to TEC can reach several percent and which cannot be neglected in
practical applications like global positioning procedures by satellites. | astro-ph_EP |
Layer formation in a stably-stratified fluid cooled from above. Towards
an analog for Jupiter and other gas giants: In 1D evolution models of gas giant planets, an outer convection zone
advances into the interior as the surface cools, and multiple convective layers
form beneath that convective front. To study layer formation below an outer
convection zone in a similar scenario, we investigate the evolution of a
stably-stratified fluid with a linear composition gradient that is constantly
being cooled from above. We use the Boussinesq approximation in a series of 2D
simulations at low and high Prandtl numbers ($\mathrm{Pr} = 0.5$ and 7),
initialized with constant temperature everywhere, and cooled at different
rates. We find that multiple convective layers form at $\mathrm{Pr} = 7$, {as
the result of an instability in the} diffusive thermal boundary layer below the
outer convection zone. At low Pr, layers do not form because the temperature
gradient within the boundary layer is much smaller than at large Pr and,
consequently, is not large enough to overcome the stabilizing effect of the
composition gradient. For the stratification used in this study, on the
long-term the composition gradient is an ineffective barrier against the
propagation of the outer convection zone and the entire fluid becomes
fully-mixed, whether layers form or not. Our results challenge 1D evolutionary
models of gas giant planets, which predict that layers are long-lived and that
the outer convective envelope stops advancing inwards. We discuss what is
needed for future work to build more realistic models. | astro-ph_EP |
The Runaway Greenhouse Effect on Hycean Worlds: Hycean worlds are a proposed subset of sub-Neptune exoplanets with
substantial water inventories, liquid surface oceans and extended
hydrogen-dominated atmospheres that could be favourable for habitability. In
this work, we aim to quantitatively define the inner edge of the Hycean
habitable zone using a 1D radiative-convective model. As a limiting case, we
model a dry hydrogen-helium envelope above a surface ocean. We find that 10 to
20 bars of atmosphere produces enough greenhouse effect to drive a liquid
surface ocean supercritical when forced with current Earth-like instellation.
Introducing water vapour into the atmosphere, we show the runaway greenhouse
instellation limit is greatly reduced due to the presence of superadiabatic
layers where convection is inhibited. This moves the inner edge of the
habitable zone from $\approx$ 1 AU for a G-star to 1.6 AU (3.85 AU) for a
Hycean world with a H$_2$-He inventory of 1 bar (10 bar). For an M-star, the
inner edge is equivalently moved from 0.17 AU to 0.28 AU (0.54 AU). Our results
suggest that most of the current Hycean world observational targets are not
likely to sustain a liquid water ocean. We present an analytical framework for
interpreting our results, finding that the maximum possible OLR scales
approximately inversely with the dry mass inventory of the atmosphere. We
discuss the possible limitations of our 1D modelling and recommend the use of
3D convection-resolving models to explore the robustness of superadiabatic
layers. | astro-ph_EP |
Effects of an eccentric inner Jupiter on the dynamical evolution of icy
body reservoirs in a planetary scattering scenario: We analyze the process of planetary scattering around M0-type stars. To do
this, we carry out N-body simulations with three Jupiter-mass planets close to
their instability limit together with an outer planetesimal disk. This paper
focuses on the analysis of systems in which a single Jupiter-mass planet
survives after the dynamical instability event. The small body reservoirs show
different dynamical behaviors. In fact, our simulations produce particles on
prograde and retrograde orbits, as well as particles whose orbital plane flips
from prograde to retrograde and back again along their evolution. Such
particles are called "Type-F particles". We find strong correlations between
the inclination $i$ and the ascending node longitude $\Omega$ of such
particles. First, $\Omega$ librates around 90$^{\circ}$ or/and 270$^{\circ}$.
This property is very important since it represents a necessary and sufficient
condition for the flipping of an orbit. Moreover, the libration periods of $i$
and $\Omega$ are equal and they are out to phase by a quarter period. We also
remark that the larger the libration amplitude of $i$, the larger the libration
amplitude of $\Omega$. Finally, we analyze the initial conditions of Type-F
particles of all our simulations immediately after the dynamical instability
event, when a single Jupiter-mass planet survives in the system. We carry out
this study with the goal to determine the parameter space that lead to the
flipping of an orbit. Our results suggest that the orbit of a test particle can
flip for any value of its initial eccentricity, although we found only two
Type-F particles with initial inclinations $i <$ 17$^{\circ}$. Moreover, our
study indicates that the minimum value of the inclination of the Type-F
particles in a given system decreases with an increase in the eccentricity of
the giant planet. | astro-ph_EP |
The abundance and thermal history of water ice in the disk surrounding
HD142527 from the DIGIT Herschel Key Program: The presence or absence of ice in protoplanetary disks is of great importance
for the formation of planets. By enhancing the solid surface density and
increasing the sticking efficiency, ice catalyzes the rapid formation of
planetesimals and decreases the time scale for giant planet core accretion.
Aims: In this paper we analyse the composition of the outer disk around the
Herbig star HD~142527. We focus on the composition of the water ice, but also
analyse the abundances of previously proposed minerals. Methods: We present new
Herschel far infrared spectra and a re-reduction of archival data from the
Infrared Space Observatory (ISO). We model the disk using full 3D radiative
transfer to obtain the disk structure. Also, we use an optically thin analysis
of the outer disk spectrum to obtain firm constraints on the composition of the
dust component. Results: The water ice in the disk around HD~142527 contains a
large reservoir of crystalline water ice. We determine the local abundance of
water ice in the outer disk (i.e. beyond 130\,AU). The re-reduced ISO spectrum
differs significantly from that previously published, but matches the new
Herschel spectrum at their common wavelength range. In particular, we do not
detect any significant contribution from carbonates or hydrous silicates, in
contrast to earlier claims. Conclusions: The amount of water ice detected in
the outer disk requires $\sim80\,$\% of the oxygen atoms. This is comparable to
the water ice abundance in the outer solar system, in comets and in dense
interstellar clouds. The water ice is highly crystalline while the temperatures
where we detect it are too low to crystallize the water on relevant time
scales. We discuss the implications of this finding. | astro-ph_EP |
Migrating super-Earths in low-viscosity discs: unveiling the roles of
feedback, vortices, and laminar accretion flows: We present the highest resolution study to date of super-Earths migrating in
inviscid and low-viscosity discs, motivated by the connection to laminar,
wind-driven models of protoplanetary discs. Our models unveil the critical role
of vortices in determining the migration behaviour for partial gap-opening
planets. Vortices form in pressure maxima at gap edges, and prevent the
disc-feedback stopping of migration for intermediate planets in low-viscosity
and inviscid discs, contrary to the concept of the `inertial limit' or `disc
feedback' halting predicted from analytical models. Vortices may also form in
the corotation region, and can dramatically modify migration behaviour through
direct gravitational interaction with the planet. These features become
apparent at high resolution, and for all but the highest viscosities there
exist significant difficulties in obtaining numerically converged results. The
migration of partial gap-opening planets, however, clearly becomes chaotic for
sufficiently low viscosities. At moderate viscosity, a smooth disc-feedback
regime is found in which migration can slow substantially, and the migration
time-scale observed corresponds to migration being driven by diffusive
relaxation of the gap edges. At high viscosity classical Type I migration is
recovered. For Jupiter-analogue planets in inviscid discs, a wide, deep gap is
formed. Transient Type II migration occurs over radial length-scales
corresponding to the gap width, beyond which migration can stall. Finally, we
examine the particle trapping driven by structures left in inviscid discs by a
migrating planet, and find that particle traps in the form of multiple rings
and vortices can persist long after the planet has passed. In this case, the
observation of particle traps by submillimetre interferometers such as ALMA
cannot be used to infer the current presence of an adjacent planet. | astro-ph_EP |
A photochemical model for the carbon-rich planet WASP-12b: The hot Jupiter WASP-12b is a heavily irradiated exoplanet in a short period
orbit around a G0-star with twice the metallicity of the Sun. A recent
thermochemical equilibrium analysis based on Spitzer and ground-based infrared
observations suggests that the presence of $\ch4$ in its atmosphere and the
lack of $\h2o$ features can only be explained if the carbon-to-oxygen ratio in
the planet's atmosphere is much greater than the solar ratio ($\ctoo = 0.54$).
Here, we use a 1-D photochemical model to study the effect of disequilibrium
chemistry on the observed abundances of $\h2o, \com, \co2$ and $\ch4$ in the
WASP-12b atmosphere. We consider two cases: one with solar $\ctoo$ and another
with $\ctoo = 1.08$. The solar case predicts that $\h2o$ and $\com$ are more
abundant than $\co2$ and $\ch4$, as expected, whereas the high $\ctoo$ model
shows that $\com$, C$_{2}$H$_{2}$ and HCN are more abundant. This indicates
that the extra carbon from the high $\ctoo$ model is in hydrocarbon species.
