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Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the
Context of Its Environment: Here we review how environmental context can be used to interpret whether O2
is a biosignature in extrasolar planetary observations. This paper builds on
the overview of current biosignature research discussed in Schwieterman et al.
(2017), and provides an in-depth, interdisciplinary example of biosignature
identification and observation that serves as a basis for the development of
the general framework for biosignature assessment described in Catling et al.,
(2017). O2 is a potentially strong biosignature that was originally thought to
be an unambiguous indicator for life at high-abundance. We describe the
coevolution of life with the early Earth's environment, and how the interplay
of sources and sinks in the planetary environment may have resulted in
suppression of O2 release into the atmosphere for several billion years, a
false negative for biologically generated O2. False positives may also be
possible, with recent research showing potential mechanisms in exoplanet
environments that may generate relatively high abundances of atmospheric O2
without a biosphere being present. These studies suggest that planetary
characteristics that may enhance false negatives should be considered when
selecting targets for biosignature searches. Similarly our ability to interpret
O2 observed in an exoplanetary atmosphere is also crucially dependent on
environmental context to rule out false positive mechanisms. We describe future
photometric, spectroscopic and time-dependent observations of O2 and the
planetary environment that could increase our confidence that any observed O2
is a biosignature, and help discriminate it from potential false positives. By
observing and understanding O2 in its planetary context we can increase our
confidence in the remote detection of life, and provide a model for
biosignature development for other proposed biosignatures. | astro-ph_EP |
The Hill Stability of Triple Minor Planets in the Solar System: The triple asteroids and triple Kuiper belt objects (collectively called the
triple minor planets) in the Solar system are of particular interest to the
scientific community since the discovery of the first triple asteroid system in
2004. In this paper, the Hill stability of the nine known triple minor planets
in the Solar system is investigated. Seven of the systems are of large size
ratio, i.e. they consist of a larger primary and two moonlets, while the other
two systems have components of comparable size. Each case is treated
separately. For the triple minor planets that have large size ratio, the
sufficient condition for Hill stability is expressed in closed form. This is
not possible for the systems with comparable size components, for which the
Hill stability is assessed by a combination of analytical and numerical means.
It is found that all the known triple minor planets are Hill stable, except
3749 Balam, for which the incomplete orbital parameters make the Hill stability
of the system uncertain. This suggests that there might be more such stable
triple minor planets in the Solar system yet to be observed. It is also shown
that the Hill stability regions increase as the mutual inclination between the
inner orbit and outer orbit decreases, the semimajor axis ratio of the inner
orbit with respect to the outer orbit decreases, and the mass ratio of the
outer satellite with respect to the inner satellite increases. This study
therefore provides useful information about dynamical properties of the triple
minor planets in the Solar system. | astro-ph_EP |
Spatially Resolved Observations of Meteor Radio Afterglows with the
OVRO-LWA: We conducted an all-sky imaging transient search with the Owens Valley Radio
Observatory Long Wavelength Array (OVRO-LWA) data collected during the Perseid
meteor shower in 2018. The data collection during the meteor shower was
motivated to conduct a search for intrinsic radio emission from meteors below
60 MHz known as the meteor radio afterglows (MRAs). The data collected were
calibrated and imaged using the core array to obtain lower angular resolution
images of the sky. These images were input to a pre-existing LWA transient
search pipeline to search for MRAs as well as cosmic radio transients. This
search detected 5 MRAs and did not find any cosmic transients. We further
conducted peeling of bright sources, near-field correction, visibility
differencing and higher angular resolution imaging using the full array for
these 5 MRAs. These higher angular resolution images were used to study their
plasma emission structures and monitor their evolution as a function of
frequency and time. With higher angular resolution imaging, we resolved the
radio emission size scales to less than 1 km physical size at 100 km heights.
The spectral index mapping of one of the long duration event showed signs of
diffusion of plasma within the meteor trails. The unpolarized emission from the
resolved radio components suggest resonant transition radiation as the possible
radiation mechanism of MRAs. | astro-ph_EP |
How long-lived grains dominate the shape of the Zodiacal Cloud: Grain-grain collisions shape the 3-dimensional size and velocity distribution
of the inner Zodiacal Cloud and the impact rates of dust on inner planets, yet
they remain the least understood sink of zodiacal dust grains. For the first
time, we combine the collisional grooming method combined with a dynamical
meteoroid model of Jupiter-family comets (JFCs) that covers four orders of
magnitude in particle diameter to investigate the consequences of grain-grain
collisions in the inner Zodiacal Cloud. We compare this model to a suite of
observational constraints from meteor radars, the Infrared Astronomical
Satellite (IRAS), mass fluxes at Earth, and inner solar probes, and use it to
derive the population and collisional strength parameters for the JFC dust
cloud. We derive a critical specific energy of $Q^*_D=5\times10^5 \pm
4\times10^5 R_\mathrm{met}^{-0.24}$ J kg$^{-1}$ for particles from
Jupiter-family comet particles, making them 2-3 orders of magnitude more
resistant to collisions than previously assumed. We find that the differential
power law size index $-4.2\pm0.1$ for particles generated by JFCs provides a
good match to observed data. Our model provides a good match to the mass
production rates derived from the Parker Solar Probe observations and their
scaling with the heliocentric distance. The higher resistance to collisions of
dust particles might have strong implications to models of collisions in solar
and exo-solar dust clouds. The migration via Poynting-Roberson drag might be
more important for denser clouds, the mass production rates of astrophysical
debris disks might be overestimated, and the mass of the source populations
might be underestimated. Our models and code are freely available online. | astro-ph_EP |
Tilting Uranus: Collisions versus Spin--Orbit Resonance: In this paper, we investigate whether Uranus's 98$^{\circ}$ obliquity was a
by-product of a secular spin-orbit resonance assuming that the planet
originated closer to the Sun. In this position, Uranus's spin precession
frequency is fast enough to resonate with another planet located beyond Saturn.
Using numerical integration, we show that resonance capture is possible in a
variety of past solar system configurations, but that the timescale required to
tilt the planet to 90$^{\circ}$ is of the order $\sim\!10^{8}$ yr -- a timespan
that is uncomfortably long. A resonance kick could tilt the planet to a
significant 40$^{\circ}$ in $\sim\!10^{7}$ yr only if conditions were ideal. We
also revisit the collisional hypothesis for the origin of Uranus's large
obliquity. We consider multiple impacts with a new collisional code that builds
up a planet by summing the angular momentum imparted from impactors. Because
gas accretion imparts an unknown but likely large part of the planet's spin
angular momentum, we compare different collisional models for tilted, untilted,
spinning, and nonspinning planets. We find that a 1 $M_{\oplus}$ strike is
sufficient to explain the planet's current spin state, but that two
$0.5\,M_{\oplus}$ collisions produce better likelihoods. Finally, we
investigate hybrid models and show that resonances must produce a tilt of at
least $\sim\!40^{\circ}$ for any noticeable improvements to the collision
model. Because it is difficult for spin-orbit resonances to drive Uranus's
obliquity to 98$^{\circ}$ even under these ideal conditions, giant impacts seem
inescapable. | astro-ph_EP |
Simulating the Formation of Carbon-rich Molecules on an idealised
Graphitic Surface: There is accumulating evidence for the presence of complex molecules,
including carbon-bearing and organic molecules, in the interstellar medium.
Much of this evidence comes to us from studies of chemical composition, photo-
and mass-spectroscopy in cometary, meteoritic and asteroid samples, indicating
a need to better understand the surface chemistry of astrophysical objects.
There is also considerable interest in the origins of life-forming and
life-sustaining molecules on Earth. Here, we perform reactive molecular
dynamics simulations to probe the formation of carbon-rich molecules and
clusters on carbonaceous surfaces resembling dust grains and meteoroids. Our
results show that large chains form on graphitic surfaces at low temperatures
(100K - 500K) and smaller fullerene-like molecules form at higher temperatures
(2000K - 3000K). The formation is faster on the surface than in the gas at low
temperatures but slower at high temperatures as surface interactions prevent
small clusters from coagulation. We find that for efficient formation of
molecular complexity, mobility about the surface is important and helps to
build larger carbon chains on the surface than in the gas phase at low
temperatures. Finally, we show that the temperature of the surface strongly
determines what kind of structures forms and that low turbulent environments
are needed for efficient formation. | astro-ph_EP |
Asteroid models from generalised projections: Essential facts for
asteroid modellers and geometric inverse problem solvers: We present a review of the problem of asteroid shape and spin reconstruction
from generalised projections; i.e., from lightcurves, disk-resolved images,
occultation silhouettes, radar range-Doppler data, and interferometry. The aim
of this text is to summarize all important mathematical facts and proofs
related to this inverse problem, to describe their implications to observers
and modellers, and to provide the reader with all relevant references. | astro-ph_EP |
Comparison of planetary Hα-emission models: A new correlation with
accretion luminosity: Accreting planets have been detected through their hydrogen-line emission,
specifically H$\alpha$. To interpret this, stellar-regime empirical
correlations between the H$\alpha$ luminosity $L_\mathrm{H\alpha}$ and the
accretion luminosity $L_\mathrm{acc}$ or accretion rate $\dot{M}$ have been
extrapolated to planetary masses, however without validation. We present a
theoretical $L_\mathrm{acc}$--$L_\mathrm{H\alpha}$ relationship applicable to a
shock at the surface of a planet. We consider wide ranges of accretion rates
and masses and use detailed spectrally-resolved, non-equilibrium models of the
postshock cooling. The new relationship gives a markedly higher
$L_\mathrm{acc}$ for a given $L_\mathrm{H\alpha}$ than fits to young stellar
objects, because Ly-$\alpha$, which is not observable, carries a large fraction
of $L_\mathrm{acc}$. Specifically, an $L_\mathrm{H\alpha}$ measurement needs
ten to 100 times higher $L_\mathrm{acc}$ and $\dot{M}$ than previously
predicted, which may explain the rarity of planetary H$\alpha$ detections. We
also compare the $\dot{M}$--$L_\mathrm{H\alpha}$ relationships coming from the
planet-surface shock or implied by accretion-funnel emission. Both can
contribute simultaneously to an observed H$\alpha$ signal but at low (high)
$\dot{M}$ the planetary-surface shock (heated funnel) dominates. Only the shock
produces Gaussian line wings. Finally, we discuss accretion contexts in which
different emission scenarios may apply, putting recent literature models in
perspective, and also present $L_\mathrm{acc}$--$L_\mathrm{line}$ relationships
for several other hydrogen lines. | astro-ph_EP |
A Harsh Test of Far-Field Scrambling with the Habitable Zone Planet
Finder and the Hobby Eberly Telescope: The Habitable zone Planet Finder (HPF) is a fiber fed precise radial velocity
spectrograph at the 10 m Hobby Eberly Telescope (HET). Due to its fixed
altitude design, the HET pupil changes appreciably across a track, leading to
significant changes of the fiber far-field illumination. HPF's fiber scrambler
is designed to suppress the impact of these illumination changes on the radial
velocities -- but the residual impact on the radial velocity measurements has
yet to be probed on sky. We use GJ 411, a bright early type (M2) M dwarf to
probe the effects of far-field input trends due to these pupil variations on
HPF radial velocities (RVs). These large changes ($\sim$ 2x) in pupil area and
centroid present a harsh test of HPF's far-field scrambling. Our results show
that the RVs are effectively decoupled from these extreme far-field input
changes due to pupil centroid offsets, attesting to the effectiveness of the
scrambler design. This experiment allows us to test the impact of these changes
with large pupil variation on-sky, something we would not easily be able to do
at a conventional optical telescope. While the pupil and illumination changes
expected at these other telescopes are small, scaling from our results enables
us to estimate and bound these effects, and show that they are controllable
even for the new and next generation of RV instruments in their quest to beat
down instrumental noise sources towards the goal of a few cm/s. | astro-ph_EP |
Localized precipitation and runoff on Mars: We use the Mars Regional Atmospheric Modeling System (MRAMS) to simulate lake
storms on Mars, finding that intense localized precipitation will occur for
lake size >=10^3 km^2. Mars has a low-density atmosphere, so deep convection
can be triggered by small amounts of latent heat release. In our reference
simulation, the buoyant plume lifts vapor above condensation level, forming a
20km-high optically-thick cloud. Ice grains grow to 200 microns radius and fall
near (or in) the lake at mean rates up to 1.5 mm/hr water equivalent (maximum
rates up to 6 mm/hr water equivalent). Because atmospheric temperatures outside
the surface layer are always well below 273K, supersaturation and condensation
begin at low altitudes above lakes on Mars. In contrast to Earth lake-effect
storms, lake storms on Mars involve continuous precipitation, and their
vertical velocities and plume heights exceed those of tropical thunderstorms on
Earth. Convection does not reach above the planetary boundary layer for lakes
<<10^3 km^2 or for atmospheric pressure >O(10^2) mbar. Instead, vapor is
advected downwind with little cloud formation. Precipitation occurs as snow,
and the daytime radiative forcing at the land surface due to plume vapor and
storm clouds is too small to melt snow directly (<+10 W/m^2). However, if
orbital conditions are favorable, then the snow may be seasonally unstable to
melting and produce runoff to form channels. We calculate the probability of
melting by running thermal models over all possible orbital conditions and
weighting their outcomes by probabilities given by Laskar et al., 2004. We
determine that for an equatorial vapor source, sunlight 15% fainter than at
present, and snowpack with albedo 0.28 (0.35), melting may occur with 4%(0.1%)
probability. This rises to 56%(12%) if the ancient greenhouse effect was
modestly (6K) greater than today. | astro-ph_EP |
An integrable model for first-order three-planet mean motion resonances: Recent works on three-planet mean motion resonances (MMRs) have highlighted
their importance for understanding the details of the dynamics of planet
formation and evolution. While the dynamics of two-planet MMRs are well
understood and approximately described by a one degree of freedom Hamiltonian,
little is known of the exact dynamics of three-bodies resonances besides the
cases of zeroth-order MMRs or when one of the body is a test particle. In this
work, I propose the first general integrable model for first-order three-planet
mean motion resonances. I show that one can generalize the strategy proposed in
the two-planet case to obtain a one degree of freedom Hamiltonian. The dynamics
of these resonances are governed by the second fundamental model of resonance.
The model is valid for any mass ratio between the planets and for every
first-order resonance. I show the agreement of the analytical model with
numerical simulations. As examples of application I show how this model could
improve our understanding of the capture into MMRs as well as their role on the
stability of planetary systems. | astro-ph_EP |
A Pluto--Charon Sonata: Dynamical Limits on the Masses of the Small
Satellites: During 2005-2012, images from Hubble Space Telescope (HST) revealed four
moons orbiting Pluto-Charon (Weaver et al 2006, Showalter et al 2011, 2012).
Although their orbits and geometric shapes are well-known, the 2$\sigma$
uncertainties in the masses of the two largest satellites - Nix and Hydra - are
comparable to their HST masses (Brozovic et al 2015, Showalter & Hamilton 2015,
Weaver et al 2016). Remarkably, gravitational $n$-body computer calculations of
the long-term system stability on 0.1-1 Gyr time scales place much tighter
constraints on the masses of Nix and Hydra, with upper limits $\sim$ 10% larger
than the HST mass. Constraints on the mass density using size measurements from
New Horizons suggest Nix and Hydra formed in icier material than Pluto and
Charon. | astro-ph_EP |
Flybys in debris disk systems with Gaia eDR3: We aim to observationally and statistically constrain the influence of flybys
in the formation and evolution of debris disks. We compiled a sample of 254
debris disks with ages between 2 Myr and 8 Gyr that are either part of an
association or isolated, drawing the binary and planetary companions of the
systems mainly from the literature. Using the Gaia eDR3 astrometric data and
radial velocities of our sample, as well as all the sources in a specific
region of the sky, we reconstructed the relative linear motions in the last 5
Myr and made predictions for the next 2 Myr. Relating the Hill radius of each
debris disk system and the closest distances reached by the two sources, we
defined the flyby events in terms of position and time. We find that in the
period between the last 5 Myrs and the next 2 Myrs, 90% of the analyzed systems
have experienced at least a close flyby, while 7% of them have experienced
flybys at distances greater than 0.5R Hill. In particular, 75% of them have
experienced at least one past close encounter and 36% multiple past close
encounters. From the sub-sample of resolved debris disk (41 out of 94), 80% of
the analyzed systems experience at least an encounter within 0.8 pc. From the
subsample of 10 debris disks with planets, half of these systems do show
misalignments between disk and planet, stirring, or asymmetries. Systems with a
misalignment between the planetary orbit and the disk do indeed experience at
least one flyby event. In particular, when the planet orbits have a difference
with the disk inclination higher than about 20 degree, as in the case of HD
38529, we find that multiple close encounters have taken place in the last 5
Myr, as theoretically predicted. The high incidence of encounters, particularly
close encounters, experienced by the systems in the last 5 Myr suggests the
fundamental impact of flybys on the evolution of debris disks. | astro-ph_EP |
Interpretation and diversity of exoplanetary material orbiting white
dwarfs: Nine metal-polluted white dwarfs are observed with medium-resolution optical
spectroscopy,where photospheric abundances are determined and interpreted
through comparison against solar system objects. An improved method of making
such comparisons is presented that overcomes potential weaknesses of prior
analyses, with the numerous sources of error considered to highlight the
limitations on interpretation. The stars are inferred to be accreting rocky,
volatile-poor asteroidal materials with origins in differentiated bodies, in
line with the consensus model. The most heavily polluted star in the sample has
14 metals detected, and appears to be accreting material from a rocky
planetesimal, whose composition is mantle-like with a small Fe-Ni core
component. Some unusual abundances are present: one star is strongly depleted
in Ca, while two others show Na abundances elevated above bulk Earth,
speculated either to reflect diversity in the formation conditions of the
source material, or to be traces of past accretion events. Another star shows
clear signs that accretion ceased around 5 Myr ago,causing Mg to dominate the
photospheric abundances, as it has the longest diffusion time of the observed
elements. Observing such post-accretion systems allows constraints to be placed
on models of the accretion process. | astro-ph_EP |
Early Mars' habitability and global cooling by H2-based methanogens: During the Noachian, Mars' crust may have provided a favorable environment
for microbial life. The porous brine-saturated regolith would have created a
physical space sheltered from UV and cosmic radiations and provided a solvent,
while the below-ground temperature and diffusion of a dense reduced atmosphere
may have supported simple microbial organisms that consume H2 and CO2 as energy
and carbon sources and produce methane as a waste. On Earth, hydrogenotrophic
methanogenesis was among the earliest metabolisms but its viability on early
Mars has never been quantitatively evaluated. Here we present a probabilistic
assessment of Mars' Noachian habitability to H2-based methanogens, and quantify
their biological feedback on Mars' atmosphere and climate. We find that
subsurface habitability was very likely, and limited mainly by the extent of
surface ice coverage. Biomass productivity could have been as high as in early
Earth's ocean. However, the predicted atmospheric composition shift caused by
methanogenesis would have triggered a global cooling event, ending potential
early warm conditions, compromising surface habitability and forcing the
biosphere deep into the Martian crust. Spatial projections of our predictions
point to lowland sites at low-to-medium latitudes as good candidates to uncover
traces of this early life at or near the surface. | astro-ph_EP |
Salty ice and the dilemma of ocean exoplanet habitability: Habitability of exoplanet's deepest oceans could be limited by the presence
of high-pressure ices at their base. New work demonstrates that efficient
chemical transport within deep planetary ice mantles is possible through
significant salt incorporation within the high-pressure ice. | astro-ph_EP |
The HARPS search for southern extra-solar planets XL. Searching for
Neptunes around metal-poor stars: Stellar metallicity -- as a probe of the metallicity of proto-planetary disks
-- is an important ingredient for giant planet formation, likely through its
effect on the timescales in which rocky/icy planet cores can form. Giant
planets have been found to be more frequent around metal-rich stars, in
agreement with predictions based on the core-accretion theory. In the
metal-poor regime, however, the frequency of planets, especially low-mass
planets, and how it depends on metallicity are still largely unknown. As part
of a planet search programme focused on metal-poor stars, we study the targets
from this survey that were observed with HARPS on more than 75 nights. The main
goals are to assess the presence of low-mass planets and provide a first
estimate of the frequency of Neptunes and super-Earths around metal-poor stars.
We perform a systematic search for planetary companions, both by analysing the
periodograms of the radial-velocities and by comparing, in a
statistically-meaningful way, models with an increasing number of Keplerians. A
first constraint on the frequency of planets in our metal-poor sample is
calculated considering the previous detection (in our sample) of a
Neptune-sized planet around HD175607 and one candidate planet (with an orbital
period of 68.42d and minimum mass $M_p \sin i = 11.14 \pm 2.47 M_{\oplus}$) for
HD87838, announced in the present study. This frequency is determined to be
close to 13% and is compared with results for solar-metallicity stars. | astro-ph_EP |
On the stability of low-mass planets with supercritical hydrospheres: Short-period and low-mass water-rich planets are subject to strong
irradiation from their host star, resulting in hydrospheres in supercritical
state. In this context, we explore the role of irradiation on small terrestrial
planets that are moderately wet in the low-mass regime (0.2--1$M_{\oplus}$). We
investigate their bulk properties for water contents in the 0.01--5\% range by
making use of an internal structure model that is coupled to an atmosphere
model. This coupling allows us to take into account both the compression of the
interior due to the weight of the hydrosphere and the possibility of
atmospheric instability in the low-mass regime. We show that even for low
masses and low water contents, these planets display inflated atmospheres. For
extremely low planetary masses and high irradiation temperatures, we find that
steam atmospheres become gravitationally unstable when the ratio $\eta$ of
their scale height to planetary radius exceeds a critical value of $\sim 0.1$.
This result is supported by observational data, as all currently detected
exoplanets exhibit values of $\eta$ smaller than 0.013. Depending on their
water content, our results show that highly irradiated and low-mass planets up
to $0.9M_{\oplus}$ with significative hydrospheres are not in stable form and
should loose their volatile envelope. | astro-ph_EP |
Dust Coagulation in the Vicinity of a Gap-Opening Jupiter-Mass Planet: We analyze the coagulation of dust in and around a gap opened by a
Jupiter-mass planet. To this end, we carry out a high-resolution
magnetohydrodynamic (MHD) simulation of the gap environment, which is turbulent
due to the magnetorotational instability. From the MHD simulation, we obtain
values of the gas velocities, densities and turbulent stresses a) close to the
gap edge, b) in one of the two gas streams that accrete onto the planet, c)
inside the low-density gap, and d) outside the gap. The MHD values are then
supplied to a Monte Carlo dust coagulation algorithm, which models grain
sticking and compaction. We consider two dust populations for each region: one
whose initial size distribution is monodisperse, with monomer radius equal to 1
$\mu$m, and another one whose initial size distribution follows the
Mathis-Rumpl-Nordsieck distribution for interstellar dust grains, with an
initial range of monomer radii between 0.5 and 10 $\mu$m. Our Monte Carlo
calculations show initial growth of dust aggregates followed by compaction in
all cases but one, that of aggregates belonging to the initially monodisperse
population subject to gas conditions outside the gap. In this latter case, the
mass-weighted (MW) average porosity of the population reaches extremely high
final values of 98\%. The final MW porosities in all other cases range between
30\% and 82\%. The efficiency of compaction is due to high turbulent relative
speeds between dust particles. Future studies will need to explore the effect
of different planet masses and electric charge on grains. | astro-ph_EP |
Influence of Stellar Multiplicity On Planet Formation. IV. Adaptive
Optics Imaging of Kepler Stars With Multiple Transiting Planet Candidates: The Kepler mission provides a wealth of multiple transiting planet systems
(MTPS). The formation and evolution of multi-planet systems are likely to be
influenced by companion stars given the abundance of multi stellar systems. We
study the influence of stellar companions by measuring the stellar multiplicity
rate of MTPS. We select 138 bright (KP < 13.5) Kepler MTPS and search for
stellar companions with AO imaging data and archival radial velocity (RV) data.
We obtain new AO images for 73 MTPS. Other MTPS in the sample have archival AO
imaging data from the Kepler Community Follow-up Observation Program (CFOP).
From these imaging data, we detect 42 stellar companions around 35 host stars.
For stellar separation 1 AU < a < 100 AU, the stellar multiplicity rate is 5.2
$\pm$ 5.0% for MTPS, which is 2.8{\sigma} lower than 21.1 $\pm$ 2.8% for the
control sample, i.e., the field stars in the solar neighborhood. We identify
two origins for the deficit of stellar companions within 100 AU to MTPS: (1) a
suppressive planet formation, and (2) the disruption of orbital coplanarity due
to stellar companions. To distinguish between the two origins, we compare the
stellar multiplicity rates of MTPS and single transiting planet systems (STPS).
However, current data are not sufficient for this purpose. For 100 AU < a <
2000 AU, the stellar multiplicity rates are comparable for MTPS (8.0 $\pm$
4.0%), STPS (6.4 $\pm$ 5.8%), and the control sample (12.5 $\pm$ 2.8%). | astro-ph_EP |
Ejecta Cloud from a Kinetic Impact on the Secondary of a Binary
Asteroid: I. Mechanical Environment and Dynamic Model: An understanding of the post-impact dynamics of ejecta clouds are crucial to
the planning of a kinetic impact mission to an asteroid, and also has great
implications for the history of planetary formation. The purpose of this
article to track the evolution of ejecta produced by AIDA mission, which
targets for kinetic impact the secondary of near-Earth binary asteroid 65803
Didymos on 2022, and to feedback essential informations to AIDA's ongoing
phase-A study. We present a detailed dynamic model for the simulation of an
ejecta cloud from a binary asteroid that synthesizes all relevant forces based
on a previous analysis of the mechanical environment. We apply our method to
gain insight into the expected response of Didymos to the AIDA impact,
including the subsequent evolution of debris and dust. The crater scaling
relations from laboratory experiments are employed to approximate the
distributions of ejecta mass and launching speed. The size composition of
fragments is modeled with a power law fitted from observations of real asteroid
surface. A full-scale demonstration is simulated using parameters specified by
the mission. We report the results of the simulation, which include the
computed spread of the ejecta cloud and the recorded history of ejecta
accretion and escape. The violent period of the ejecta evolution is found to be
short, and is followed by a stage where the remaining ejecta is gradually
cleared. Solar radiation pressure proves to be efficient in cleaning dust-size
ejecta, and the simulation results after two weeks shows that large debris on
polar orbits (perpendicular to the binary orbital plane) has a survival
advantage over smaller ejecta and ejecta that keep to low latitudes. | astro-ph_EP |
Nonlinear behaviour of warped discs around a central object with a
quadrupole moment: The nonlinear behaviour of low-viscosity warped discs is poorly understood.
We verified a nonlinear bending-wave theory, in which fluid columns undergo
affine transformations, with direct 3D hydrodynamical simulations. We employed
a second-order Godunov-type scheme, Meshless Finite Mass (MFM), and also the
Smoothed Particle Hydrodynamics (SPH) method, with up to 128M particles. For
moderate nonlinearity, MFM maintains well the steady nonlinear warp predicted
by the affine model for a tilted inviscid disc around a central object with a
quadrupole moment. However, numerical dissipation in SPH is so severe that even
a low-amplitude nonlinear warp degrades at a resolution where MFM performs
well. A low-amplitude arbitrary warp tends to evolve towards a nonlinear steady
state. However, no such state exists in our thin disc with an angular
semi-thickness H/R = 0.02 when the outer tilt angle is beyond about 14 degrees.