$\h2o$ photolysis is the dominant disequilibrium mechanism that alters the
chemistry at higher altitudes in the solar $\ctoo$ case, whereas
photodissociation of C$_{2}$H$_{2}$ and HCN is significant in the super-solar
case. Furthermore, our analysis indicates that $\c2h2$ is the major absorber in
the atmosphere of WASP-12b and the absorption features detected near 1.6 and 8
micron may be arising from C$_{2}$H$_{2}$ rather than $\ch4$. The Hubble Space
Telescope's WFC3 can resolve this discrepancy, as $\c2h2$ has absorption
between $1.51 - 1.54$ microns, while $\ch4$ does not. | astro-ph_EP |
Volatile transport on inhomogeneous surfaces: II. Numerical calculations
(VT3D): Several distant icy worlds have atmospheres that are in vapor-pressure
equilibrium with their surface volatiles, including Pluto, Triton, and,
probably, several large KBOs near perihelion. Studies of the volatile and
thermal evolution of these have been limited by computational speed, especially
for models that treat surfaces that vary with both latitude and longitude. In
order to expedite such work, I present a new numerical model for the seasonal
behavior of Pluto and Triton which (i) uses initial conditions that improve
convergence, (ii) uses an expedient method for handling the transition between
global and non-global atmospheres, (iii) includes local conservation of energy
and global conservation of mass to partition energy between heating,
conduction, and sublimation or condensation, (iv) uses time-stepping algorithms
that ensure stability while allowing larger timesteps, and (v) can include
longitudinal variability. This model, called VT3D, has been used in Young
(2012), Young (2013), Olkin et al. (2015), Young and McKinnon (2013), and
French et al. (2015). | astro-ph_EP |
Transmission Spectroscopy of the Hot-Jupiter WASP-12b from 0.7 to 5
microns: Since the first report of a potentially non-solar carbon-to-oxygen ratio
(C/O) in its dayside atmosphere, the highly irradiated exoplanet WASP-12b has
been under intense scrutiny and the subject of many follow-up observations.
Additionally, the recent discovery of stellar binary companions ~1" from
WASP-12 has obfuscated interpretation of the observational data. Here we
present new ground-based multi-object transmission-spectroscopy observations of
WASP-12b that we acquired over two consecutive nights in the red optical with
Gemini-N/GMOS. After correcting for the influence of WASP-12's stellar
companions, we find that these data rule out a cloud-free, H2 atmosphere with
no additional opacity sources. We detect features in the transmission spectrum
that may be attributed to metal oxides (such as TiO and VO) for an O-rich
atmosphere or to metal hydrides (such as TiH) for a C-rich atmosphere. We also
reanalyzed NIR transit-spectroscopy observations of WASP-12b from HST/WFC3 and
broadband transit photometry from Warm Spitzer. We attribute the broad spectral
features in the WFC3 data to either H2O or CH4 and HCN for an O-rich or C-rich
atmosphere, respectively. The Spitzer data suggest shallower transit depths
than the models predict at infrared wavelengths, albeit at low statistical
significance. A multi-instrument, broad-wavelength analysis of WASP-12b
suggests that the transmission spectrum is well approximated by a simple
Rayleigh scattering model with a planet terminator temperature of 1870 +/- 130
K. We conclude that additional high-precision data and isolated spectroscopic
measurements of the companion stars are required to place definitive
constraints on the composition of WASP-12b's atmosphere. | astro-ph_EP |
A Framework for Prioritizing the TESS Planetary Candidates Most Amenable
to Atmospheric Characterization: A key legacy of the recently launched TESS mission will be to provide the
astronomical community with many of the best transiting exoplanet targets for
atmospheric characterization. However, time is of the essence to take full
advantage of this opportunity. JWST, although delayed, will still complete its
nominal five year mission on a timeline that motivates rapid identification,
confirmation, and mass measurement of the top atmospheric characterization
targets from TESS. Beyond JWST, future dedicated missions for atmospheric
studies such as ARIEL require the discovery and confirmation of several hundred
additional sub-Jovian size planets (R_p < 10 R_Earth) orbiting bright stars,
beyond those known today, to ensure a successful statistical census of
exoplanet atmospheres. Ground-based ELTs will also contribute to surveying the
atmospheres of the transiting planets discovered by TESS. Here we present a set
of two straightforward analytic metrics, quantifying the expected
signal-to-noise in transmission and thermal emission spectroscopy for a given
planet, that will allow the top atmospheric characterization targets to be
readily identified among the TESS planet candidates. Targets that meet our
proposed threshold values for these metrics would be encouraged for rapid
follow-up and confirmation via radial velocity mass measurements. Based on the
catalog of simulated TESS detections by Sullivan et al. (2015), we determine
appropriate cutoff values of the metrics, such that the TESS mission will
ultimately yield a sample of $\sim300$ high-quality atmospheric
characterization targets across a range of planet size bins, extending down to
Earth-size, potentially habitable worlds. | astro-ph_EP |
Combining astrometry and JUICE -- Europa Clipper radio science to
improve the ephemerides of the Galilean moons: The upcoming JUICE and Europa Clipper missions to Jupiter's Galilean
satellites will provide radio science tracking measurements of both spacecraft.
Such data are expected to significantly help estimating the moons' ephemerides
and related dynamical parameters. However, the two missions will yield an
imbalanced dataset, with no flybys planned at Io, condensed over less than six
years. Current ephemerides' solutions for the Galilean moons, on the other
hand, rely on ground-based astrometry collected over more than a century which,
while being less accurate, bring very valuable constraints on the long-term
dynamics of the system. An improved solution for the Galilean satellites'
complex dynamics could however be achieved by exploiting the existing synergies
between these different observation sets. To quantify this, we merged simulated
JUICE and Clipper radio science data with existing ground-based astrometric and
radar observations, and performed the inversion. Our study specifically
focusses on the resulting formal uncertainties in the moons' states, as well as
Io's and Jupiter's tidal dissipation parameters. Adding astrometry stabilises
the moons' state solution, especially beyond the missions' timelines. It
furthermore reduces the uncertainties in $1/Q$ (inverse of the tidal quality
factor) by a factor two to four for Jupiter, and about 30-35\% for Io. Among
all data types, classical astrometry data prior to 1960 proved particularly
beneficial. We also show that ground observations of Io add the most to the
solution, confirming that ground observations can fill the lack of radio
science data for this specific moon. We obtained a noticeable solution
improvement when exploiting the complementarity between all different
observation sets. These promising simulation results thus motivate future
efforts to achieve a global solution from actual JUICE and Clipper radio
science data. | astro-ph_EP |
Formation of dust-rich planetesimals from sublimated pebbles inside of
the snow line: Content: For up to a few millions of years, pebbles must provide a
quasi-steady inflow of solids from the outer parts of protoplanetary disks to
their inner regions. Aims: We wish to understand how a significant fraction of
the pebbles grows into planetesimals instead of being lost to the host star.
Methods:We examined analytically how the inward flow of pebbles is affected by
the snow line and under which conditions dust-rich (rocky) planetesimals form.
When calculating the inward drift of solids that is due to gas drag, we
included the back-reaction of the gas to the motion of the solids. Results: We
show that in low-viscosity protoplanetary disks (with a monotonous surface
density similar to that of the minimum-mass solar nebula), the flow of pebbles
does not usually reach the required surface density to form planetesimals by
streaming instability. We show, however, that if the pebble-to-gas-mass flux
exceeds a critical value, no steady solution can be found for the solid-to-gas
ratio. This is particularly important for low-viscosity disks (alpha < 10^(-3))
where we show that inside of the snow line, silicate-dust grains ejected from
sublimating pebbles can accumulate, eventually leading to the formation of
dust-rich planetesimals directly by gravitational instability. Conclusions:
This formation of dust-rich planetesimals may occur for extended periods of
time, while the snow line sweeps from several au to inside of 1 au. The
rock-to-ice ratio may thus be globally significantly higher in planetesimals
and planets than in the central star. | astro-ph_EP |
Large eccentricity, low mutual inclination: the three-dimensional
architecture of a hierarchical system of giant planets: We establish the three-dimensional architecture of the Kepler-419 (previously
KOI-1474) system to be eccentric yet with a low mutual inclination. Kepler-419b
is a warm Jupiter at semi-major axis a = 0.370 +0.007/-0.006 AU with a large
eccentricity e=0.85 +0.08/-0.07 measured via the "photoeccentric effect." It
exhibits transit timing variations induced by the non-transiting Kepler-419c,
which we uniquely constrain to be a moderately eccentric (e=0.184 +/- 0.002),
hierarchically-separated (a=1.68 +/- 0.03 AU) giant planet (7.3 +/- 0.4 MJup).
We combine sixteen quarters of Kepler photometry, radial-velocity (RV)
measurements from the HIgh Resolution Echelle Spectrometer (HIRES) on Keck, and
improved stellar parameters that we derive from spectroscopy and
asteroseismology. From the RVs, we measure the mass of inner planet to be
2.5+/-0.3MJup and confirm its photometrically-measured eccentricity, refining
the value to e=0.83+/-0.01. The RV acceleration is consistent with the
properties of the outer planet derived from TTVs. We find that, despite their
sizable eccentricities, the planets are coplanar to within 9 +8/-6 degrees, and
therefore the inner planet's large eccentricity and close-in orbit are unlikely
to be the result of Kozai migration. Moreover, even over many secular cycles,
the inner planet's periapse is most likely never small enough for tidal
circularization. Finally, we present and measure a transit time and impact
parameter from four simultaneous ground-based light curves from 1m-class
telescopes, demonstrating the feasibility of ground-based follow-up of Kepler
giant planets exhibiting large TTVs. | astro-ph_EP |
Asteroid Photometry: Asteroid photometry has three major applications: providing clues about
asteroid surface physical properties and compositions, facilitating photometric
corrections, and helping design and plan ground-based and spacecraft
observations. The most significant advances in asteroid photometry in the past
decade were driven by spacecraft observations that collected spatially resolved
imaging and spectroscopy data. In the mean time, laboratory measurements and
theoretical developments are revealing controversies regarding the physical
interpretations of models and model parameter values. We will review the new
developments in asteroid photometry that have occurred over the past decade in
the three complementary areas of observations, laboratory work, and theory.