The warp breaks tenuously and reconnects in adiabatic simulations, or breaks
into distinct annuli in isothermal simulations. The breaking radius lies close
to the location with the most extreme nonlinear deformation. Parametric
instability is captured only in our highest-resolution simulation, leading to
ring structures that may serve as incubators for planets around binaries. | astro-ph_EP |
Surveying the Inner Solar System with an Infrared Space Telescope: We present an analysis of surveying the inner Solar System for objects that
may pose some threat to the Earth. Most of the analysis is based on
understanding the capability provided by Sentinel, a concept for an infrared
space-based telescope placed in a heliocentric orbit near the distance of
Venus. From this analysis, we show 1) the size range being targeted can affect
the survey design, 2) the orbit distribution of the target sample can affect
the survey design, 3) minimum observational arc length during the survey is an
important metric of survey performance, and 4) surveys must consider objects as
small as D=15-30 m to meet the goal of identifying objects that have the
potential to cause damage on Earth in the next 100 years. Sentinel will be able
to find 50% of all impactors larger than 40 meters in a 6.5 year survey. The
Sentinel mission concept is shown to be as effective as any survey in finding
objects bigger than D=140 m but is more effective when applied to finding
smaller objects on Earth-impacting orbits. Sentinel is also more effective at
finding objects of interest for human exploration that benefit from lower
propulsion requirements. To explore the interaction between space and ground
search programs, we also study a case where Sentinel is combined with the Large
Synoptic Survey Telescope and show the benefit of placing a space-based
observatory in an orbit that reduces the overlap in search regions with a
ground-based telescope. In this case, Sentinel+LSST can find more than 70% of
the impactors larger than 40 meters assuming a 6.5 year lifetime for Sentinel
and 10 years for LSST. | astro-ph_EP |
Diversity of planetary systems in low-mass disks: Terrestrial-type
planet formation and water delivery: Several studies, observational and theoretical, suggest that planetary
systems with only rocky planets should be the most common in the Universe. We
study the diversity of planetary systems that might form around Sun-like stars
in low-mass disks without giant planets. We focus on the formation process of
terrestrial planets in the habitable zone (HZ) and analyze their water contents
with the goal to determine systems of astrobiological interest. Besides, we
study the formation of planets on wide orbits because they can be detected with
the microlensing technique. N-body simulations of high resolution (embryos +
planetesimals) are developed for a wide range of surface density profiles. The
surface density profile combines a power law to the inside of the disk of the
form r^{-gamma}, with an exponential decay to the outside. We adopt a disk of
0.03M_sun and values of gamma = 0.5, 1 and 1.5. All our simulations form
planets in the HZ with different masses and final water contents depending on
the 3 profiles. For gamma = 0.5, we produce 3 planets in the HZ with masses
between 0.03 M_e to 0.1 M_e and water contents between 0.2 and 16 Earth oceans.
For gamma = 1, 3 planets form in the HZ with masses between 0.18 M_e and 0.52
M_e and water contents from 34 to 167 Earth oceans. For gamma = 1.5, we find 4
planets in the HZ with masses from 0.66 M_e to 2.21 M_e and water contents
between 192 and 2326 Earth oceans. This profile shows distinctive results
because it is the only one of those studied here that leads to the formation of
water worlds. Since planetary systems with gamma = 1 and 1.5 present planets in
the HZ with suitable masses to retain a long-live atmosphere and to maintain
plate tectonics, they seem to be the most outstanding candidates to be
potentially habitable. Particularly, these systems form Earths and Super-Earths
near the snow line which can be discovered by microlensing. | astro-ph_EP |
Effect of Dust Size on the Near-Infrared Spectra (1.0-5.0 $μ$m) of
Brown Dwarf Atmospheres: In this study, we demonstrate the dependence of atmospheric dust size on the
near-infrared spectra of ten L dwarfs, and constrain the sizes of dust grains
in each L dwarf atmosphere. In previous studies, by comparing observed and
modeled spectra, it was suggested that the deviations of their spectral shapes
from theoretical prediction are general characteristics. Here, we focus on the
dust size in brown dwarf atmospheres to understand the observed spectra. We
confirm that changing the dust size changes the temperature-pressure structure
of the atmosphere, with the shape of the spectrum changing accordingly. At the
wavelength at which dust is the main absorber of radiation (the dust-dominated
regime), a large dust opacity combined with a medium grain size, e.g., 0.1
$\mu$m, results in a low photospheric temperature, and thus a small flux.
Conversely, for the wavelength at which gas absorption is dominant (the
gas-dominated regime), a large dust opacity modifies the temperature-pressure
structure, resulting in a high photospheric temperature, which corresponds to
large flux emissions. Taking into account the size effect, we compare the model
spectral fluxes in the wavelength range 1-5 $\mu$m with the observational ones
to constrain the main dust size in the atmosphere of each of the ten L dwarfs
observed with AKARI and SpeX or CGS4. Ultimately, we reveal that the observed
data are reproduced with higher fidelity by models based on a medium dust size
of 0.1-3.0 $\mu$m for six of these L dwarfs; therefore, we suggest that such
atmospheric dust sizes apply to the majority of L dwarfs. | astro-ph_EP |
EPIC Simulations of Neptune's Dark Spots Using an Active Cloud
Microphysical Model: The Great Dark Spot (GDS-89) observed by Voyager 2 was the first of several
large-scale vortices observed on Neptune, the most recent of which was observed
in 2018 in the northern hemisphere (NDS-2018). Ongoing observations of these
features are constraining cloud formation, drift, shape oscillations, and other
dynamic properties. In order to effectively model these characteristics, an
explicit calculation of methane cloud microphysics is needed. Using an updated
version of the Explicit Planetary Isentropic Coordinate General Circulation
Model (EPIC GCM) and its active cloud microphysics module to account for the
condensation of methane, we investigate the evolution of large scale vortices
on Neptune. We model the effect of methane deep abundance and cloud formation
on vortex stability and dynamics. In our simulations, the vortex shows a sharp
contrast in methane vapor density inside compared to outside the vortex.
Methane vapor column density is analogous to optical depth and provides a more
consistent tracer to track the vortex, so we use that variable over potential
vorticity. We match the meridional drift rate of the GDS and gain an initial
insight into the evolution of vortices in the northern hemisphere, such as the
NDS-2018. | astro-ph_EP |
Tidal Evolution of Close Binary Asteroid Systems: We provide a generalized discussion of tidal evolution to arbitrary order in
the expansion of the gravitational potential between two spherical bodies of
any mass ratio. To accurately reproduce the tidal evolution of a system at
separations less than five times the radius of the larger primary component,
the tidal potential due to the presence of a smaller secondary component is
expanded in terms of Legendre polynomials to arbitrary order rather than
truncated at leading order as is typically done in studies of well-separated
system like the Earth and Moon. The equations of tidal evolution including
tidal torques, the changes in spin rates of the components, and the change in
semimajor axis (orbital separation) are then derived for binary asteroid
systems with circular and equatorial mutual orbits. Accounting for higher-order
terms in the tidal potential serves to speed up the tidal evolution of the
system leading to underestimates in the time rates of change of the spin rates,
semimajor axis, and mean motion in the mutual orbit if such corrections are
ignored. Special attention is given to the effect of close orbits on the
calculation of material properties of the components, in terms of the rigidity
and tidal dissipation function, based on the tidal evolution of the system. It
is found that accurate determinations of the physical parameters of the system,
e.g., densities, sizes, and current separation, are typically more important
than accounting for higher-order terms in the potential when calculating
material properties. In the scope of the long-term tidal evolution of the
semimajor axis and the component spin rates, correcting for close orbits is a
small effect, but for an instantaneous rate of change in spin rate, semimajor
axis, or mean motion, the close-orbit correction can be on the order of tens of
percent. | astro-ph_EP |
A Bayesian Periodogram Finds Evidence for Three Planets in 47 Ursae
Majoris: A Bayesian analysis of 47 Ursae Majoris (47 UMa) radial velocity data
confirms and refines the properties of two previously reported planets with
periods of 1079 and 2325 days and finds evidence for an additional long period
planet with a period of approximately 10000 days. The three planet model is
found to be 10^5 times more probable than the next most probable model which is
a two planet model. The nonlinear model fitting is accomplished with a new
hybrid Markov chain Monte Carlo (HMCMC) algorithm which incorporates parallel
tempering, simulated annealing and genetic crossover operations. Each of these
features facilitate the detection of a global minimum in chi-squared. By
combining all three, the HMCMC greatly increases the probability of realizing
this goal. When applied to the Kepler problem it acts as a powerful
multi-planet Kepler periodogram. The measured periods are 1078 \pm 2,
2391{+100}{-87}, and 14002{+4018}{-5095}d, and the corresponding eccentricities
are 0.032 \pm 0.014, 0.098{+.047}{-.096}, and 0.16{+.09}{-.16}. The results
favor low eccentricity orbits for all three. Assuming the three signals (each
one consistent with a Keplerian orbit) are caused by planets, the corresponding
limits on planetary mass (M sin i) and semi-major axis are (2.53{+.07}{-.06}MJ,
2.10\pm0.02au), (0.54\pm0.07MJ, 3.6\pm0.1au), and (1.6{+0.3}{-0.5}MJ,
11.6{+2.1}{-2.9}au), respectively. We have also characterized a noise induced
eccentricity bias and designed a correction filter that can be used as an
alternate prior for eccentricity, to enhance the detection of planetary orbits
of low or moderate eccentricity. | astro-ph_EP |
On the nature of the transition disk around LkCa 15: We present CARMA 1.3 mm continuum observations of the T Tauri star LkCa
15,which resolve the circumstellar dust continuum emission on angular scales
between 0.2-3 arcsec, corresponding to 28-420 AU at the distance of the star.
The observations resolve the inner gap in the dust emission and reveal an
asymmetric dust distribution in the outer disk. (Abridge) We calculate that 90%
of the dust emission arises from an azimuthally symmetric ring that contains
about 5x10^{-4} M_sun of dust. A low surface-brightness tail that extends to
the northwest out to a radius of about 300 AU contains the remaining 10% of the
observed continuum emission. The ring is modeled with a rather flat surface
density profile between 40 and 120 AU, while the inner cavity is consistent
with either a sharp drop of the 1.3 mm dust optical depth at about 42 AU or a
smooth inward decrease between 3 and 85 AU. (Abridge). Within 40 AU, the
observations constrain the amount of dust between 10^{-6} and 7 Earth masses,
where the minimum and maximum limits are set by the near-IR SED modeling and by
the mm-wave observations of the dust emission respectively. In addition, we
confirm the discrepancy in the outer disk radius inferred from the dust and
gas, which corresponds to 150 AU and 900 AU respectively. We cannot reconcile
this difference by adopting an exponentially tapered surface density profile as
suggested for other systems, but we instead suggest that the gas surface
density in the outer disk decreases less steeply than that predicted by model
fits to the dust continuum emission. The lack of continuum emission at radii
lager than 120 AU suggests a drop of at least a factor of 5 in the dust-to-gas
ratio, or in the dust opacity. We show that a sharp dust opacity drop of this
magnitude is consistent with a radial variation of the grain size distribution
as predicted by existing grain growth models. | astro-ph_EP |
Model for Nitric oxide and its dayglow emission in the Martian upper
atmosphere using NGIMS/MAVEN measured neutral and ion densities: A comprehensive study of Nitric oxide (NO) chemistry in the Martian upper
atmosphere is restricted due to the lack of requisite measurements. NO is an
abundant form of odd nitrogen species in the Martian lower atmosphere and its
density depends on several photochemical processes. We have developed a
photochemical model to study the NO density in the dayside of Martian upper
atmosphere by accounting for various production and loss mechanisms. By
utilizing the Neutral Gas and Ion Mass Spectrometer (NGIMS) on-board Mars
Atmosphere and Volatile Evolution (MAVEN) mission measured neutral and ion
densities during deep dip 8 and 9 campaigns, we modelled NO number density in
the Martian sunlit upper atmosphere for the altitudes between 120 and 200 km.
The modelled NO densities are employed to calculate NO (1,0) gamma band
emission intensity profiles in the dayside upper atmosphere of Mars. The
calculated NO density and its gamma band intensity profiles are found to be
consistent with Imaging Ultraviolet Spectrograph (IUVS) onboard MAVEN
observations and also with other modelling studies. We found that the local CO2
and N2 density variations can lead to a change in NO density and consequently
its dayglow intensity by a factor of 2 to 5. Since NO is a trace constituent
and also its dayglow emissions are strongly obscured by CO Cameron band
emissions, we suggest that the derivation of NO number density based on our
approach can constrain its abundance in the dayside upper atmosphere of Mars.
More observations of (1-0) gamma band emission along with modelling will help
to study the global distribution of NO in the Martian atmosphere. | astro-ph_EP |
The M3 project: 2 -- Global distributions of mafic mineral abundances on
Mars: A radiative transfer model was used to reproduce several millions of OMEGA
(Observatoire pour la Min\'eralogie, l'Eau, les Glaces et l'Activit\'e) spectra
representative of igneous terrains of Mars. This task provided the modal
composition and grain sizes at a planetary scale. The lithology can be
summarized in five mineral maps at km-scale. We found that the low albedo
equatorial regions of the Martian surface (from 60{\deg}S to 30{\deg}N) are
globally dominated by plagioclase with average abundance ~50 vol% and pyroxenes
with total averaged abundance close to 40 vol%. An evolution of the
LCP/(LCP+HCP) ratio is observed with time at the global scale, suggesting an
evolution of the degree of partial melting throughout the Martian eras. Olivine
and Martian dust are minor components of the modelled terrains. The olivine
distribution is quite different from the other minerals because it is found on
localized areas with abundance reaching 20 vol%. A statistical approach, to
classify the pixels of the abundances maps, using k-means clustering,
highlighted seven distinct mineral assemblages on the surface. This
classification illustrates that diverse mineralogical units are found in the
Noachian and Hesperian terrains, which suggests the presence of various and
complex magmatic processes at a global scale during the two oldest eras. The
chemical composition was derived from the modal composition maps. The
OMEGA-derived chemical composition is quite consistent with several distinctive
geochemical characteristics previously considered as fingerprints of the
Martian surface. A major discrepancy is in regards to the Fe content that is
significantly smaller than soil and rock analyses from GRS and in situ
measurements. The discrepancy could be partly explained by the assumptions used
for the spectral modelling or could also indicate surface alteration rinds. | astro-ph_EP |
Model for Cameron band emission in comets: A case for EPOXI mission
target comet 103P/Hartley 2: The CO2 production rate has been derived in comets using the Cameron band
(a3Pi - X1Sigma) emission of CO molecule assuming that photodissociative
excitation of CO2 is the main production mechanism of CO in a3Pi metastable
state. We have devoloped a model for the production and loss of CO(a3Pi) which
has been applied to comet 103P/Hartley 2: the target of EPOXI mission. Our
model calculations show that photoelectron impact excitation of CO and
dissociative excitation of CO2 can together contribute about 60-90% to the
Cameron band emission. The modeled brightness of (0-0) Cameron band emission on
comet Hartley 2 is consistent with Hubble Space Telescope observations for 3-5%
CO2 (depending on model input solar flux) and 0.5% CO relative to water, where
photoelectron impact contribution is about 50-75%. We suggest that estimation
of CO2 abundances on comets using Cameron band emission may be reconsidered. We
predict the height integrated column brightness of Cameron band of ~1300 R
during EPOXI mission encounter period. | astro-ph_EP |
Chemical Constraints on the Oxygen Abundances in Jupiter and Saturn: We perform a comparative analysis of the chemical kinetics of CO and $\rm
PH_3$ in Jupiter and Saturn to assess the full set of constraints available on
the troposphere water abundance in the two giant planets. For carbon monoxide
we employ both a widely used CO kinetic scheme from Yung et al, and a newly
identified CO chemical scheme from Visscher and Moses. For $\rm PH_3$ chemical
scheme, we use the same chemical scheme as in Visscher and Fegley. Yung's
chemical scheme for CO yields a water enrichment of 0.95 - 23.0 times solar
abundance on Jupiter, and an upper limit of 14.0 for Saturn. Visscher's
chemical scheme in contrast produces a water enrichment of 0.24 - 2.6 times
solar abundance in Jupiter, and for Saturn an upper limit for water enrichment
of 8.0. From this scheme, which takes advantage of the most up-to-date
kinetics, we preclude high water enrichments on Jupiter and Saturn, and show
that the kinetics approach yields Jovian bulk abundance in which values of C/O
elevated relative to solar are admissible. Our result is consistent with recent
reinterpretation of Galileo Probe data in which Jupiter formed in a
water-depleted portion of the protoplanetary disk (Mousis et al). | astro-ph_EP |
Alternative Sample Mass Measurement Technique for OSIRIS-REX Sample
Collection Phase: The Origins, Spectral Interpretation, Resource Identification, and
Security-Regolith Explorer (OSIRIS-REx) spacecraft is the third NASA New
Frontiers Program mission and arrived at the near-Earth asteroid (101955) Bennu
in December 2018. Following completion of sample collection in October 2020,
otherwise known as Touch-And-Go (TAG), the OSIRIS-REx spacecraft was set to
verify its collected sample mass requirement (> 60g of material). The
thoroughly tested Sample Mass Measurement (SMM) method was to be used for this
verification. Imaging of the Touch-And-Go Sample Acquisition Mechanism (TAGSAM)
was received shortly following the TAG event, intended to ensure mechanism
health prior to moving forward with the SMM activity. These images displayed
sample leakage, prompting discussion for alternative paths forward. Risk of
continued sample loss and a desire to retain as much material as possible lead
the team to pursue an accelerated sample stow schedule and forgo the planned
SMM activity. Once the sample was safely stowed in the return capsule an
alternative SMM method was proposed. The alternative SMM technique utilized
reaction wheel momentum data from identical TAGSAM movements prior to and
following the TAG event to estimate changes in spacecraft moment of inertia.
Conservation of momentum was used to isolate the sample mass from this inertia
change. Using this new method, the spacecraft team was able to successfully
estimate collected sample mass to be 250.37 +/- 101 g. | astro-ph_EP |
The evolution of dust-disk sizes from a homogeneous analysis of 1-10
Myr-old stars: We utilize ALMA archival data to estimate the dust disk size of 152
protoplanetary disks in Lupus (1-3 Myr), Chamaeleon I (2-3 Myr), and Upper-Sco
(5-11 Myr). We combine our sample with 47 disks from Tau/Aur and Oph whose dust
disk radii were estimated, as here, through fitting radial profile models to
visibility data. We use these 199 homogeneously derived disk sizes to identify
empirical disk-disk and disk-host property relations as well as to search for
evolutionary trends. In agreement with previous studies, we find that dust disk
sizes and millimeter luminosities are correlated, but show for the first time
that the relationship is not universal between regions. We find that disks in
the 2-3 Myr-old Cha I are not smaller than disks in other regions of similar
age, and confirm the Barenfeld et al. (2017) finding that the 5-10 Myr USco
disks are smaller than disks belonging to younger regions. Finally, we find
that the outer edge of the Solar System, as defined by the Kuiper Belt, is
consistent with a population of dust disk sizes which have not experienced
significant truncation. | astro-ph_EP |
Growing the seeds of pebble accretion through planetesimal accretion: We explore the growth of planetary embryos by planetesimal accretion up to
and beyond the point where pebble accretion becomes efficient at the so-called
Hill-transition mass. Both the transition mass and the characteristic mass of
planetesimals formed by the streaming instability increase with increasing
distance from the star. We developed a model for the growth of a large
planetesimal (embryo) embedded in a population of smaller planetesimals formed
in a filament by the streaming instability. The model includes in a
self-consistent way the collisional mass growth of the embryo, the
fragmentation of the planetesimals, the velocity evolution of all involved
bodies, as well as the viscous spreading of the filament. We find that the
embryo accretes all available material in the filament during the lifetime of
the protoplanetary disc only in the inner regions of the disc. In contrast, we
find little or no growth in the outer parts of the disc beyond 5--10 AU.
Overall, our results demonstrate very long timescales for collisional growth of
planetesimals in the regions of the protoplanetary disc where giant planets
form. As such, in order to form giant planets in cold orbits, pebble accretion
must act directly on the largest bodies present in the initial mass-function of
planetesimals with little or no help from mutual collisions. | astro-ph_EP |
The missing cavities in the SEEDS polarized scattered light images of
transitional protoplanetary disks: a generic disk model: Transitional circumstellar disks around young stellar objects have a
distinctive infrared deficit around 10 microns in their Spectral Energy
Distributions (SED), recently measured by the Spitzer Infrared Spectrograph
(IRS), suggesting dust depletion in the inner regions. These disks have been
confirmed to have giant central cavities by imaging of the submillimeter
(sub-mm) continuum emission using the Submillimeter Array (SMA). However, the
polarized near-infrared scattered light images for most objects in a systematic
IRS/SMA cross sample, obtained by HiCIAO on the Subaru telescope, show no
evidence for the cavity, in clear contrast with SMA and Spitzer observations.
Radiative transfer modeling indicates that many of these scattered light images
are consistent with a smooth spatial distribution for micron-sized grains, with
little discontinuity in the surface density of the micron-sized grains at the
cavity edge. Here we present a generic disk model that can simultaneously
account for the general features in IRS, SMA, and Subaru observations.
Particularly, the scattered light images for this model are computed, which
agree with the general trend seen in Subaru data. Decoupling between the
spatial distributions of the micron-sized dust and mm-sized dust inside the
cavity is suggested by the model, which, if confirmed, necessitates a
mechanism, such as dust filtration, for differentiating the small and big dust
in the cavity clearing process. Our model also suggests an inwardly increasing
gas-to-dust-ratio in the inner disk, and different spatial distributions for
the small dust inside and outside the cavity, echoing the predictions in grain
coagulation and growth models. | astro-ph_EP |
The Apparently Decaying Orbit of WASP-12: We present new transit and occultation times for the hot Jupiter WASP-12b.
The data are compatible with a constant period derivative: $\dot{P}=-29 \pm 3$
ms yr$^{-1}$ and $P/\dot{P}= 3.2$ Myr. However, it is difficult to tell whether
we have observed orbital decay, or a portion of a 14-year apsidal precession
cycle. If interpreted as decay, the star's tidal quality parameter $Q_\star$ is
about $2\times 10^5$. If interpreted as precession, the planet's Love number is
$0.44\pm 0.10$. Orbital decay appears to be the more parsimonious model: it is
favored by $\Delta\chi^2=5.5$ despite having two fewer free parameters than the
precession model. The decay model implies that WASP-12 was discovered within
the final $\sim$0.2% of its existence, which is an unlikely coincidence but
harmonizes with independent evidence that the planet is nearing disruption.
Precession does not invoke any temporal coincidence, but does require some
mechanism to maintain an eccentricity of $\approx$0.002 in the face of rapid
tidal circularization. To distinguish unequivocally between decay and
precession will probably require a few more years of monitoring. Particularly
helpful will be occultation timing in 2019 and thereafter. | astro-ph_EP |
The physics of wind-blown sand and dust: The transport of sand and dust by wind is a potent erosional force, creates
sand dunes and ripples, and loads the atmosphere with suspended dust aerosols.
This article presents an extensive review of the physics of wind-blown sand and
dust on Earth and Mars. Specifically, we review the physics of aeolian
saltation, the formation and development of sand dunes and ripples, the physics
of dust aerosol emission, the weather phenomena that trigger dust storms, and
the lifting of dust by dust devils and other small-scale vortices. We also
discuss the physics of wind-blown sand and dune formation on Venus and Titan. | astro-ph_EP |
HD45364, a pair of planets in a 3:2 mean motion resonance: Precise radial-velocity measurements with the HARPS spectrograph reveal the
presence of two planets orbiting the solar-type star HD45364. The companion
masses are 0.187 Mjup and 0.658 Mjup, with semi-major axes of 0.681 AU and
0.897 AU, and eccentricities of 0.168 and 0.097, respectively. A dynamical
analysis of the system further shows a 3:2 mean motion resonance between the
two planets, which prevents close encounters and ensures the stability of the
system over 5 Gyr. This is the first time that such a resonant configuration
has been observed for extra-solar planets, although there is an analogue in our
Solar System formed by Neptune and Pluto. This singular planetary system may
provide important constraints on planetary formation and migration scenarios. | astro-ph_EP |
Discovery of a Two-Armed Spiral Structure in the Gapped Disk in HD
100453: We present VLT/SPHERE adaptive optics imaging in Y$-$, J$-$, H$-$, and
K-bands of the HD 100453 system and the discovery of a two-armed spiral
structure in a disk extending to 0.37" ($\sim$42 AU) from the star, with highly
symmetric arms to the Northeast and Southwest. Inside of the spiral arms, we
resolve a ring of emission from 0.18"-0.25" ($\sim$21-29 AU). By assuming that
the ring is intrinsically circular we estimate an inclination of $\sim$34$^{o}$
from face-on. We detect dark crescents on opposite sides (NW and SE) which
begin at 0.18" and continue to radii smaller than our inner working angle of
0.15", which we interpret as the signature of a gap at $\lesssim$21 AU that has
likely been cleared by forming planets. We also detect the $\sim$120 AU
companion HD 100453 B, and by comparing our data to 2003 HST/ACS and VLT/NACO
images we estimate an orbital period of $\sim$850 yr. We discuss what
implications the discovery of the spiral arms and finer structures of the disk
may have on our understanding of the possible planetary system in HD 100453,
and how the morphology of this disk compares to other related objects. | astro-ph_EP |
Supermassive Hot Jupiters Provide More Favourable Conditions for the
Generation of Radio Emission via the Cyclotron Maser Instability - A Case
Study Based on Tau Bootis b: We investigate under which conditions supermassive hot Jupiters can sustain
source regions for radio emission, and whether this emission could propagate to
an observer outside the system. We study Tau Bootis b-like planets (a
supermassive hot Jupiter with 5.84 Jupiter masses and 1.06 Jupiter radii), but
located at different orbital distances (between its actual orbit of 0.046 AU
and 0.2 AU). Due to the strong gravity of such planets and efficient radiative
cooling, the upper atmosphere is (almost) hydrostatic and the exobase remains
very close to the planet, which makes it a good candidate for radio
observations. We expect similar conditions as for Jupiter, i.e. a region
between the exobase and the magnetopause that is filled with a depleted plasma
density compared with cases where the whole magnetosphere cavity is filled with
hydrodynamically outward flowing ionospheric plasma. Thus, unlike classical hot
Jupiters like the previously studied planets HD 209458b and HD 189733b,
supermassive hot Jupiters should be in general better targets for radio
observations. | astro-ph_EP |
A Past Episode of Rapid Tidal Evolution of Enceladus?: Saturn possesses a dynamically rich system containing numerous moons and
impressive rings. Whether the rings of Saturn are much younger than the planet
itself has been a long-open question; more recently a young age has been
proposed for some moons. Recent detection of the fast orbital evolution of Rhea
and Titan strongly suggest a highly frequency-dependent tidal response of
Saturn, possibly through excitation of inertial waves within the planet's
convective envelope. Here we show that the resonance locking to inertial waves
cannot explain the dynamical structure of the Saturnian system or the current
tidal heating of Enceladus. On the other hand, both the observation and our
modelling results indicate that the system is not consistent with evolution
under equilibrium tides. We propose that the system's architecture can best be
explained by relatively high "background" tidal response coupled with discrete
resonant modes. In this view, only Titan may be in a true long-term resonance
lock with a tidal mode of Saturn. Rhea is most likely currently experiencing a
transient period of fast tidal evolution as it passes through a mode, rather
than being locked to it. Assuming that Enceladus went through a temporary
period of fast tidal evolution, we can reproduce its present resonance with
Dione and satisfy other dynamical constraints. Additionally, we conclude that
the long-term tidal response of Saturn to Tethys must be weaker than expected
from frequency-independent tides, as already found by observations. | astro-ph_EP |
Secondary eclipses of WASP-18b -- Near Infrared observations with the
Anglo Australian Telescope, the Magellan Clay Telescope and the LCOGT network: We present new eclipse observations for one of the hottest "hot Jupiters"
WASP-18b, for which previously published data from HST WFC3 and Spitzer have
led to radically conflicting conclusions about the composition of this planet's
atmosphere. We measure eclipse depths of $0.15\pm0.02\%$ at $Ks$ and
$0.07\pm0.01\%$ at $z'$ bands. Using the VSTAR line-by-line radiative transfer
code and both these new observations with previously published data, we derive
a new model of the planetary atmosphere. We have varied both the metallicity
and C/O ratio in our modelling, and find no need for the extreme metallicity
suggested by Sheppard et al.(2017). Our best fitting models slightly
underestimate the emission at $z'$ band and overestimate the observed flux at
$Ks$-band. To explain these discrepancies, we examine the impact on the
planetary emission spectrum of the presence of several types of hazes which
could form on the night-side of the planet. Our $Ks$ band eclipse flux
measurement is lower than expected from clear atmosphere models and this could
be explained by a haze particles larger than 0.2 $\mu$m with the optical
properties of Al$_{2}$O$_{3}$, CaTiO$_{3}$ or MgSiO$_{3}$. We find that $z'$
band measurements are important for understanding the contribution of
photochemical hazes with particles smaller than 0.1 $\mu$m at the top of the
atmosphere. | astro-ph_EP |
Finding Signs of Life in Transit: High-resolution Transmission Spectra
of Earth-like Planets around FGKM Host Stars: Thousands of transiting exoplanets have already been detected orbiting a wide
range of host stars, including the first planets that could potentially be
similar to Earth. The upcoming Extremely Large Telescopes and the James Webb
Space Telescope will enable the first searches for signatures of life in
transiting exoplanet atmospheres. Here, we quantify the strength of spectral
features in transit that could indicate a biosphere similar to the modern Earth
on exoplanets orbiting a wide grid of host stars (F0 to M8) with effective
temperatures between 2,500 and 7,000K: transit depths vary between about
6,000ppm (M8 host) to 30 ppm (F0 host) due to the different sizes of the host
stars. CO2 possesses the strongest spectral features in transit between 0.4 and
20microns. The atmospheric biosignature pairs O2+CH4 and O3+CH4 - which
identify Earth as a living planet - are most prominent for Sun-like and cooler
host stars in transit spectra of modern Earth analogs. Assessing biosignatures
and water on such planets orbiting hotter stars than the Sun will be extremely
challenging even for high-resolution observations. All high-resolution transit
spectra and model profiles are available online: they provide a tool for
observers to prioritize exoplanets for transmission spectroscopy, test
atmospheric retrieval algorithms, and optimize observing strategies to find
life in the cosmos. In the search for life in the cosmos, transiting planets
provide the first opportunity to discover whether or not we are alone, with
this database as one of the keys to optimize the search strategies. | astro-ph_EP |
Stability of Planetary Motion in Binary Star Systems: We considered the problem of stability for planets of finite mass in binary
star systems. We selected a huge set of initial conditions for planetary orbits
of the S-type, to perform high precision and very extended in time
integrations.