Finally we will summarize and discuss the implications of recent findings. | astro-ph_EP |
Electrostatic lofting of dust grains from the surfaces of Thebe and
Amalthea: Energetic electrons from the inner radiation belt provide significant
electric charging of the surfaces of Jupiter's moons Thebe and Amalthea whose
orbits are located within this radiation belt. We estimate theoretically the
electric fields in the vicinity of the polar regions of Thebe and Amalthea and
argue that these fields are sufficient for lofting of micron and
submicron-sized dust grains from the surfaces of the moons. Thus, the lofting
of charged dust grains in the electric fields can be considered as an
additional source supplying dust to the gossamer rings in addition to dust
ejection by micrometeoroid impacts onto the moons' surfaces. The suggested
mechanism can explain qualitatively some peculiarities of the dust grain
distributions within the gossamer rings. | astro-ph_EP |
Constraining the origin of the planetary debris surrounding ZTF
J0139+5245 through rotational fission of a triaxial asteroid: White dwarfs containing orbiting planetesimals or their debris represent
crucial benchmarks by which theoretical investigations of post-main-sequence
planetary systems may be calibrated. The photometric transit signatures of
likely planetary debris in the ZTF J0139+5245 white dwarf system has an orbital
period of about 110 days. An asteroid which breaks up to produce this debris
may spin itself to destruction through repeated close encounters with the star
without entering its Roche radius and without influence from the white dwarf's
luminosity. Here, we place coupled constraints on the orbital pericentre ($q$)
and the ratio ($\beta$) of the middle to longest semiaxes of a triaxial
asteroid which disrupts outside of this white dwarf's Roche radius ($r_{\rm
Roche}$) soon after attaining its 110-day orbit. We find that disruption within
tens of years is likely when $\beta \lesssim 0.6$ and $q\approx 1.0-2.0r_{\rm
Roche}$, and when $\beta \lesssim 0.2$ out to $q\approx 2.5r_{\rm Roche}$.
Analysing the longer-timescale disruption of triaxial asteroids around ZTF
J0139+5245 is desirable but may require either an analytical approach relying
on ergodic theory or novel numerical techniques. | astro-ph_EP |
HAT-P-28b and HAT-P-29b: Two Sub-Jupiter Mass Transiting Planets: We present the discovery of two transiting exoplanets. HAT-P-28b orbits a
V=13.03 G3 dwarf star with a period P = 3.2572 d and has a mass of 0.63 +- 0.04
MJ and a radius of 1.21 + 0.11 -0.08 RJ yielding a mean density of 0.44 +- 0.09
g cm-3. HAT-P-29b orbits a V=11.90 F8 dwarf star with a period P = 5.7232 d and
has a mass of 0.78 +0.08 -0.04 MJ and a radius of 1.11 +0.14 -0.08 RJ yielding
a mean density of 0.71 +- 0.18 g cm-3. We discuss the properties of these
planets in the context of other known transiting planets. | astro-ph_EP |
Constraining spatial pattern of early activity of comet 67P/C-G with 3D
modeling of the MIRO observations: Our aim is to investigate early activity (July 2014) of 67P/CG with 3D coma
and radiative transfer modeling of MIRO measurements, accounting for nucleus
shape, illumination, and orientation of the comet. We investigate MIRO line
shape information for spatial distribution of water activity on the nucleus
during the onset of activity. During this period we show that MIRO line shape
have enough information to clearly isolate contribution from Hapi and Inhotep
independently, and compare it to the nominal case of activity from the entire
illuminated surface. We also demonstrate that spectral line shapes differ from
the 1D model for different viewing geometries and coma conditions relevant to
this study. Specifically, line shapes are somewhat sensitive to the location of
the terminator in the coma. At last, fitting the MIRO observations we show that
the Imhotep region (possible extended source of H$_{2}$O due to CO$_{2}$
activity) contributes only a small fraction of the total number of water
molecules into MIRO beam in the early activity. On the other hand, a strong
enhancement of water activity from the Hapi region seems required to fit the
MIRO line shapes. This is consistent with earlier Rosetta results.
Nevertheless, within the assumption of our coma and surface boundary
conditions, we cannot get a reasonable fit to all MIRO mapping observations in
July 2014, which may illustrate that a more sophisticated coma model or more
accurate temperature/velocity distribution is needed. | astro-ph_EP |
Detection of an atmosphere around the super-Earth 55 Cancri e: We report the analysis of two new spectroscopic observations of the
super-Earth 55 Cancri e, in the near infrared, obtained with the WFC3 camera
onboard the HST. 55 Cancri e orbits so close to its parent star, that
temperatures much higher than 2000 K are expected on its surface. Given the
brightness of 55 Cancri, the observations were obtained in scanning mode,
adopting a very long scanning length and a very high scanning speed. We use our
specialized pipeline to take into account systematics introduced by these
observational parameters when coupled with the geometrical distortions of the
instrument. We measure the transit depth per wavelength channel with an average
relative uncertainty of 22 ppm per visit and find modulations that depart from
a straight line model with a 6$\sigma$ confidence level. These results suggest
that 55 Cancri e is surrounded by an atmosphere, which is probably
hydrogen-rich. Our fully Bayesian spectral retrieval code, T-REx, has
identified HCN to be the most likely molecular candidate able to explain the
features at 1.42 and 1.54 $\mu$m. While additional spectroscopic observations
in a broader wavelength range in the infrared will be needed to confirm the HCN
detection, we discuss here the implications of such result. Our chemical model,
developed with combustion specialists, indicates that relatively high mixing
ratios of HCN may be caused by a high C/O ratio. This result suggests this
super-Earth is a carbon-rich environment even more exotic than previously
thought. | astro-ph_EP |
Hypotheses for near-surface exchange of methane on Mars: The Curiosity rover recently detected a background of 0.7 ppb and spikes of 7
ppb of methane on Mars. This in situ measurement reorients our understanding of
the Martian environment and its potential for life, as the current theories do
not entail any geological source or sink of methane that varies sub-annually.
In particular, the 10-fold elevation during the southern winter indicates
episodic sources of methane that are yet to be discovered. Here we suggest a
near-surface reservoir could explain this variability. Using the temperature
and humidity measurements from the rover, we find that perchlorate salts in the
regolith deliquesce to form liquid solutions, and deliquescence progresses to
deeper subsurface in the season of the methane spikes. We therefore formulate
the following three testable hypotheses. The first scenario is that the
regolith in Gale Crater adsorbs methane when dry and releases this methane to
the atmosphere upon deliquescence. The adsorption energy needs to be 36 kJ/mol
to explain the magnitude of the methane spikes, higher than existing laboratory
measurements. The second scenario is that microorganisms convert organic matter
in the soil to methane when they are in liquid solutions. This scenario does
not require regolith adsorption, but entails extant life on Mars. The third
scenario is that deep subsurface aquifers produce the bursts of methane.
Continued in situ measurements of methane and water, as well as laboratory
studies of adsorption and deliquescence, will test these hypotheses and inform
the existence of the near-surface reservoir and its exchange with the
atmosphere. | astro-ph_EP |
Insights into planet formation from debris disks: II. Giant impacts in
extrasolar planetary systems: Giant impacts refer to collisions between two objects each of which is
massive enough to be considered at least a planetary embryo. The putative
collision suffered by the proto-Earth that created the Moon is a prime example,
though most Solar System bodies bear signatures of such collisions. Current
planet formation models predict that an epoch of giant impacts may be
inevitable, and observations of debris around other stars are providing
mounting evidence that giant impacts feature in the evolution of many planetary
systems. This chapter reviews giant impacts, focussing on what we can learn
about planet formation by studying debris around other stars. Giant impact
debris evolves through mutual collisions and dynamical interactions with
planets. General aspects of this evolution are outlined, noting the importance
of the collision-point geometry. The detectability of the debris is discussed
using the example of the Moon-forming impact. Such debris could be detectable
around another star up to 10Myr post-impact, but model uncertainties could
reduce detectability to a few 100yr window. Nevertheless the 3% of young stars
with debris at levels expected during terrestrial planet formation provide
valuable constraints on formation models; implications for super-Earth
formation are also discussed. Variability recently observed in some bright
disks promises to illuminate the evolution during the earliest phases when
vapour condensates may be optically thick and acutely affected by the
collision-point geometry. The outer reaches of planetary systems may also
exhibit signatures of giant impacts, such as the clumpy debris structures seen
around some stars. | astro-ph_EP |
Outburst activity in comets: II. A multi-band photometric monitoring of
comet 29p/Schwassmann-Wachmann 1: We have carried out a continuous multi-band photometric monitoring of the
nuclear activity of comet 29P/Schwassmann-Wachmann 1 from 2008 to 2010. Our
main aim has been to study the outburst mechanism on the basis of a follow-up
of the photometric variations associated with the release of dust. We used a
standardized method to obtain the 10 arc-sec nucleus photometry in the V, R,
and I filters of the Johnson-Kron-Cousins system, being accurately calibrated
with standard Landolt stars. Production of dust in the R and I bands during the
2010 Feb. 3 outburst has been also computed. We conclude that the massive
ejection of large (optically-thin) particles from the surface at the time of
the outburst is the triggering mechanism to produce the outburst. Ulterior
sublimation of these ice-rich dust particles during the following days induces
fragmentation, generating micrometer-sized grains that increase the dust
spatial density to produce the outburst in the optical range due to scattering
of sun light. The material leaving the nucleus adopts a fan-like dust feature,
formed by micrometer-sized particles that are decaying in brightness as it
evolved outwards. By analyzing the photometric signal measured in a
standardized 10-arcsec aperture using the Phase Dispersion Minimization
technique we have found a clear periodicity of 50 days. Remarkably, this value
is also consistent with an outburst frequency of 7.4 outbursts/year deduced
from the number of outbursts noticed during the effective observing time. | astro-ph_EP |
The Radial Distribution of Dust Particles in the HL Tau Disk from ALMA
and VLA Observations: Understanding planet formation requires to discern how dust grows in
protoplanetary disks. An important parameter to measure in disks is the maximum
dust grain size present. This is usually estimated through measurements of the
dust opacity at different millimeter wavelengths assuming optically thin
emission and dust opacity dominated by absorption. However, ALMA observations
have shown that these assumptions might not be correct in the case of
protoplanetary disks, leading to overestimation of particle sizes and to
underestimation of the disk's mass. Here, we present an analysis of high
quality ALMA and VLA images of the HL Tau protoplanetary disk, covering a wide
range of wavelengths, from 0.8 mm to 1 cm, and with a physical resolution of
$\sim$7.35 au. We describe a procedure to analyze a set of millimeter images
without any assumption about the optical depth of the emission, and including
the effects of absorption and scattering in the dust opacity. This procedure
allows us to obtain the dust temperature, the dust surface density and the
maximum particle size at each radius. In the HL Tau disk, we found that
particles have already grown up to a few millimeters in size. We detect
differences in the dust properties between dark and bright rings, with dark
rings containing low dust density and small dust particles. Different features
in the HL Tau disk seem to have different origins. Planet-disk interactions can
explain substructure at the external half of the disk, but the internal rings
seem to be associated to the presence of snow lines of several molecules. | astro-ph_EP |
Instantaneous Three-dimensional Thermal Structure of the South Polar
Vortex of Venus: The Venus thermal radiation spectrum exhibits the signature of $CO_2$
absorption bands. By means of inversion techniques, those bands enable the
retrieval of atmospheric temperature profiles. We have analyzed VIRTIS-M-IR
night-side data obtaining high-resolution thermal maps of Venus south polar
region between 55 and 85 km altitudes for three dynamical configurations of the
vortex. The cold collar is clearly distinguishable at $\sim 62$ km altitude
level, and it is more than 15 K colder than the pole on average. The South
Polar Vortex appears as a vertically extended hot region close to the pole and
squeezed by the cold collar between altitudes 55 and 67 km but spreading
equatorward at about 74 km. Both the instantaneous temperature maps and their
zonal averages show that the top altitude limit of the thermal signature of the
vortex is at $\sim 80$ km altitude, at least on the night-side of the planet.