For our numerical integrations, we resorted to the use of a 15th order
integration scheme (IAS15, available within the REBOUND framework), that
provides an optimal solution for long-term time integrations. We estimated the
probability of different types of instability: planet collisions with the
primary or secondary star or planet ejected away from the binary star system.
We confirm and generalize to massive planets the dependence of the critical
semi-major axis on eccentricity and mass ratio of the binary already found by
Holman and Wiegert (1999). We were also able to pick a significant number of
orbits that are only `marginally' stable, according to the classification
introduced by Musielak et al. (2005).
A, natural, extension of this work has been the study of the effect of
perturbations induced to circumbinary planet motion by a passing-by star, like
it often happens in a star cluster. One of the targets of this analysis is the
investigation of the possibility that a planet, formerly on a stable S-type
orbit around one of the two stars, could transit to a stable P-type orbit (or
viceversa). We performed a series of more than 4500 scattering experiments with
different initial conditions typical of encounters in small star clusters. We
found some interesting behaviors of the systems after perturbation and showed
how a transition from an inner (S-type) stable orbit to a circumbinary (P-type)
(and vice-versa) has a very low (but non null) probability. | astro-ph_EP |
Nano dust impacts on spacecraft and boom antenna charging: High rate sampling detectors measuring the potential difference between the
main body and boom antennas of interplanetary spacecraft have been shown to be
efficient means to measure the voltage pulses induced by nano dust impacts on
the spacecraft body itself (see Meyer-Vernet et al, Solar Phys. 256, 463
(2009)). However, rough estimates of the free charge liberated in post impact
expanding plasma cloud indicate that the cloud's own internal electrostatic
field is too weak to account for measured pulses as the ones from the TDS
instrument on the STEREO spacecraft frequently exceeding 0.1 V/m. In this paper
we argue that the detected pulses are not a direct measure of the potential
structure of the plasma cloud, but are rather the consequence of a transitional
interruption of the photoelectron return current towards the portion of the
antenna located within the expanding cloud. | astro-ph_EP |
Binary planet formation by gas-assisted encounters of planetary embryos: We present radiation hydrodynamic simulations in which binary planets form by
close encounters in a system of several super-Earth embryos. The embryos are
embedded in a protoplanetary disk consisting of gas and pebbles and evolve in a
region where the disk structure supports convergent migration due to Type I
torques. As the embryos accrete pebbles, they become heated and thus affected
by the thermal torque (Ben\'{i}tez-Llambay et al. 2015) and the hot-trail
effect (Chrenko et al. 2017) which excites orbital eccentricities. Motivated by
findings of Eklund & Masset (2017), we assume the hot-trail effect operates
also vertically and reduces the efficiency of inclination damping. Non-zero
inclinations allow the embryos to become closely packed and also vertically
stirred within the convergence zone. Subsequently, close encounters of two
embryos assisted by the disk gravity can form transient binary planets which
quickly dissolve. Binary planets with a longer lifetime $\sim$$10^{4}$ yr form
in 3-body interactions of a transient pair with one of the remaining embryos.
The separation of binary components generally decreases in subsequent
encounters and due to pebble accretion until the binary merges, forming a giant
planet core. We provide an order-of-magnitude estimate of the expected
occurrence rate of binary planets, yielding one binary planet per
$\simeq$$2$--$5\times10^{4}$ planetary systems. Therefore, although rare, the
binary planets may exist in exoplanetary systems and they should be
systematically searched for. | astro-ph_EP |
Searching sub-stellar objects in DR1-TGAS, effectiveness and efficiency
of Gaias' astrometry: We used 1,477,047 data from DR1-TGAS, in order to analyse the minimum
require- ments of accuracy, necessary to detect sub-stellar objects in the
astrometric mea- surements of Gaia. We found that the first set of data (DR1)
does not have enough accuracy, so sub-stellar objects can not be easily
detected. Barely, it would be possible to detect jovian and higher mass
objects, with orbital periods over 5 years. We made the calculations of the
minimum values of the astrometric angle produced by an or- biting sub-stellar
object using a range of different masses. We estimate the efficiency and
effectiveness of the DR1-TGAS data in order to detect sub-stellar objects and
the minimum accuracy that Gaia would be required to detect these objects using
the datasets that the mission will release in the near future. | astro-ph_EP |
Modelling the optical energy profile of the 2021 October Jupiter impact
flash: We have conducted numerical simulations to reproduce the observed optical
energy profile of the 15 October 2021 (UT) impact flash on Jupiter, which was
the largest and the most well-observed flash event detected by ground-based
movie observations. The observed long-duration ($\sim 5.5~{\rm s}$) optical
emission can be reproduced by an impact of an object with an exceptionally
small angle of entry relative to the horizontal. The apparent lack of the
impact debris feature despite the large impact object was possibly due to the
shallower angle of entry ($\le 12^\circ$), which resulted in the lower ablation
per unit volume at altitudes higher than $50 \, {\rm km}$, and the volume
densities of the ablated materials were too low to allow the debris
particulates to coagulate. The absence of temporal methane absorption change in
the observed flash spectrum is consistent with the best-fit results. The model
better fits the observed optical energy profile for weaker material (cometary
and stony) cases than for metallic ones. Based on the simulation results,
prospects for future observations of impact flashes are discussed. | astro-ph_EP |
Laboratory light scattering from regolith surface and simulation of data
by Hapke model: The small atmosphereless objects of our solar system, such as asteroids, the
moon are covered by layer of dust particles known as regolith, formed by
meteoritic impact. The light scattering studies of such dust layer by
laboratory experiment and numerical simulation are two important tools to
investigate their physical properties. In the present work, the light scattered
from a layer of dust particles, containing 0.3{\mu}m Al2O3 at wavelength 632.8
nm is analysed. This work has been performed by using a light scattering
instrument 'ellipsometer', at the Department of Physics, Assam Universiy,
Silchar, India. Through this experiment, we generated in laboratory the
photometric and polarimetric phase curves of light scattered from such a layer.
In order to numerically simulate this data, we used Hapke's model combined with
Mie's single particle scattering properties. The perpendicular and parallel
components of single particle albedo and the phase function were derived from
Mie theory. By using the Hapke's model combined with Mie theory, the physical
properties of the dust grain such as grain size, optical constant (n,k) and
wavelength can be studied through this scheme. In literature, till today no
theoretical model to represent polarisation caused due to scattering from rough
surface is available, which can successfully explain the scattering process. So
the main objective of this work is to develop a model which can theoretically
estimate polarisation as caused due to scattering from rough surface and also
to validate our model with the laboratory data generated in the present work. | astro-ph_EP |
$V$-band photometry of asteroids from ASAS-SN: Finding asteroids with
slow spin: We present $V$-band photometry of the 20,000 brightest asteroids using data
from the All-Sky Automated Survey for Supernovae (ASAS-SN) between 2012 and
2018. We were able to apply the convex inversion method to more than 5,000
asteroids with more than 60 good measurements in order to derive their sidereal
rotation periods, spin axis orientations, and shape models. We derive unique
spin state and shape solutions for 760 asteroids, including 163 new
determinations. This corresponds to a success rate of about 15%, which is
significantly higher than the success rate previously achieved using photometry
from surveys. We derive the first sidereal rotation periods for additional 69
asteroids. We find good agreement in spin periods and pole orientations for
objects with prior solutions. We obtain a statistical sample of asteroid
physical properties that is sufficient for the detection of several previously
known trends, such as the underrepresentation of slow rotators in current
databases, and the anisotropic distribution of spin orientations driven by the
nongravitational forces. We also investigate the dependence of spin
orientations on the rotation period. Since 2018, ASAS-SN has been observing the
sky with higher cadence and deeper limiting magnitude, which will lead to many
more new solutions in just a few years. | astro-ph_EP |
Ejection of iron-bearing giant-impact fragments and the dynamical and
geochemical influence of the fragment re-accretion: The Earth was born in violence. Many giant collisions of protoplanets are
thought to have occurred during the terrestrial planet formation. Here we
investigated the giant impact stage by using a hybrid code that consistently
deals with the orbital evolution of protoplanets around the Sun and the details
of processes during giant impacts between two protoplanets. A significant
amount of materials (up to several tens of percent of the total mass of the
protoplanets) is ejected by giant impacts. We call these ejected fragments the
giant-impact fragments (GIFs). In some of the erosive hit-and-run and
high-velocity collisions, metallic iron is also ejected, which comes from the
colliding protoplanets' cores. From ten numerical simulations for the giant
impact stage, we found that the mass fraction of metallic iron in GIFs ranges
from ~ 1wt% to ~ 25wt%. We also discussed the effects of the GIFs on the
dynamical and geochemical characteristics of formed terrestrial planets. We
found that the GIFs have the potential to solve the following dynamical and
geochemical conflicts: (1) The Earth, currently in a near circular orbit, is
likely to have had a highly eccentric orbit during the giant impact stage. The
GIFs are large enough in total mass to lower the eccentricity of the Earth to
its current value via their dynamical friction. (2) The concentrations of
highly siderophile elements (HSEs) in the Earth's mantle are greater than what
was predicted experimentally. Re-accretion of the iron-bearing GIFs onto the
Earth can contribute to the excess of HSEs. In addition, the estimated amount
of iron-bearing GIFs provides significant reducing agent that could transform
primitive CO2-H2O atmosphere and ocean into more reducing H2-bearing
atmosphere. Thus, GIFs are important for the origin of Earth's life and its
early evolution. | astro-ph_EP |
Mineral cloud and hydrocarbon haze particles in the atmosphere of the
hot Jupiter JWST target WASP-43b: Having a short orbital period and being tidally locked makes WASP-43b an
ideal candidate for JWST observations. Phase curve observations of an entire
orbit will enable the mapping of the atmospheric structure across the planet,
with different wavelengths of observation allowing different atmospheric depths
to be seen. We provide insight into the details of the clouds that may form on
WASP-43b in order to prepare the forthcoming interpretation of the JWST and
follow-up data. We utilize 3D GCM results as input for a kinetic,
non-equilibrium model for mineral cloud particles, and for a kinetic model to
study a photochemicaly-driven hydrocarbon haze component. Mineral condensation
seeds form throughout the atmosphere of WASP-43b. This is in stark contrast to
the ultra-hot Jupiters, like WASP-18b and HAT-P-7b. The dayside is loaded with
few but large mineral cloud particles in addition to hydrocarbon haze particles
of comparable abundance. Photochemically driven hydrocarbon haze appears on the
dayside, but does not contribute to the cloud formation on the nightside. The
geometrical cloud extension differs across the globe due to the changing
thermodynamic conditions. Day and night differ by 6000km in pressure scale
height. As reported for other planets, the C/O is not constant throughout the
atmosphere. The mean molecular weight is approximately constant in a
H2-dominated WASP-43b. WASP-43b is expected to be fully covered in clouds which
are not homogeneously distributed throughout the atmosphere. The dayside and
the terminator clouds will be a combination of mineral particles of locally
varying size and composition, and of hydrocarbon hazes. The optical depth of
hydrocarbon hazes is considerably lower than that of mineral cloud particles
such that a wavelength-dependent radius measurement of WASP-43b would be
determined by the mineral cloud particles but not by hazes. | astro-ph_EP |
Protoplanetary Disk Heating and Evolution Driven by the Spiral Density
Waves: High-resolution imaging of some protoplanetary disks in scattered light
reveals presence of the global spiral arms of significant amplitude, likely
excited by massive planets or stellar companions. Assuming that these arms are
density waves, evolving into spiral shocks, we assess their effect on the
thermodynamics, accretion, and global evolution of the disk. We derive
analytical expressions for the direct (irreversible) heating, angular momentum
transport, and mass accretion rate induced by the disk shocks of arbitrary
strength. We find these processes to be very sensitive to the shock amplitude.
Focusing on the waves of moderate strength (density jump at the shock
$\Delta\Sigma/\Sigma\sim 1$) we show the associated disk heating to be
negligible (contributing at $\sim 1\%$ level to the energy budget) in passive,
irradiated protoplanetary disks on $\sim 100$ AU scales, but becoming important
within several AU from the star. At the same time, shock heating can be a
significant (or even dominant) energy source in disks of cataclysmic variables,
stellar X-ray binaries, and supermassive black hole binaries, heated mainly by
viscous dissipation. Mass accretion induced by the global spiral shocks is
comparable to (or exceeds) the mass inflow due to viscous stresses.
Protoplanetary disks featuring prominent global spirals must be evolving
rapidly, in $\lesssim 0.5$ Myr at $\sim 100$ AU. A direct upper limit on the
disk evolution timescale can be established via the measurement of the
gravitational torque due to the spiral arms from the imaging data. Our findings
suggest that, regardless of their origin, global spiral waves must be important
agents of the protoplanetary disk evolution. They may serve as an effective
mechanism of disk dispersal and could be related to the transitional disk
phenomenon. | astro-ph_EP |
Revisiting Lambert's Problem: The orbital boundary value problem, also known as Lambert Problem, is
revisited. Building upon Lancaster and Blanchard approach, new relations are
revealed and a new variable representing all problem classes, under
L-similarity, is used to express the time of flight equation. In the new
variable, the time of flight curves have two oblique asymptotes and they mostly
appear to be conveniently approximated by piecewise continuous lines. We use
and invert such a simple approximation to provide an efficient initial guess to
an Householder iterative method that is then able to converge, for the single
revoltuion case, in only two iterations. The resulting algorithm is compared to
Gooding's procedure revealing to be numerically as accurate, while having a
smaller computational complexity. | astro-ph_EP |
Close Stellar Encounters in Young, Substructured, Dissolving Star
Clusters: Statistics and Effects on Planetary Systems: Both simulations and observations indicate that stars form in filamentary,
hierarchically clustered associations, most of which disperse into their
galactic field once feedback destroys their parent clouds. However, during
their early evolution in these substructured environments, stars can undergo
close encounters with one another that might have significant impacts on their
protoplanetary disks or young planetary systems. We perform N-body simulations
of the early evolution of dissolving, substructured clusters with a wide range
of properties, with the aim of quantifying the expected number and orbital
element distributions of encounters as a function of cluster properties. We
show that the presence of substructure both boosts the encounter rate and
modifies the distribution of encounter velocities compared to what would be
expected for a dynamically relaxed cluster. However, the boost only lasts for a
dynamical time, and as a result the overall number of encounters expected
remains low enough that gravitational stripping is unlikely to be a significant
effect for the vast majority of star-forming environments in the Galaxy. We
briefly discuss the implications of this result for models of the origin of the
Solar System, and of free-floating planets. We also provide tabulated encounter
rates and orbital element distributions suitable for inclusion in population
synthesis models of planet formation in a clustered environment. | astro-ph_EP |
The impact of ultraviolet heating and cooling on the dynamics and
observability of lava planet atmospheres: Lava planets have non-global, condensible atmospheres similar to icy bodies
within the solar system. Because they depend on interior dynamics, studying the
atmospheres of lava planets can lead to understanding unique geological
processes driven by their extreme environment. Models of lava planet
atmospheres have thus far focused on either radiative transfer or
hydrodynamics. In this study, we couple the two processes by introducing
ultraviolet and infrared radiation to a turbulent boundary layer model. We also
test the effect of different vertical temperature profiles on atmospheric
dynamics. Results from the model show that UV radiation affects the atmosphere
much more than IR. UV heating and cooling work together to produce a
horizontally isothermal atmosphere away from the sub-stellar point regardless
of the vertical temperature profile. We also find that stronger temperature
inversions induce stronger winds and hence cool the atmosphere. Our simulated
transmission spectra of the bound atmosphere show a strong SiO feature in the
UV that would be challenging to observe in the planet's transit spectrum due to
the precision required. Our simulated emission spectra are more promising, with
significant SiO spectral features at 4.5 and 9 $\mu$m that can be observed with
the James Webb Space Telescope. Different vertical temperature profiles produce
discernible dayside emission spectra, but not in the way one would expect. | astro-ph_EP |
A Deep Search for Additional Satellites around the Dwarf Planet Haumea: Haumea is a dwarf planet with two known satellites, an unusually high spin
rate, and a large collisional family, making it one of the most interesting
objects in the outer solar system. A fully self-consistent formation scenario
responsible for the satellite and family formation is still elusive, but some
processes predict the initial formation of many small moons, similar to the
small moons recently discovered around Pluto. Deep searches for regular
satellites around KBOs are difficult due to observational limitations, but
Haumea is one of the few for which sufficient data exist. We analyze Hubble
Space Telescope (HST) observations, focusing on a ten-consecutive-orbit
sequence obtained in July 2010, to search for new very small satellites. To
maximize the search depth, we implement and validate a non-linear
shift-and-stack method. No additional satellites of Haumea are found, but by
implanting and recovering artificial sources, we characterize our sensitivity.
At distances between $\sim$10,000 km and $\sim$350,000 km from Haumea,
satellites with radii as small as $\sim$10 km are ruled out, assuming an albedo
($p \simeq 0.7$) similar to Haumea. We also rule out satellites larger than
$\gtrsim$40 km in most of the Hill sphere using other HST data. This search
method rules out objects similar in size to the small moons of Pluto. By
developing clear criteria for determining the number of non-linear rates to
use, we find that far fewer shift rates are required ($\sim$35) than might be
expected. The non-linear shift-and-stack method to discover satellites (and
other moving transients) is tractable, particularly in the regime where
non-linear motion begins to manifest itself. | astro-ph_EP |
The infrared colors of 51 Eridani b: micrometereoid dust or chemical
disequilibrium?: We reanalyze near-infrared spectra of the young extrasolar giant planet 51
Eridani b which was originally presented in (Macintosh et al. 2015) and (Rajan
et al. 2017) using modern atmospheric models which include a self-consistent
treatment of disequilibrium chemistry due to turbulent vertical mixing. In
addition, we investigate the possibility that significant opacity from
micrometeors or other impactors in the planet's atmosphere may be responsible
for shaping the observed spectral energy distribution (SED). We find that
disequilibrium chemistry is useful for describing the mid-infrared colors of
the planet's spectra, especially in regards to photometric data at M band
around 4.5 $\mu$m which is the result of super-equilibrium abundances of carbon
monoxide, while the micrometeors are unlikely to play a pivotal role in shaping
the SED. The best-fitting, micrometeroid-dust-free, disequilibrium chemistry,
patchy cloud model has the following parameters: effective temperature
$T_\textrm{eff} = 681$ K with clouds (or without clouds, i.e. the grid
temperature $T_\textrm{grid}$ = 900 K), surface gravity $g$ = 1000 m/s$^2$,
sedimentation efficiency $f_\textrm{sed}$ = 10, vertical eddy diffusion
coefficient $K_\textrm{zz}$ = 10$^3$ cm$^2$/s, cloud hole fraction
$f_\textrm{hole}$ = 0.2, and planet radius $R_\textrm{planet}$ = 1.0
R$_\textrm{Jup}$. | astro-ph_EP |
Five New Post-Main-Sequence Debris Disks with Gaseous Emission: Observations of debris disks, the products of the collisional evolution of
rocky planetesimals, can be used to trace planetary activity across a wide
range of stellar types. The most common end points of stellar evolution are no
exception as debris disks have been observed around several dozen white dwarf
stars. But instead of planetary formation, post-main-sequence debris disks are
a signpost of planetary destruction, resulting in compact debris disks from the
tidal disruption of remnant planetesimals. In this work, we present the
discovery of five new debris disks around white dwarf stars with gaseous debris
in emission. All five systems exhibit excess infrared radiation from dusty
debris, emission lines from gaseous debris, and atmospheric absorption features
indicating on-going accretion of metal-rich debris. In four of the systems, we
detect multiple metal species in emission, some of which occur at strengths and
transitions previously unseen in debris disks around white dwarf stars. Our
first year of spectroscopic follow-up hints at strong variability in the
emission lines that can be studied in the future, expanding the range of
phenomena these post-main-sequence debris disks exhibit. | astro-ph_EP |
Reflected light from giant planets in habitable zones: Tapping into the
power of the Cross-Correlation Function: The direct detection of reflected light from exoplanets is an excellent probe
for the characterization of their atmospheres. The greatest challenge for this
task is the low planet-to-star flux ratio, which even in the most favourable
case is of the order of $10^{-4}$ in the optical. This ratio decreases even
more for planets in their host habitable zone, typically lower than $10^{-7}$.
To reach the signal-to-noise level required for such detections, we propose to
unleash the power of the Cross Correlation Function in combination with the
collecting power of next generation observing facilities. The technique we
propose has already yielded positive results by detecting the reflected
spectral signature of 51 Pegasi b (see Martins et al. 2015). In this work, we
attempted to infer the number of hours required for the detection of several
planets in their host habitable zone using the aforementioned technique from
theoretical EELT observations. Our results show that for 5 of the selected
planets it should be possible to directly recover their reflected spectral
signature. | astro-ph_EP |
A self-consistent cloud model for brown dwarfs and young giant
exoplanets: comparison with photometric and spectroscopic observations: We developed a simple, physical and self-consistent cloud model for brown
dwarfs and young giant exoplanets. We compared different parametrisations for
the cloud particle size, by either fixing particle radii, or fixing the mixing
efficiency (parameter fsed) or estimating particle radii from simple
microphysics. The cloud scheme with simple microphysics appears as the best
parametrisation by successfully reproducing the observed photometry and spectra
of brown dwarfs and young giant exoplanets. In particular, it reproduces the
L-T transition, due to the condensation of silicate and iron clouds below the
visible/near-IR photosphere. It also reproduces the reddening observed for
low-gravity objects, due to an increase of cloud optical depth for low gravity.
In addition, we found that the cloud greenhouse effect shifts chemical
equilibriums, increasing the abundances of species stable at high temperature.
This effect should significantly contribute to the strong variation of methane
abundance at the L-T transition and to the methane depletion observed on young
exoplanets. Finally, we predict the existence of a continuum of brown dwarfs
and exoplanets for absolute J magnitude=15-18 and J-K color=0-3, due to the
evolution of the L-T transition with gravity. This self-consistent model
therefore provides a general framework to understand the effects of clouds and
appears well-suited for atmospheric retrievals. | astro-ph_EP |
Exoplanetary Interiors: The first mass-estimate of an exoplanet around a Sun-like star, 51 Peg b and
the first radius measurement of an exoplanet, HD209458b pointed to the
challenges of understanding the atmosphere, interior, and evolution of
exoplanets including the possibility of mass loss of planets on close-orbits
that are exposed to strong irradiation. These discoveries raised the question
of heating and inflation mechanisms, and of the nature of these objects in
terms of composition compared to the known planets in the Solar system. The
field of exoplanet interior modeling was born. Here, we outline and discuss
current big science questions: (i) What is the amount of heavy elements in a
planet and do all planets possess an iron-rock core? We suggest that a
promising and novel approach for exoplanets can be measuring their tidal
response in form of the Love numbers h2 and k2. (ii) How much and through what
mechanisms are the interiors of planets heated or delayed from cooling? Many
strongly irradiated gaseous planets require an additional heat source to
explain their large radii. (iii) What is the origin of the observed populations
in the radius-period diagram? Objects in and along the radius valley are
excellent targets to study planetary formation and evaporation. (iv) What does
the composition of rocky planets tell us about their formation? Planets more
iron-rich than Mercury are found, as well as planets that if rocky, are
depleted in iron with respect to Earth. We do not have yet a reliable formation
theory that would explain their existence. | astro-ph_EP |
Using the Sun to estimate Earth-like planets detection capabilities.I.