The upper part of the atmosphere (67 - 85 km) is more homogeneous and has
long-scale horizontal temperature differences of about 25 K over horizontal
distances of $\sim 2,000$ km. The lower part (55 - 67 km) shows more fine-scale
structure, creating the vortex' morphology, with thermal differences of up to
about 50 K over $\sim 500$ km horizontal distances. We also study the vertical
stability of different atmospheric layers within the 55 - 85 km altitude range
for the three vortex configurations. It is always positive, but the cold collar
is the most vertically stable structure at polar latitudes, while the vortex
and sub-polar latitudes show lower stability values. Furthermore, the hot
filaments present within the vortex exhibit lower stability values than their
surroundings. The layer between 62 and 67 km resulted to be the most stable.
These results are in good agreement with conclusions from previous radio
occultation analyses. | astro-ph_EP |
Gravito-turbulence in irradiated protoplanetary discs: Using radiation hydrodynamics simulations in a local stratified shearing box
with realistic equations of state and opacities, we explored the outcome of
self-gravity at 50 AU in a protoplanetary disc irradiated by the central star.
We found that gravito-turbulence is sustained for a finite range of the surface
density, from $\sim 80$ to $\sim$ 250 gcm$^{-2}$. The disk is laminar below the
range while fragments above it. In the range of gravito-turbulence, the Toomre
parameter decreases monotonically from $\sim 1$ to $\sim 0.7$ as the surface
density increases while an effective cooling time is almost constant at $\sim
4$ in terms of the inverse of the orbital frequency. The turbulent motions are
supersonic at all heights, which dissipates through both shock waves and
compressional heating. The compressional motions, occurring near the midplane,
create upward flows, which not only contribute to supporting the disc but also
to transporting the dissipated energy to the disc surfaces. The irradiation
does not affect much the gravito-turbulence near the midplane unless the
grazing angle is larger than 0.32. We also show that a simple cooling function
with a constant cooling time does not approximate the realistic cooling. | astro-ph_EP |
The Architecture of the V892 Tau System: the Binary and its Circumbinary
Disk: We present high resolution millimeter continuum and CO line observations for
the circumbinary disk around V892 Tau to constrain the stellar and disk
properties. The total mass of the two near-equal-mass A stars is estimated to
be $6.0\pm0.2\,M_{\odot}$ based on our models of the Keplerian-dominated gas
disk rotation. The detection of strong ionized gas emission associated with the
two stars at 8 mm, when combined with previous astrometric measurements in the
near-infrared, provides an updated view of the binary orbit with $a=7.1\pm0.1$
au, $e=0.27\pm0.1$, and $P=7.7\pm0.2$ yr, which is about half of a previously
reported orbital period. The binary orbital plane is proposed to be near
coplanar to the circumbinary disk plane (with a mutual inclination of only
$\Delta=8\pm4.2$ deg; another solution with $\Delta=113$ deg is less likely
given the short re-alignment timescale). An asymmetric dust disk ring peaking
at a radius of 0.''2 is detected at 1.3 mm and its fainter counterparts are
also detected at the longer 8 and 9.8 mm. The CO gas disk, though dominated by
Keplerian rotation, presents a mild inner and outer disk misalignment, such
that the inner disk to the SW and outer disk to the NE appear brighter than
their counterparts at the opposite disk sides. The radial extension of the
disk, its asymmetric dust ring, and the presence of a disk warp could all be
explained by the interaction between the eccentric binary and the circumbinary
disk, which we assume were formed with non-zero mutual inclination. Some
tentatively detected gas spirals in the outer disk are likely produced by
interactions with the low mass tertiary component located 4'' to the northeast.
Our analyses demonstrate the promising usage of V892 Tau as an excellent
benchmark system to study the details of binary--disk interactions. | astro-ph_EP |
XO-2b: a Prograde Planet with a Negligible Eccentricity, and an
Additional Radial Velocity Variation: We present precise radial velocities of XO-2 taken with the Subaru HDS,
covering two transits of XO-2b with an interval of nearly two years. The data
suggest that the orbital eccentricity of XO-2b is consistent with zero within
2$\sigma$ ($e=0.045\pm0.024$) and the orbit of XO-2b is prograde (the
sky-projected spin-orbit alignment angle $\lambda=10^{\circ}\pm72^{\circ}$).
The poor constraint of $\lambda$ is due to a small impact parameter (the
orbital inclination of XO-2b is almost 90$^{\circ}$). The data also provide an
improved estimate of the mass of XO-2b as $0.62\pm0.02$ $M_{\rm Jup}$. We also
find a long-term radial velocity variation in this system. Further radial
velocity measurements are necessary to specify the cause of this additional
variation. | astro-ph_EP |
Particle transport in evolving protoplanetary disks: Implications for
results from Stardust: Samples returned from comet 81P/Wild 2 by Stardust confirm that substantial
quantities of crystalline silicates were incorporated into the comet at
formation. We investigate the constraints that this observation places upon
protoplanetary disk physics, assuming that outward transport of particles
processed at high temperatures occurs via advection and turbulent diffusion in
an evolving disk. We also look for constraints on particle formation locations.
Our results are based upon 1D disk models that evolve with time under the
action of viscosity and photoevaporation, and track solid transport using an
ensemble of individual particle trajectories. We find that two classes of disk
model are consistent with the Stardust findings. One class features a high
particle diffusivity (a Schmidt number Sc < 1), which suffices to diffuse
particles up to 20 microns in size outward against the mean gas flow. For Sc >
1, such models are unlikely to be viable, and significant outward transport
requires that the particles of interest settle into a midplane layer that
experiences an outward gas flow. In either class of models, the mass of inner
disk material that reaches the outer disk is a strong function of the disk's
initial compactness. Hence, models of grain transport within steady-state disks
underestimate the efficiency of outward transport. Neither model results in
sustained outward transport of very large particles exceeding a mm in size. We
show that the transport efficiency generally falls off rapidly with time.
Hence, high-temperature material must be rapidly incorporated into icy bodies
to avoid fallback, and significant radial transport may only occur during the
initial phase of rapid disk evolution. It may also vary substantially between
disks depending upon their initial mass distributions. We discuss implications
for Spitzer observations of crystalline silicates in T Tauri disks. | astro-ph_EP |
Ground-based detections of thermal emission from CoRoT-1b and WASP-12b: We report a new detection of the H-band thermal emission of CoRoT-1b and two
confirmation detections of the Ks-band thermal emission of WASP-12b at
secondary eclipses. The H-band measurement of CoRoT-1b shows an eclipse depth
of 0.145%\pm0.049% with a 3-{\sigma} percentile between 0.033% - 0.235%. This
depth is consistent with the previous conclusions that the planet has an
isother- mal region with inefficient heat transport from dayside to nightside,
and has a dayside thermal inversion layer at high altitude. The two Ks band
detections of WASP-12b show a joint eclipse depth of 0.299%\pm0.065%. This
result agrees with the measurement of Croll & collaborators, providing
independent confirmation of their measurement. The repeatability of the
WASP-12b measurements also validates our data analysis method. Our
measurements, in addition to a number of previous results made with other
telescopes, demonstrate that ground-based observations are becoming widely
available for characterization of atmospheres of hot Jupiters. | astro-ph_EP |
Shedding Light on the Eccentricity Valley: Gap Heating and Eccentricity
Excitation of Giant Planets in Protoplanetary Disks: We show that the first order (non co-orbital) corotation torques are
significantly modified by entropy gradients in a non-barotropic protoplanetary
disk. Such non-barotropic torques can dramatically alter the balance that, for
barotropic cases, results in the net eccentricity damping for giant
gap-clearing planets embedded in the disk. We demonstrate that stellar
illumination can heat the gap enough for the planet's orbital eccentricity to
instead be excited. We also discuss the "Eccentricity Valley" noted in the
known exoplanet population, where low-metallicity stars have a deficit of
eccentric planets between $\sim 0.1$ and $\sim 1$ AU compared to metal-rich
systems (Dawson & Murray-Clay 2013). We show that this feature in the planet
distribution may be due to the self-shadowing of the disk by a rim located at
the dust sublimation radius $\sim 0.1$ AU, which is known to exist for several
T Tauri systems. In the shadowed region between $\sim 0.1$ and $\sim 1$ AU lack
of gap insolation allows disk interactions to damp eccentricity. Outside such
shadowed regions stellar illumination can heat the planetary gaps and drive
eccentricity growth for giant planets. We suggest that the self-shadowing does
not arise at higher metallicity due to the increased optical depth of the gas
interior to the dust sublimation radius. | astro-ph_EP |
Origin Scenarios for the Kepler 36 Planetary System: We explore scenarios for the origin of two different density planets in the
Kepler 36 system in adjacent orbits near the 7:6 mean motion resonance. We find
that fine tuning is required in the stochastic forcing amplitude, the migration
rate and planet eccentricities to allow two convergently migrating planets to
bypass mean motion resonances such as the 4:3, 5:4 and 6:5, and yet allow
capture into the 7:6 resonance. Stochastic forcing can eject the system from
resonance causing a collision between the planets, unless the disk inducing
migration and stochastic forcing is depleted soon after resonance capture.
We explore a scenario with approximately Mars mass embryos originating
exterior to the two planets and migrating inwards toward two planets. We find
that gravitational interactions with embryos can nudge the system out of
resonances. Numerical integrations with about a half dozen embryos can leave
the two planets in the 7:6 resonance. Collisions between planets and embryos
have a wide distribution of impact angles and velocities ranging from
accretionary to disruptive. We find that impacts can occur at sufficiently high
impact angle and velocity that the envelope of a planet could have been
stripped, leaving behind a dense core. Some of our integrations show the two
planets exchanging locations, allowing the outer planet that had experienced
multiple collisions with embryos to become the innermost planet. A scenario
involving gravitational interactions and collisions with embryos may account
for both the proximity of the Kepler 36 planets and their large density
contrast. | astro-ph_EP |
Friends of Hot Jupiters II: No Correspondence Between Hot-Jupiter
Spin-Orbit Misalignment and the Incidence of Directly Imaged Stellar
Companions: Multi-star systems are common, yet little is known about a stellar
companion's influence on the formation and evolution of planetary systems. For
instance, stellar companions may have facilitated the inward migration of hot
Jupiters towards to their present day positions. Many observed short period gas
giant planets also have orbits that are misaligned with respect to their star's
spin axis, which has also been attributed to the presence of a massive outer
companion on a non-coplanar orbit. We present the results of a multi-band
direct imaging survey using Keck NIRC2 to measure the fraction of short period
gas giant planets found in multi-star systems. Over three years, we completed a
survey of 50 targets ("Friends of Hot Jupiters") with 27 targets showing some
signature of multi-body interaction (misaligned or eccentric orbits) and 23
targets in a control sample (well-aligned and circular orbits). We report the
masses, projected separations, and confirmed common proper motion for the 19
stellar companions found around 17 stars. Correcting for survey incompleteness,
we report companion fractions of $48\%\pm9\%$, $47\%\pm12\%$, and $51\%\pm13\%$
in our total, misaligned/eccentric, and control samples, respectively. This
total stellar companion fraction is $2.8\,\sigma$ larger than the fraction of
field stars with companions approximately $50-2000\,$AU. We observe no
correlation between misaligned/eccentric hot Jupiter systems and the incidence
of stellar companions. Combining this result with our previous radial velocity
survey, we determine that $72\% \pm 16\%$ of hot Jupiters are part of
multi-planet and/or multi-star systems. | astro-ph_EP |
Identification of a new spectral signature at 3 $μ$m over Martian
northern high latitudes: implications for surface composition: Mars northern polar latitudes are known to harbor an enhanced 3 ${\mu}$m
spectral signature when observed from orbit. This may indicate a greater amount
of surface adsorbed or bound water, although it has not yet been possible to
easily reconcile orbital observations with ground measurements by Phoenix. Here
we re-analyzed OMEGA/Mars Express observations acquired during the Northern
summer to further characterize this 3 ${\mu}$m absorption band increase. We
identify the presence of a new specific spectral signature composed of an
additional narrow absorption feature centered at 3.03 ${\mu}$m coupled with an
absorption at ${\lambda}$ ${\geq}$ 3.8 ${\mu}$m. This signature is
homogeneously distributed over a high-albedo open ring surrounding the
circumpolar low-albedo terrains between ~ 68{\deg}N and 76{\deg}N and ~
0{\deg}E and 270{\deg}E. This location includes the Phoenix landing site. This
feature shows no time variability and can be confidently attributed to a
seasonally stable surface component. All together, the stability, spectral
shape and absence of significant correlation with other signatures in the 1 $-$
2.5 ${\mu}$m range discard interpretations relying on water ice or easily
exchangeable adsorbed water. Sulfates, notably anhydrite, provide interesting
comparisons to several sections of the spectrum. Analogies with Earth samples
also show that the spectral signature could result from a latitudinal
modification of the hydration state and/or grains size of salts contaminants.
While the exact full spectral shape cannot be easily reproduced, plausible
explanations to this observation seem to involve geologically recent water
alteration at high northern latitudes. | astro-ph_EP |
Direct Imaging of Extra-solar Planets - Homogeneous Comparison of
Detected Planets and Candidates: Searching the literature, we found 25 stars with directly imaged planets and
candidates. We gathered photometric and spectral information for all these
objects to derive their luminosities in a homogeneous way, taking a bolometric
correction into account. Using theoretical evolutionary models, one can then
estimate the mass from luminosity, temperature, and age. According to our mass
estimates, all of them can have a mass below 25 Jup masses, so that they are
considered as planets. | astro-ph_EP |
On the co-orbital asteroids in the solar system: medium-term timescale
analysis of the quasi-coplanar objects: The focus of this work is the current distribution of asteroids in co-orbital
motion with Venus, Earth and Jupiter, under a quasi-coplanar configuration and
for a medium-term timescale of the order of 900 years. A co-orbital trajectory
is a heliocentric orbit trapped in a 1:1 mean-motion resonance with a given
planet. As such, to model it this work considers the Restricted Three-Body
Problem in the circular-planar case with the help of averaging techniques. The
domain of each co-orbital regime, that is, the quasi-satellite motion, the
horseshoe motion and the tadpole motion, can be neatly defined by means of an
integrable model and a simple bi-dimensional map, that is invariant with
respect to the mass parameter of the planet, and turns out to be a remarkable
tool to investigate the distribution of the co-orbitals objects of interest.
The study is based on the data corresponding to the ephemerides computed by the
JPL Horizons system for asteroids with a sufficient low orbital inclination
with respect to the Sun-planet orbital plane. These objects are cataloged
according to their current dynamics, together with the transitions that occur
in the given time frame from a given type of co-orbital motion to another. The
results provide a general catalog of co-orbital asteroids in the solar system,
the first one to our knowledge, and an efficient mean to study transitions. | astro-ph_EP |
Giant Impact onto a Vesta-Like Asteroid and Formation of Mesosiderites
through Mixing of Metallic Core and Surface Crust: Mesosiderites are a type of stony-iron meteorites composed of a mixture of
silicates and Fe-Ni metals. The mesosiderite silicates and metals are
considered to have originated from the crust and metal core, respectively, of a
differentiated asteroid. In contrast, mesosiderites rarely contain the olivine
that is mainly included in a mantle. Although a giant impact onto a
differentiated asteroid is considered to be a probable mechanism to mix crust
and metal materials to form mesosiderites, it is not obvious how such a giant
impact can form mesosiderite-like materials without including mantle materials.