Impact of cold spots: Stellar spots may in some cases produce radial velocity (RV) signatures
similar to those of exoplanets. To further investigate the impact of spots, we
aim at studying the detectability of Earth mass planets in the habitable zone
(HZ) of solar type stars, if covered by spots similar to the sunspots. We have
used the Sunspots properties recorded over one solar cycle between 1993 and
2003 to build the RV curve that a solar type star seen edge-on would show, if
covered by such spots with Tsun -Tspot = 550K. We also simulate the RV of such
a spotted star surrounded by an Earth mass planet located in the HZ. Under
present assumptions, the detection of a 1 M Earth planet located between 0.8
and 1.2 AU requires an intensive monitoring (weekly or better), during several
years of low activity phasis. The temporal sampling is more crucial than the
precision of the data (assuming precisions in the range [1-10] cm/s). Cooler
spots may become a problem for such detections. Also, we anticipate that
plages, not considered in this paper, could further complicate or even
compromise the detections. | astro-ph_EP |
Cool Gaseous Exoplanets: surveying the new frontier with Twinkle: Cool gaseous exoplanets ($1.75\ R_\oplus < R_\text{p} < 3\ R_\text{J}$, $200$
K $<T_\text{eq} < 1000$~K) are an as-yet understudied population, with great
potential to expand our understanding of planetary atmospheres and formation
mechanisms. In this paper, we outline the basis for a homogeneous survey of
cool gaseous planets with Twinkle, a 0.45-m diameter space telescope with
simultaneous spectral coverage from 0.5-4.5~$\mu$m, set to launch in 2025. We
find that Twinkle has the potential to characterise the atmospheres of 36 known
cool gaseous exoplanets (11~sub-Neptunian, 11~Neptunian, 14~Jovian) at an SNR
$\geq$ 5 during its 3-year primary mission, with the capability of detecting
most major molecules predicted by equilibrium chemistry to > $5\sigma$
significance. We find that an injected mass-metallicity trend is
well-recovered, demonstrating Twinkle's ability to elucidate this fundamental
relationship into cool regime. We also find Twinkle will be able to detect
cloud layers at 3$\sigma$ or greater in all cool gaseous planets for clouds at
$\leq$ 10 Pa pressure level, but will be insensitive to clouds deeper than
$10^4$ Pa in all cases. With these results we demonstrate the capability of the
Twinkle mission to greatly expand the current knowledge of cool gaseous
planets, enabling key insights and constraints to be obtained for this
poorly-charted region of exoplanet parameter space. | astro-ph_EP |
On the Detection of Exomoons Transiting Isolated Planetary-Mass Objects: All-sky imaging surveys have identified several dozen isolated planetary-mass
objects (IPMOs), far away from any star. Here, we examine the prospects for
detecting transiting moons around these objects. We expect transiting moons to
be common, occurring around 10-15% of IPMOs, given that close-orbiting moons
have a high geometric transit probability and are expected to be a common
outcome of giant planet formation. IPMOs offer an advantage over other directly
imaged planets in that high-contrast imaging is not necessary to detect the
photometric transit signal. For at least 30 (>50%) of the currently known
IPMOs, observations of a single transit with the James Webb Space Telescope
would have low enough forecasted noise levels to allow for the detection of an
Io-like or Titan-like moon. Intrinsic variability of the IPMOs will be an
obstacle. Using archival time-series photometry of IPMOs with the Spitzer Space
Telescope as a proof-of-concept, we found evidence for a fading event of 2MASS
J1119-1137 AB that might have been caused by intrinsic variability, but is also
consistent with a single transit of a habitable-zone 1.7$R_\oplus$ exomoon.
Although the interpretation of this particular event is inconclusive, the
characteristics of the data and the candidate signal suggest that Earth-sized
habitable-zone exomoons around IPMOs are detectable with existing
instrumentation. | astro-ph_EP |
Ethyl cyanide on Titan: Spectroscopic detection and mapping using ALMA: We report the first spectroscopic detection of ethyl cyanide (C$_2$H$_5$CN)
in Titan's atmosphere, obtained using spectrally and spatially resolved
observations of multiple emission lines with the Atacama Large
Millimeter/submillimeter array (ALMA). The presence of C$_2$H$_5$CN in Titan's
ionosphere was previously inferred from Cassini ion mass spectrometry
measurements of C$_2$H$_5$CNH$^+$. Here we report the detection of 27
rotational lines from C$_2$H$_5$CN (in 19 separate emission features detected
at $>3\sigma$ confidence), in the frequency range 222-241 GHz. Simultaneous
detections of multiple emission lines from HC$_3$N, CH$_3$CN and CH$_3$CCH were
also obtained. In contrast to HC$_3$N, CH$_3$CN and CH$_3$CCH, which peak in
Titan's northern (spring) hemisphere, the emission from C$_2$H$_5$CN is found
to be concentrated in the southern (autumn) hemisphere, suggesting a distinctly
different chemistry for this species, consistent with a relatively short
chemical lifetime for C$_2$H$_5$CN. Radiative transfer models show that most of
the C$_2$H$_5$CN is concentrated at altitudes 300-600 km, suggesting production
predominantly in the mesosphere and above. Vertical column densities are found
to be in the range (2-5)$\times10^{14}$ cm$^{-2}$. | astro-ph_EP |
Spherical Harmonics for the 1D Radiative Transfer Equation II: Thermal
Emission: Approximate methods to estimate solutions to the radiative transfer equation
are essential for the understanding of atmospheres of exoplanets and brown
dwarfs. The simplest and most popular choice is the "two-stream method" which
is often used to produce simple yet effective models for radiative transfer in
scattering and absorbing media. Toon et al. (1989) (Toon89) outlined a
two-stream method for computing reflected light and thermal spectra and was
later implemented in the open-source radiative transfer model PICASO. In Part~I
of this series, we developed an analytical spherical harmonics method for
solving the radiative transfer equation for reflected solar radiation (Rooney
et al. 2023), which was implemented in PICASO to increase the accuracy of the
code by offering a higher-order approximation. This work is an extension of
this spherical harmonics derivation to study thermal emission spectroscopy. We
highlight the model differences in the approach for thermal emission and
benchmark the 4-term method (SH4) against Toon89 and a high-stream
discrete-ordinates method, CDISORT. By comparing the spectra produced by each
model we demonstrate that the SH4 method provides a significant increase in
accuracy, compared to Toon89, which can be attributed to the increased order of
approximation and to the choice of phase function. We also explore the
trade-off between computational time and model accuracy. We find that our
4-term method is twice as slow as our 2-term method, but is up to five times
more accurate, when compared with CDISORT. Therefore, SH4 provides excellent
improvement in model accuracy with minimal sacrifice in numerical expense. | astro-ph_EP |
Seasonal Water "Pump" in the Atmosphere of Mars: Vertical Transport to
the Thermosphere: We present results of simulations with the Max Planck Institute general
circulation model (MPI-MGCM) implementing a hydrological cycle scheme. The
simulations reveal a seasonal water "pump" mechanism responsible for the upward
transport of water vapor. This mechanism occurs in high latitudes above
60$^\circ$ of the southern hemisphere at perihelion, when the upward branch of
the meridional circulation is particularly strong. A combination of the mean
vertical flux with variations induced by solar tides facilitates penetration of
water across the "bottleneck" at approximately 60 km. The meridional
circulation then transports water across the globe to the northern hemisphere.
Since the intensity of the meridional cell is tightly controlled by airborne
dust, the water abundance in the thermosphere strongly increases during dust
storms. | astro-ph_EP |
Boundary Layer Circumplanetary Accretion: How Fast Could an Unmagnetized
Planet Spin Up Through Its Disk?: Gas giant planets are expected to accrete most of their mass via a
circumplanetary disk. If the planet is unmagnetized and initially slowly
rotating, it will accrete gas via a radially narrow boundary layer and rapidly
spin up. Radial broadening of the boundary layer as the planet spins up reduces
the specific angular momentum of accreted gas, allowing the planet to find a
terminal rotation rate short of the breakup rate. Here, we use axisymmetric
viscous hydrodynamic simulations to quantify the terminal rotation rate of
planets accreting from their circumplanetary disks. For an isothermal
planet-disk system with a disk scale height $h/r =0.1$ near the planetary
surface, spin up switches to spin down at between 70\% and 80\% of the planet's
breakup angular velocity. In a qualitative difference from vertically-averaged
models -- where spin down can co-exist with mass accretion -- we observe
\emph{decretion} accompanying solutions where angular momentum is being lost.
The critical spin rate depends upon the disk thickness near the planet. For an
isothermal system with a disk scale height of $h/r = 0.15$ near the planet, the
critical spin rate drops to between 60\% and 70\% of the planet's breakup
angular velocity. In the disk outside the boundary layer, we identify
meridional circulation flows, which are unsteady and instantaneously asymmetric
across the mid-plane. The simulated flows are strong enough to vertically
redistribute solid material in early-stage satellite formation. We discuss how
extrasolar planetary rotation measurements, when combined with spectroscopic
and variability studies of protoplanets with circumplanetary disks, could
determine the role of magnetic and non-magnetic processes in setting giant
planet spins. | astro-ph_EP |
Seeking echoes of circumstellar disks in Kepler light curves: Light echoes of flares on active stars offer the opportunity for direct
detection of circumstellar dust. We revisit the problem of identifying faint
echoes in post-flare light curves, focusing on debris disks from on-going
planet formation. Starting with simulations, we develop an algorithm for
estimating the radial extent and total mass from disk echo profiles. We apply
this algorithm to light curves from over 2,100 stars observed by NASA's Kepler
mission, selected for multiple, short-lived flares in either the long-cadence
or short-cadence data sets. While flux uncertainties in light curves from
individual stars preclude useful mass limits on circumstellar disks,
catalog-averaged light curves yield constraints on disk mass that are
comparable to estimates from known debris disks. The average mass in micron- to
millimeter-sized dust around the Kepler stars cannot exceed 10% of an Earth
mass in exo-Kuiper belts or 10% of a Lunar mass in the terrestrial zone. We
group stars according to IR excess, based on WISE W1-W3 color, as an indicator
for the presence of circumstellar dust. The mass limits are greater for stars
with strong IR excess, a hint that echoes are lurking not far beneath the noise
in post-flare light curves. With increased sensitivity, echo detection will let
time-domain astronomy complement spectroscopic and direct-imaging studies in
mapping how, when, and where planets form. | astro-ph_EP |
A Fast Approximate Approach to Microlensing Survey Analysis: Microlensing can be used to discover exoplanets of a wide range of masses
with orbits beyond ~ 1 AU, and even free-floating planets. The WFIRST mission
will use microlensing to discover approximately 1600 planets by monitoring ~100
million stars to find ~50000 microlensing events. Modelling each microlensing
event, especially the ones involving two or more lenses, is typically
complicated and time-consuming, and analyzing thousands of WFIRST microlensing
events is possibly infeasible using current methods. Here, we present an
algorithm that is able to rapidly evaluate thousands of simulated WFIRST
binary-lens microlensing light curves, returning an estimate for the physical
parameters of the lens systems. We find that this algorithm can recover
projected separations between the planet and the star very well for
low-mass-ratio events, and can also estimate mass ratios within an order of
magnitude for events with wide and close caustic topologies. | astro-ph_EP |
The composition of hot Jupiter atmospheres assembled within chemically
evolved protoplanetary discs: The radial-dependent positions of snowlines of abundant oxygen- and
carbon-bearing molecules in protoplanetary discs will result in systematic
radial variations in the C/O ratios in the gas and ice. This variation is
proposed as a tracer of the formation location of gas-giant planets. However,
disc chemistry can affect the C/O ratios in the gas and ice, thus potentially
erasing the chemical fingerprint of snowlines in gas-giant atmospheres. We
calculate the molecular composition of hot Jupiter atmospheres using elemental
abundances extracted from a chemical kinetics model of a disc midplane where we
have varied the initial abundances and ionization rates. The models predict a
wider diversity of possible atmospheres than those predicted using elemental
ratios from snowlines only. As found in previous work, as the C/O ratio exceeds
the solar value, the mixing ratio of CH$_{4}$ increases in the lower
atmosphere, and those of C$_{2}$H$_{2}$ and HCN increase mainly in the upper
atmosphere. The mixing ratio of H$_{2}$O correspondingly decreases. We find
that hot Jupiters with C/O$>1$ can only form between the CO$_{2}$ and CH$_{4}$
snowlines. Moreover, they can only form in a disc which has fully inherited
interstellar abundances, and where negligible chemistry has occurred. Hence,
carbon-rich planets are likely rare, unless efficient transport of
hydrocarbon-rich ices via pebble drift to within the CH$_{4}$ snowline is a
common phenomenon. We predict combinations of C/O ratios and elemental
abundances that can constrain gas-giant planet formation locations relative to
snowline positions, and that can provide insight into the disc chemical
history. | astro-ph_EP |
Cooling Requirements for the Vertical Shear Instability in
Protoplanetary Disks: The vertical shear instability (VSI) offers a potential hydrodynamic
mechanism for angular momentum transport in protoplanetary disks (PPDs). The
VSI is driven by a weak vertical gradient in the disk's orbital motion, but
must overcome vertical buoyancy, a strongly stabilizing influence in cold
disks, where heating is dominated by external irradiation. Rapid radiative
cooling reduces the effective buoyancy and allows the VSI to operate. We
quantify the cooling timescale $t_c$ needed for efficient VSI growth, through a
linear analysis of the VSI with cooling in vertically global, radially local
disk models. We find the VSI is most vigorous for rapid cooling with
$t_c<\Omega_\mathrm{K}^{-1}h|q|/(\gamma -1)$ in terms of the Keplerian orbital
frequency, $\Omega_\mathrm{K}$; the disk's aspect-ratio, $h\ll1$; the radial
power-law temperature gradient, $q$; and the adiabatic index, $\gamma$. For
longer $t_c$, the VSI is much less effective because growth slows and shifts to
smaller length scales, which are more prone to viscous or turbulent decay. We
apply our results to PPD models where $t_c$ is determined by the opacity of
dust grains. We find that the VSI is most effective at intermediate radii, from
$\sim5$AU to $\sim50$AU with a characteristic growth time of $\sim30$ local
orbital periods. Growth is suppressed by long cooling times both in the opaque
inner disk and the optically thin outer disk. Reducing the dust opacity by a
factor of 10 increases cooling times enough to quench the VSI at all disk
radii. Thus the formation of solid protoplanets, a sink for dust grains, can
impede the VSI. | astro-ph_EP |
The case of HD 106906 debris disc: A binary's revenge: Debris disc architecture presents [exo-]planetary scientists with precious
clues for processes of planet formation and evolution, including constraints on
planetary mass perturbers. This is particularly true of the disc in HD 106906,
which in early HST, then follow up polarimetric observations, presented
asymmetries and needle-like features that have been attributed to perturbations
by a massive, and unusually distant external planetary companion. Here, we
revisit the long-term secular dynamical evolution of the HD 106906 disc
allowing for the combined gravitational action of the planetary companion and
the inner stellar binary which holds the system together. We argue that the
binary is strong enough to impose a dynamical break at the disc's location,
resulting in distinctive observational signatures which we render via simulated
surface density maps and vertical structure profiles. Within uncertainties on
the planet's orbit, we show that the disc can go from being fully dominated by
the inner binary to significantly so, and is hardly ever outside its reach. The
extent of binary dominance impacts the disc's mean eccentricity, a metric which
we map as a function of the planet's semi-major axis and orbital eccentricity,
with and without radiation pressure. We can thus constrain the planet's orbit
to ease the tension between evident axisymmetry in the millimeter, and apparent
asymmetry in scattered light. We discuss phase space structure, then
inclination distribution, arguing for the relevance of our results to a variety
of hierarchical systems, as we set the stage for generalizations that allow for
disc self-gravity and collisional evolution. | astro-ph_EP |
A pair of TESS planets spanning the radius valley around the nearby
mid-M dwarf LTT 3780: We present the confirmation of two new planets transiting the nearby mid-M
dwarf LTT 3780 (TIC 36724087, TOI-732, $V=13.07$, $K_s=8.204$, $R_s$=0.374
R$_{\odot}$, $M_s$=0.401 M$_{\odot}$, d=22 pc). The two planet candidates are
identified in a single TESS sector and are validated with reconnaissance
spectroscopy, ground-based photometric follow-up, and high-resolution imaging.