We conducted numerical simulations of giant impacts onto differentiated
asteroids, using the smoothed particle hydrodynamics method, to investigate the
detailed distribution of mixed materials on the resultant bodies. For the
internal structure of a target body, we used a thin-crust model derived from
the magma ocean crystallization model of the asteroid Vesta and a thick-crust
and a large-core model suggested from the proximity observation of Vesta by the
Dawn probe. In the simulations with the former model, excavation of the metal
core requires nearly catastrophic impacts and mantle is exposed over large
surface areas. Thus, stony-iron materials produced on its surface are likely to
include mantle materials and it is difficult to produce mesosiderite-like
materials. Conversely, in the simulations with the latter model, mantle
materials are exposed only at impact sites, even when the impacts excavate the
metal core, and the formation of a surface with little mantle material and the
formation of mesosiderite-like materials are possible. Therefore, our
simulations suggest that an internal structure with a thick crust and a large
core is more likely as a mesosiderite parent body rather than the thin-crust
internal structure inferred from the conventional magma ocean model. | astro-ph_EP |
The giant nature of WD 1856 b implies that transiting rocky planets are
rare around white dwarfs: White dwarfs (WDs) have roughly Earth-sized radii - a fact long recognized to
facilitate the potential discovery of sub-Earth sized planets via transits, as
well atmospheric characterization including biosignatures. Despite this, the
first (and still only) transiting planet discovered in 2020 was a roughly
Jupiter-sized world, found using TESS photometry. Given the relative paucity of
giant planets compared to terrestrials indicated by both exoplanet demographics
and theoretical simulations (a "bottom-heavy" radius distribution), this is
perhaps somewhat surprising. Here, we quantify the surprisingness of this fact
accounting for geometric bias and detection bias assuming 1) a bottom-heavy
Kepler derived radius distribution, and 2) a top-heavy radial velocity inspired
radius distribution. Both are concerning, with the latter implying rocky
planets are highly unusual and the former implying WD 1856 b would have to be
highly surprising event at the <0.5% level. Using an HBM, we infer the implied
power-law radius distribution conditioned upon WD 1856 b and arrive at a
top-heavy distribution, such that 0.1-2 REarth planets are an
order-of-magnitude less common than 2-20 REarth planets in the period range of
0.1-10 days. The implied hypothesis is that transiting WD rocky planets are
rare. We discuss ways to reconcile this with other evidence for minor bodies
around WDs, and ultimately argue that it should be easily testable. | astro-ph_EP |
Cometary glycolaldehyde as a source of pre-RNA molecules: Over 200 molecules have been detected in multiple extraterrestrial
environments, including glycolaldehyde (C2(H2O)2, GLA), a two-carbon sugar
precursor that has been detected in regions of the interstellar medium. Its
recent in situ detection on the nucleus of comet 67P/Churyumov-Gerasimenko and
via remote observations in the comae of others, provides tantalizing evidence
that it is common on most (if not all) comets. Impact experiments conducted at
the Experimental Impact Laboratory at NASA's Johnson Space Center have shown
that samples of GLA and GLA mixed with montmorillonite clays can survive impact
delivery in the pressure range of 4.5 GPa to 25 GPa. Extrapolated to amounts of
GLA observed on individual comets and assuming a monotonic impact rate in the
first billion years of solar system history, these experimental results show
that up to 10^23 kg of cometary GLA could have survived impact delivery, with
substantial amounts of threose, erythrose, glycolic acid, and ethylene glycol
also produced or delivered. Importantly, independent of the profile of the
impact flux in the early solar system, comet delivery of GLA would have
provided (and may continue to provide) a reservoir of starting material for the
formose reaction (to form ribose) and the Strecker reaction (to form amino
acids). Thus, comets may have been important delivery vehicles for starting
molecules necessary for life as we know it. | astro-ph_EP |
Asteroseismology of iota Draconis and Discovery of an Additional
Long-Period Companion: Giant stars as known exoplanet hosts are relatively rare due to the potential
challenges in acquiring precision radial velocities and the small predicted
transit depths. However, these giant host stars are also some of the brightest
in the sky and so enable high signal-to-noise follow-up measurements. Here we
report on new observations of the bright (V ~ 3.3) giant star $\iota$ Draconis
($\iota$ Dra), known to host a planet in a highly eccentric ~511 day period
orbit. TESS observations of the star over 137 days reveal asteroseismic
signatures, allowing us to constrain the stellar radius, mass, and age to ~2%,
~6%, and ~28%, respectively. We present the results of continued radial
velocity monitoring of the star using the Automated Planet Finder over several
orbits of the planet. We provide more precise planet parameters of the known
planet and, through the combination of our radial velocity measurements with
Hipparcos and Gaia astrometry, we discover an additional long-period companion
with an orbital period of ~$68^{+60}_{-36}$ years. Mass predictions from our
analysis place this sub-stellar companion on the border of the planet and brown
dwarf regimes. The bright nature of the star combined with the revised orbital
architecture of the system provides an opportunity to study planetary orbital
dynamics that evolve as the star moves into the giant phase of its evolution. | astro-ph_EP |
Dynamic Limits on Planar Libration-Orbit Coupling Around an Oblate
Primary: This paper explores the dynamic properties of the planar system of an
ellipsoidal satellite in an equatorial orbit about an oblate primary. In
particular, we investigate the conditions for which the satellite is bound in
librational motion or when the satellite will circulate with respect to the
primary. We find the existence of stable equilibrium points about which the
satellite can librate, and explore both the linearized and non-linear dynamics
around these points. Absolute bounds are placed on the phase space of the
libration-orbit coupling through the use of zero-velocity curves that exist in
the system. These zero-velocity curves are used to derive a sufficient
condition for when the satellite's libration is bound to less than 90 degrees.
When this condition is not satisfied so that circulation of the satellite is
possible, the initial conditions at zero libration angle are determined which
lead to circulation of the satellite. Exact analytical conditions for
circulation and the maximum libration angle are derived for the case of a small
satellite in orbits of any eccentricity. | astro-ph_EP |
Accessible Carbon on the Moon: Carbon is one of the most essential elements to support a sustained human
presence in space, and more immediately, several large-scale methalox-based
transport systems will begin operating in the near future. This raises the
question of whether indigenous carbon on the Moon is abundant and concentrated
to the extent where it could be used as a viable resource including as
propellant. Here, I assess potential sources of lunar carbon based on previous
work focused on polar water ice. A simplified model is used to estimate the
temperature-dependent Carbon Content of Ices at the lunar poles, and this is
combined with remote sensing data to estimate the total amount of carbon and
generate a Carbon Favorability Index that highlights promising deposits for
future ground-based prospecting. Hotspots in the index maps are identified, and
nearby staging areas are analyzed using quantitative models of trafficability
and solar irradiance. Overall, the Moon is extremely poor in carbon sources
compared to more abundant and readily accessible options at Mars. However, a
handful of polar regions may contain appreciable amounts of subsurface
carbon-bearing ices that could serve as a rich source in the near term, but
would be easily exhausted on longer timescales. Four of those regions were
found to have safe nearby staging areas with equatorial-like illumination at a
modest height above the surface. Any one of these sites could yield enough C, H
and O to produce propellant for hundreds of refuelings of a large spacecraft.
Other potential lunar carbon sources including bulk regolith and pyroclastic
glasses are less viable due to their low carbon concentrations. | astro-ph_EP |
The Absence of Cold Dust around Warm Debris Disk Star HD 15407A: We report Herschel and AKARI photometric observations at far-infrared (FIR)
wavelengths of the debris disk around the F3V star HD 15407A, in which the
presence of an extremely large amount of warm dust (~500-600 K) has been
suggested by mid-infrared (MIR) photometry and spectroscopy. The observed flux
densities of the debris disk at 60-160 micron are clearly above the
photospheric level of the star, suggesting excess emission at FIR as well as at
MIR wavelengths previously reported. The observed FIR excess emission is
consistent with the continuum level extrapolated from the MIR excess,
suggesting that it originates in the inner warm debris dust and cold dust
(~50-130 K) is absent in the outer region of the disk. The absence of cold dust
does not support a late heavy bombardment-like event as an origin of the large
amount of warm debris dust around HD 15047A. | astro-ph_EP |
Information Content Analysis for Selection of Optimal JWST Observing
Modes for Transiting Exoplanet Atmospheres: The James Webb Space Telescope (JWST) is nearing its launch date of 2018, and
will undoubtedly revolutionize our knowledge of exoplanet atmospheres. While
several studies have explored what the limits of the telescope will be in terms
of exoplanet characterization, less has been done to specifically identify
which observing modes will be most useful for characterizing a diverse range of
exoplanetary atmospheres. We use an information content based approach commonly
used in the studies of Solar System atmospheres. We develop a system based upon
these information content methods to trace the instrumental and atmospheric
model phase space in order to identify which observing modes are best suited
for particular classes of planets, focusing on transmission spectra.
Specifically, the atmospheric parameter space we cover is T=600-1800 K,
C/O=0.55-1, [M/H]=1-100$\times$Solar for a R=1.39 R$_J$, M=0.59 M$_J$ planet
orbiting WASP-62-like star. We find that obtaining broader wavelength coverage
over multiple modes is preferred over higher precision in a single mode given
the same amount of observing time. Regardless of the planet temperature and
composition, the best modes for constraining terminator temperatures, C/O
ratios, and metalicity are NIRISS SOSS+NIRSpec G395. If the target's host star
is dim enough such that the NIRSpec prism can be used, then it cam be used
instead of NIRISS SOSS+NIRSpec G395. Lastly, observations that use more than
two modes, should be carefully analyzed because sometimes the addition of a
third mode results in no gain of information. In these cases, higher precision
in the original two modes is favorable. | astro-ph_EP |
Collisional Grooming Models of the Kuiper Belt Dust Cloud: We modeled the 3-D structure of the Kuiper Belt dust cloud at four different
dust production rates, incorporating both planet-dust interactions and
grain-grain collisions using the collisional grooming algorithm. Simulated
images of a model with a face-on optical depth of ~10^-4 primarily show an
azimuthally-symmetric ring at 40-47 AU in submillimeter and infrared
wavelengths; this ring is associated with the cold classical Kuiper Belt. For
models with lower optical depths (10^-6 and 10^-7), synthetic infrared images
show that the ring widens and a gap opens in the ring at the location of of
Neptune; this feature is caused by trapping of dust grains in Neptune's mean
motion resonances. At low optical depths, a secondary ring also appears
associated with the hole cleared in the center of the disk by Saturn. Our
simulations, which incorporate 25 different grain sizes, illustrate that
grain-grain collisions are important in sculpting today's Kuiper Belt dust, and
probably other aspects of the Solar System dust complex; collisions erase all
signs of azimuthal asymmetry from the submillimeter image of the disk at every
dust level we considered. The model images switch from being dominated by
resonantly-trapped small grains ("transport dominated") to being dominated by
the birth ring ("collision dominated") when the optical depth reaches a
critical value of tau ~ v/c, where v is the local Keplerian speed. | astro-ph_EP |
The EChO science case: The discovery of almost 2000 exoplanets has revealed an unexpectedly diverse
planet population. Observations to date have shown that our Solar System is
certainly not representative of the general population of planets in our Milky
Way. The key science questions that urgently need addressing are therefore:
What are exoplanets made of? Why are planets as they are? What causes the
exceptional diversity observed as compared to the Solar System?
EChO (Exoplanet Characterisation Observatory) has been designed as a
dedicated survey mission for transit and eclipse spectroscopy capable of
observing a large and diverse planet sample within its four-year mission
lifetime. EChO can target the atmospheres of super-Earths, Neptune-like, and
Jupiter-like planets, in the very hot to temperate zones (planet temperatures
of 300K-3000K) of F to M-type host stars. Over the next ten years, several new
ground- and space-based transit surveys will come on-line (e.g. NGTS, CHEOPS,
TESS, PLATO), which will specifically focus on finding bright, nearby systems.
The current rapid rate of discovery would allow the target list to be further
optimised in the years prior to EChO's launch and enable the atmospheric
characterisation of hundreds of planets. Placing the satellite at L2 provides a
cold and stable thermal environment, as well as a large field of regard to
allow efficient time-critical observation of targets randomly distributed over
the sky. A 1m class telescope is sufficiently large to achieve the necessary
spectro-photometric precision. The spectral coverage (0.5-11 micron, goal 16
micron) and SNR to be achieved by EChO, thanks to its high stability and
dedicated design, would enable a very accurate measurement of the atmospheric
composition and structure of hundreds of exoplanets. | astro-ph_EP |
The Rate of Planet-star Coalescences Due to Tides and Stellar Evolution: Orbits of close-in planets can shrink significantly due to dissipation of
tidal energy in a host star. This process can result in star-planet coalescence
within the Galactic lifetime. In some cases, such events can be accompanied by
an optical or/and UV/X-ray transient. Potentially, these outbursts can be
observed in near future with new facilities such as LSST from distances about
few Mpc. We use a population synthesis model to study this process and derive
the rate of star-planet mergers of different types. Mostly, planets are
absorbed by red giants. However, these events, happening with the rate about 3
per year, mostly do not produce detectable transients. The rate of mergers with
main sequence stars depends on the effectiveness of tidal dissipation; for
reasonable values of stellar tidal quality factor, such events happen in a
Milky Way-like galaxy approximately once in 70 yrs or more rarely. This rate is
dominated by planets with low masses. Such events do not produce bright
transients having maximum luminosities $\lesssim 10^{36.5}$erg s$^{-1}$.
Brighter events, related to massive planets, with maximum luminosity $\sim
10^{37.5}$--$10^{38}$erg s$^{-1}$, have the rate nearly five times smaller. | astro-ph_EP |
Global Calculations of Density Waves and Gap Formation in Protoplanetary
Disks using a Moving Mesh: We calculate the global quasi-steady state of a thin disk perturbed by a
low-mass protoplanet orbiting at a fixed radius using extremely high-resolution
numerical integrations of Euler's equations in two dimensions. The calculations
are carried out using a moving computational domain, which greatly reduces
advection errors and allows for much longer time-steps than a fixed grid. We
calculate the angular momentum flux and the torque density as a function of
radius and compare them with analytical predictions. We discuss the
quasi-steady state after 100 orbits and the prospects for gap formation by low
mass planets. | astro-ph_EP |
Millimetre spectral indices of transition disks and their relation to
the cavity radius: Transition disks are protoplanetary disks with inner depleted dust cavities
and excellent candidates to investigate the dust evolution under the existence
of a pressure bump. A pressure bump at the outer edge of the cavity allows dust
grains from the outer regions to stop their rapid inward migration towards the
star and efficiently grow to millimetre sizes. Dynamical interactions with
planet(s) have been one of the most exciting theories to explain the clearing
of the inner disk. We look for evidence of the presence of millimetre dust
particles in transition disks by measuring their spectral index with new and
available photometric data. We investigate the influence of the size of the
dust depleted cavity on the disk integrated millimetre spectral index. We
present the 3mm photometric observations carried out with PdBI of four
transition disks: LkHa330, UXTauA, LRLL31, and LRLL67. We use available values
of their fluxes at 345GHz to calculate their spectral index, as well as the
spectral index for a sample of twenty transition disks. We compare the
observations with two kind of models. In the first set of models, we consider
coagulation and fragmentation of dust in a disk in which a cavity is formed by
a massive planet located at different positions. The second set of models
assumes disks with truncated inner parts at different radius and with power-law
dust size distributions, where the maximum size of grains is calculated
considering turbulence as the source of destructive collisions. We show that
the integrated spectral index is higher for transition disks than for regular
protoplanetary disks. For transition disks, the probability that the measured
spectral index is positively correlated with the cavity radius is 95%. High
angular resolution imaging of transition disks is needed to distinguish between
the dust trapping scenario and the truncated disk case. | astro-ph_EP |
Debiasing the Minimum-Mass Extrasolar Nebula: On the Diversity of Solid
Disk Profiles: A foundational idea in the theory of in situ planet formation is the "minimum
mass extrasolar nebula" (MMEN), a surface density profile ($\Sigma$) of disk
solids that is necessary to form the planets in their present locations. While
most previous studies have fit a single power-law to all exoplanets in an
observed ensemble, it is unclear whether most exoplanetary systems form from a
universal disk template. We use an advanced statistical model for the
underlying architectures of multi-planet systems to reconstruct the MMEN. The
simulated physical and Kepler-observed catalogs allows us to directly assess
the role of detection biases, and in particular the effect of non-transiting or
otherwise undetected planets, in altering the inferred MMEN. We find that
fitting a power-law of the form $\Sigma = \Sigma_0^* (a/a_0)^\beta$ to each
multi-planet system results in a broad distribution of disk profiles;
$\Sigma_0^* = 336_{-291}^{+727}$ g/cm$^2$ and $\beta = -1.98_{-1.52}^{+1.55}$
encompass the 16th-84th percentiles of the marginal distributions in an
underlying population, where $\Sigma_0^*$ is the normalization at $a_0 = 0.3$
AU. Around half of inner planet-forming disks have minimum solid masses of
$\gtrsim 40 M_\oplus$ within 1 AU. While transit observations do not tend to
bias the median $\beta$, they can lead to both significantly over- and
under-estimated $\Sigma_0^*$ and thus broaden the inferred distribution of disk
masses. Nevertheless, detection biases cannot account for the full variance in
the observed disk profiles; there is no universal MMEN if all planets formed in
situ. The great diversity of solid disk profiles suggests that a substantial
fraction ($\gtrsim 23\%$) of planetary systems experienced a history of
migration. | astro-ph_EP |
The Far-Ultraviolet "Continuum" in Protoplanetary Disk Systems I:
Electron-Impact H2 and Accretion Shocks: We present deep spectroscopic observations of the classical T Tauri stars DF
Tau and V4046 Sgr in order to better characterize two important sources of
far-ultraviolet continuum emission in protoplanetary disks. These new Hubble
Space Telescope-Cosmic Origins Spectrograph observations reveal a combination
of line and continuum emission from collisionally excited H2 and emission from
accretion shocks. H2 is the dominant emission in the 1400-1650 A band spectrum
of V4046 Sgr, while an accretion continuum contributes strongly across the
far-ultraviolet spectrum of DF Tau. We compare the spectrum of V4046 Sgr to
models of electron-impact induced H2 emission to constrain the physical
properties of the emitting region, after making corrections for attenuation
within the disk. We find reasonable agreement with the broad spectral
characteristics of the H2 model, implying N(H2) ~ 10^{18} cm^{-2}, T(H2) =
3000^{+1000}_{-500} K, and a characteristic electron energy in the range of ~
50 - 100 eV. We propose that self-absorption and hydrocarbons provide the
dominant attenuation for H2 line photons originating within the disk. For both
DF Tau and V4046 Sgr, we find that a linear fit to the far-UV data can
reproduce near-UV/optical accretion spectra. We discuss outstanding issues
concerning how these processes operate in protostellar/protoplanetary disks,
including the effective temperature and absolute strength of the radiation
field in low-mass protoplanetary environments. We find that the 912-2000A
continuum in low-mass systems has an effective temperature of ~10^{4} K with
fluxes 10^{5-7} times the interstellar level at 1 AU. | astro-ph_EP |
Water delivery to the TRAPPIST-1 planets: Three of the seven rocky planets (e, f, and g) in TRAPPIST-1 system orbit in
the habitable zone of the host star. Therefore, water can be in liquid state at
their surface being essential for life. Recent studies suggest that these
planets formed beyond the snow line in a water-rich region. The initial water
reservoir can be lost during the planet formation due to the stellar activity
of the infant low-mass star. However, a potential subsequent water delivery
event, like the late heavy bombardment (LHB) in the Solar System, can replenish
planetary water reservoirs. To study this water delivery process, we set up a
simple model in which an additional 5 M_Earth--50 M_Earth planet is embedded in
a water-rich asteroid belt beyond the snow line of TRAPPIST-1. Asteroids
perturbed out from the chaotic zone of the putative planet can enter into the
inner system and accreted by the known planets. Our main finding is that the
larger is the orbital distance of planet, the higher is the amount of water
delivered to the planet by an LHB-like event. | astro-ph_EP |
Nebular history of an ultrarefractory phase bearing CAI from a reduced
type CV chondrite: Ultrarefractory (UR) phases in CAIs could have formed at higher T compared to
common CAI minerals and thus they potentially provide constraints on very
high-T processes in the solar nebula. We report a detailed characterization of
an UR phase davisite bearing CAI from a reduced type CV chondrite. Absence of
secondary iron- and/or alkali-rich phases and occurrence of LIME olivine
indicate that primitive chemical and isotopic compositions are preserved in the
CAI. Davisite occur only in one lithological unit that consists of three
chemically and isotopically distinct parts: i) $^{16}$O-poor regions with
reversely-zoned melilite and davisite; ii) $^{16}$O-rich regions consisting of
unzoned, gehlenitic melilite, diopside and spinel; and iii) spinel framboids
composed of $^{16}$O-rich spinel and $^{16}$O-poor melilite. Random
distribution of chemical and isotopic heterogeneities with sharp boundaries in
the CAI indicates its formation by an aggregation of mineral assemblages formed
and processed separately at different time and/or space. Although isotope
exchange prior to the final agglomeration of the CAI cannot be ruled out, we
suggest that modification of chemical and isotopic composition of porous CAI
precursors or aggregation of isotopically distinct mineral assemblages are
alternative scenarios for the origin of O-isotopic heterogeneity in CAIs. In
either case, coexistence of spatially and/or temporally distinct $^{16}$O-rich
and -poor gaseous reservoirs at the earliest stage of the solar system
formation is required. The grain-scale oxygen isotopic disequilibrium in the
CAI indicate that post-formation heating of the CAI was short, which can be
achieved by rapid outward transport of the CAI. High
Ti$^{3+}$/Ti$^{\mathrm{tot}}$ ratios of pyroxene and presence of LIME olivine
document that the entire CAI formation process took place under highly reducing
conditions. | astro-ph_EP |
LHS6343C: A Transiting Field Brown Dwarf Discovered by the Kepler
Mission: We report the discovery of a brown dwarf that transits one member of the M+M
binary system LHS6343AB every 12.71 days. The transits were discovered using
photometric data from the Kelper public data release. The LHS6343 stellar
system was previously identified as a single high-proper-motion M dwarf. We use
high-contrast imaging to resolve the system into two low-mass stars with masses
0.45 Msun and 0.36 Msun, respectively, and a projected separation of 55 arcsec.
High-resolution spectroscopy shows that the more massive component undergoes
Doppler variations consistent with Keplerian motion, with a period equal to the
transit period and an amplitude consistent with a companion mass of M_C = 62.8
+/- 2.3 Mjup. Based on an analysis of the Kepler light curve we estimate the
radius of the companion to be R_C = 0.832 +/- 0.021 Rjup, which is consistent
with theoretical predictions of the radius of a > 1 Gyr brown dwarf. | astro-ph_EP |
Radiative transfer models of mid-infrared H2O lines in the
Planet-forming Region of Circumstellar Disks: The study of warm molecular gas in the inner regions of protoplanetary disks
is of key importance for the study of planet formation and especially for the
transport of H2O and organic molecules to the surfaces of rocky
planets/satellites. Recent Spitzer observations have shown that the
mid-infrared spectra of protoplanetary disks are covered in emission lines due
to water and other molecules. Here, we present a non-LTE 2D radiative transfer
model of water lines in the 10-36 mum range that can be used to constrain the
abundance structure of water vapor, given an observed spectrum, and show that
an assumption of local thermodynamic equilibrium (LTE) does not accurately
estimate the physical conditions of the water vapor emission zones. By applying
the model to published Spitzer spectra we find that: 1) most water lines are
subthermally excited, 2) the gas-to-dust ratio must be one to two orders of
magnitude higher than the canonical interstellar medium ratio of 100-200, and
3) the gas temperature must be higher than the dust temperature, and 4) the
water vapor abundance in the disk surface must be truncated beyond ~ 1 AU. A
low efficiency of water formation below ~ 300 K may naturally result in a lower
water abundance beyond a certain radius. However, we find that chemistry, may
not be sufficient to produce an abundance drop of many orders of magnitude and
speculate that the depletion may also be caused by vertical turbulent diffusion
of water vapor from the superheated surface to regions below the snow line,
where the water can freeze out and be transported to the midplane as part of
the general dust settling. Such a vertical cold finger effect is likely to be
efficient due to the lack of a replenishment mechanism of large, water-ice
coated dust grains to the disk surface. | astro-ph_EP |
Improvements on analytic modelling of stellar spots: In this work we present the solution of the stellar spot problem using the
Kelvin-Stokes theorem. Our result is applicable for any given location and
dimension of the spots on the stellar surface. We present explicitely the
result up to the second degree in the limb darkening law. This technique can be
used to calculate very efficiently mutual photometric effects produced by
eclipsing bodies occulting stellar spots and to construct complex spot shapes. | astro-ph_EP |
Reconstructing the photometric light curves of Earth as a planet along
its history: By utilizing satellite-based estimations of the distribution of clouds, we
have studied the Earth's large-scale cloudiness behavior according to latitude
and surface types (ice, water, vegetation and desert). These empirical
relationships are used here to reconstruct the possible cloud distribution of
historical epochs of the Earth's history such as the Late Cretaceous (90 Ma
ago), the Late Triassic (230 Ma ago), the Mississippian (340 Ma ago), and the
Late Cambrian (500 Ma ago), when the landmass distributions were different from
today's. With this information, we have been able to simulate the
globally-integrated photometric variability of the planet at these epochs. We
find that our simple model reproduces well the observed cloud distribution and
albedo variability of the modern Earth. Moreover, the model suggests that the
photometric variability of the Earth was probably much larger in past epochs.
This enhanced photometric variability could improve the chances for the
difficult determination of the rotational period and the identification of
continental landmasses for a distant planets. | astro-ph_EP |
Following the TraCS of exoplanets with Pan-Planets: Wendelstein-1b and
Wendelstein-2b: Hot Jupiters seem to get rarer with decreasing stellar mass. The goal of the
Pan-Planets transit survey was the detection of such planets and a statistical
characterization of their frequency. Here, we announce the discovery and
validation of two planets found in that survey, Wendelstein-1b and
Wendelstein-2b, which are two short-period hot Jupiters that orbit late K host
stars. We validated them both by the traditional method of radial velocity
measurements with the HIgh Resolution Echelle Spectrometer (HIRES) and the
Habitable-zone Planet Finder (HPF) instruments and then by their Transit Color
Signature (TraCS). We observed the targets in the wavelength range of $4000 -
24000$ Angstr\"om and performed a simultaneous multiband transit fit and
additionally determined their thermal emission via secondary eclipse
observations. Wendelstein-1b is a hot Jupiter with a radius of
$1.0314_{-0.0061}^{+0.0061}$ $R_J$ and mass of $0.592_{-0.129}^{+0.165}$ $M_J$,
orbiting a K7V dwarf star at a period of $2.66$ d, and has an estimated surface
temperature of about $1727_{-90}^{+78}$ K. Wendelstein-2b is a hot Jupiter with
a radius of $1.1592_{-0.0210}^{+0.0204}$ $R_J$ and a mass of
$0.731_{-0.311}^{+0.541}$ $M_J$, orbiting a K6V dwarf star at a period of
$1.75$ d, and has an estimated surface temperature of about
$1852_{-140}^{+120}$ K. With this, we demonstrate that multiband photometry is
an effective way of validating transiting exoplanets, in particular for fainter
targets since radial velocity (RV) follow-up becomes more and more costly for
those targets. | astro-ph_EP |
How initial and boundary conditions affect protoplanetary migration in a
turbulent sub-Keplerian accretion disc: 2D non viscous SPH simulations: Current theories on planetary formation establish that giant planet formation
should be contextual to their quick migration towards the central star due to
the protoplanets-disc interactions on a timescale of the order of $10^5$ years,
for objects of nearly 10 terrestrial masses. Such a timescale should be smaller
by an order of magnitude than that of gas accretion onto the protoplanet during
the hierarchical growing-up of protoplanets by collisions with other minor
objects. These arguments have recently been analysed using N-body and/or
fluid-dynamics codes or a mixing of them. In this work, inviscid 2D simulations
are performed, using the SPH method, to study the migration of one protoplanet,
to evaluate the effectiveness of the accretion disc in the protoplanet dragging
towards the central star, as a function of the mass of the planet itself, of
disc tangential kinematics. To this purpose, the SPH scheme is considered
suitable to study the roles of turbulence, kinematic and boundary conditions,
due to its intrinsic advective turbulence, especially in 2D and in 3D codes.
Simulations are performed both in disc sub-Keplerian and in Keplerian kinematic
conditions as a parameter study of protoplanetary migration if moderate and
consistent deviations from Keplerian Kinematics occur. Our results show
migration times of a few orbital periods for Earth-like planets in
sub-Keplerian conditions, while for Jupiter-like planets estimates give that
about $10^4$ orbital periods are needed to half the orbital size. Timescales of
planet migration are strongly dependent on the relative position of the planet
with respect to the shock region near the centrifugal barrier of the disc flow. | astro-ph_EP |
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