With measured orbital periods of $P_b=0.77$ days, $P_c=12.25$ days and sizes
$r_{p,b}=1.33\pm 0.07$ R$_{\oplus}$, $r_{p,c}=2.30\pm 0.16$ R$_{\oplus}$, the
two planets span the radius valley in period-radius space around low mass stars
thus making the system a laboratory to test competing theories of the emergence
of the radius valley in that stellar mass regime. By combining 63 precise
radial-velocity measurements from HARPS and HARPS-N, we measure planet masses
of $m_{p,b}=2.62^{+0.48}_{-0.46}$ M$_{\oplus}$ and $m_{p,c}=8.6^{+1.6}_{-1.3}$
M$_{\oplus}$, which indicates that LTT 3780b has a bulk composition consistent
with being Earth-like, while LTT 3780c likely hosts an extended H/He envelope.
We show that the recovered planetary masses are consistent with predictions
from both photoevaporation and from core-powered mass loss models. The
brightness and small size of LTT 3780, along with the measured planetary
parameters, render LTT 3780b and c as accessible targets for atmospheric
characterization of planets within the same planetary system and spanning the
radius valley. | astro-ph_EP |
The inner solar system cratering record and the evolution of impactor
populations: We review previously published and newly obtained crater size-frequency
distributions in the inner solar system. These data indicate that the Moon and
the terrestrial planets have been bombarded by two populations of objects.
Population 1, dominating at early times, had nearly the same size distribution
as the present-day asteroid belt, and produced the heavily cratered surfaces
with a complex, multi-sloped crater size-frequency distribution. Population 2,
dominating since about 3.8-3.7 Ga, has the same size distribution as near-Earth
objects (NEOs), had a much lower impact flux, and produced a crater size
distribution characterized by a differential -3 single-slope power law in the
crater diameter range 0.02 km to 100 km. Taken together with the results from a
large body of work on age-dating of lunar and meteorite samples and theoretical
work in solar system dynamics, a plausible interpretation of these data is as
follows. The NEO population is the source of Population 2 and it has been in
near-steady state over the past ~3.7-3.8 gigayears; these objects are derived
from the main asteroid belt by size-dependent non-gravitational effects that
favor the ejection of smaller asteroids. However, Population 1 were main belt
asteroids ejected from their source region in a size-independent manner,
possibly by means of gravitational resonance sweeping during giant planet orbit
migration; this caused the so-called Late Heavy Bombardment (LHB). The LHB
began some time before ~3.9 Ga, peaked and declined rapidly over the next ~100
to 300 megayears, and possibly more slowly from about 3.8-3.7 Ga to ~2 Ga. A
third crater population (Population S) consists of secondary impact craters
that can dominate the cratering record at small diameters. | astro-ph_EP |
Signatures of an eccentric disc cavity: Dust and gas in IRS 48: We test the hypothesis that the disc cavity in the `transition disc' Oph IRS
48 is carved by an unseen binary companion. We use 3D dust-gas
smoothed-particle hydrodynamics simulations to demonstrate that marginally
coupled dust grains concentrate in the gas over-density that forms in in the
cavity around a low binary mass ratio binary. This produces high contrast ratio
dust asymmetries at the cavity edge similar to those observed in the disc
around IRS 48 and other transition discs. This structure was previously assumed
to be a vortex. However, we show that the observed velocity map of IRS 48
displays a peculiar asymmetry that is not predicted by the vortex hypothesis.
We show the unusual kinematics are naturally explained by the non-Keplerian
flow of gas in an eccentric circumbinary cavity. We further show that
perturbations observed in the isovelocity curves of IRS 48 may be explained as
the product of the dynamical interaction between the companion and the disc.
The presence of a $\sim$0.4 M$_{\odot}$ companion at a $\sim$10 au separation
can qualitatively explain these observations. High spatial resolution line and
continuum imaging should be able to confirm this hypothesis. | astro-ph_EP |
Insight from laboratory measurements on dust in debris discs: Extreme adaptive optics instruments have revealed exquisite details on debris
discs, allowing to extract the optical properties of the dust particles such as
the phase function, the degree of polarisation and the spectral reflectance.
These are three powerful diagnostic tools to understand the physical properties
of the dust : the size, shape and composition of the dust particles. This can
inform us on the population of parent bodies, also called planetesimals, which
generate those particles through collisions. It is however very rare to be able
to combine all those three observables for the same system, as this requires
different high-contrast imaging techniques to suppress the starlight and reveal
the faint scattered light emission from the dust. Due to its brightness, the
ring detected around the A-type star HR 4796 is a notable exception, with both
unpolarised and polarised images covering near-infrared wavelengths. Here, we
show how measurements of dust particles in the laboratory can reproduce the
observed near-infrared photo-polarimetric properties of the HR 4796 disc.
Experimental characterisation of dust allows to bypass the current limitations
of dust models to reproduce simultaneously the phase function, the degree of
polarisation and the spectral reflectance. | astro-ph_EP |
Oort cloud Ecology II: Extra-solar Oort clouds and the origin of
asteroidal interlopers: We simulate the formation and evolution of Oort clouds around the 200 nearest
stars (within 16pc according to the Gaia DR2) database. This study is performed
by numerically integrating the planets and minor bodies in orbit around the
parent star and in the Galactic potential. The calculations start 1\,Gyr ago
and continue for 100Myr into the future. In this time frame, we simulate how
asteroids (and planets) are ejected from the star's vicinity and settle in an
Oort cloud and how they escape the local stellar gravity to form tidal steams.
A fraction of 0.0098 to 0.026 of the asteroids remain bound to their parent
star. The orbits of these asteroids isotropizes and circularizes due to the
influence of the Galactic tidal field to eventually form an Oort cloud between
10^4 and 2 10^5au. We estimate that 6% of the nearby stars may have a planet in
its Oort cloud. The majority of asteroids (and some of the planets) become
unbound from the parent star to become free floating in the Galactic potential.
These soli lapides remain in a similar orbit around the Galactic center as
their host star, forming dense streams of rogue interstellar asteroids and
planets.
The Solar system occasionally passes through such tidal streams, potentially
giving rise to occasional close encounters with object in this stream. The two
recently discovered objects, 1I/(2017 Q3) 'Oumuamua and 2I/(2019 Q4) Borisov,
may be such objects. Although the direction from which an individual solus
lapis originated cannot easily be traced back to the original host, multiple
such objects coming from the same source might help to identify their origin.
At the moment the Solar system is in the bow or wake of the tidal stream of 10
of the nearby stars which might contribute considerably to the interaction
rate. (abridged) | astro-ph_EP |
Traditional formation scenarios fail to explain 4:3 mean motion
resonances: At least two multi-planetary systems in a 4:3 mean motion resonance have been
found by radial velocity surveys. These planets are gas giants and the systems
are only stable when protected by a resonance. Additionally the Kepler mission
has detected at least 4 strong candidate planetary systems with a period ratio
close to 4:3.
This paper investigates traditional dynamical scenarios for the formation of
these systems. We systematically study migration scenarios with both N-body and
hydro-dynamic simulations. We investigate scenarios involving the in-situ
formation of two planets in resonance. We look at the results from finely tuned
planet-planet scattering simulations with gas disk damping. Finally, we
investigate a formation scenario involving isolation-mass embryos.
Although the combined planet-planet scattering and damping scenario seems
promising, none of the above scenarios is successful in forming enough systems
in 4:3 resonance with planetary masses similar to the observed ones. This is a
negative result but it has important implications for planet formation.
Previous studies were successful in forming 2:1 and 3:2 resonances. This is
generally believed to be evidence of planet migration. We highlight the main
differences between those studies and our failure in forming a 4:3 resonance.
We also speculate on more exotic and complicated ideas. These results will
guide future investigators toward exploring the above scenarios and alternative
mechanisms in a more general framework. | astro-ph_EP |
Using Star Spots to Measure the Spin-orbit Alignment of Transiting
Planets: Spectroscopic follow-up of dozens of transiting planets has revealed the
degree of alignment between the equators of stars and the orbits of the planets
they host. Here we determine a method, applicable to spotted stars, that can
reveal the same information from the photometric discovery data, with no need
for follow-up. A spot model fit to the global light curve, parametrized by the
spin orientation of the star, predicts when the planet will transit the spots.
Observing several spot crossings during different transits then leads to
constraints on the spin-orbit alignment. In cases where stellar spots are
small, the stellar inclination, and hence the true alignment, rather than just
the sky projection, can be obtained. This method has become possible with the
advent of space telescopes such as CoRoT and Kepler, which photometrically
monitor transiting planets over a nearly continuous, long time baseline. We
apply our method to CoRoT-2, and find the projected spin-orbit alignment angle,
lambda= 4.7 deg +/- 12.3 deg, in excellent agreement with a previous
determination that employed the Rossiter-McLaughlin effect. The large spots of
the parent star, CoRoT-2, limit our precision on the stellar inclination: i_s =
84 deg +/- 36 deg, where i_s < 90 deg (> 90 deg) indicates the rotation axis is
tilted towards (away from) the line of sight. | astro-ph_EP |
Searching for the HR 8799 Debris Disk with HST/STIS: We present a new algorithm for space telescope high contrast imaging of
close-to-face-on planetary disks called Optimized Spatially Filtered (OSFi)
normalization. This algorithm is used on HR 8799 Hubble Space Telescope (HST)
coronagraphic archival data, showing an over-luminosity after reference star
point spread function (PSF) subtraction that may be from the inner disk and/or
planetesimal belt components of this system. The PSF-subtracted radial profiles
in two separate epochs from 2011 and 2012 are consistent with one another, and
self-subtraction shows no residual in both epochs. We explore a number of
possible false-positive scenarios that could explain this residual flux,
including telescope breathing, spectral differences between HR 8799 and the
reference star, imaging of the known warm inner disk component, OSFi algorithm
throughput and consistency with the standard spider normalization HST PSF
subtraction technique, and coronagraph misalignment from pointing accuracy. In
comparison to another similar STIS dataset, we find that the over-luminosity is
likely a result of telescope breathing and spectral difference between HR 8799
and the reference star. Thus, assuming a non-detection, we derive upper limits
on the HR 8799 dust belt mass in small grains. In this scenario, we find that
the flux of these micron-sized dust grains leaving the system due to radiation
pressure is small enough to be consistent with measurements of other debris
disk halos. | astro-ph_EP |
Organic hazes as a source of life's building blocks to warm little ponds
on the Hadean Earth: Over 4 billion years ago, Earth is thought to have been a hazy world akin to
Saturn's moon Titan. The organic hazes in the atmosphere at this time could
contain a vast inventory of life's building blocks, and thus may have seeded
warm little ponds for life. In this work, we produce organic hazes in the lab
in atmospheres with high (5%) and low (0.5%) CH4 abundances and analyze the
solid particles for nucleobases, amino acids, and a few other organics using
GC/MS/MS to obtain their concentrations. We also analyze heated (200
$^{\circ}$C) samples from the high methane organic haze experiment to simulate
these particles sitting on an uninhabitable surface. Finally, we use our
experimental results and estimates of atmospheric haze production as inputs for
a comprehensive numerical pond model to calculate the concentrations of
nucleobases from organic hazes in these environments. We find that organic
hazes typically provide up to 0.2-6.5 $\mu$M concentrations of nucleobases to
warm little ponds for potentially habitable Hadean conditions. However, without
seepage, uracil and thymine can reach ~100 $\mu$M concentrations, which is the
present lower experimental limit to react these species to form nucleotides.
Heating samples leads to partial or complete decay of biomolecules, suggesting
that biomolecule stockpiling on the hot surface is unlikely. The ideal
conditions for the delivery of life's building blocks from organic hazes would
be when the Hadean atmosphere is rich in methane, but not so rich as to create
an uninhabitable surface. | astro-ph_EP |
Realistic collisional water transport during terrestrial planet
formation: Self-consistent modeling by an N-body--SPH hybrid code: According to current evidence the water inventory of Earth (and perhaps
similar exoplanets) was transported inwards via (giant) collisions during the
chaotic final phase of planet formation. In dynamical simulations water
delivery is still studied almost exclusively by assuming oversimplified perfect
merging (PM), even though it is particularly prone to collisional transfer and
loss. To close this gap we have developed a framework to model collisional
water transport by direct combination of long-term N-body computations with
dedicated 3D SPH simulations for each collision. Post-collision water
inventories are self-consistently traced further, in accretionary or erosive as
well as hit-and-run encounters. The latter are frequent outcomes among
protoplanets, where besides collisional losses, water transfer between the
encountering bodies has to be considered. This hybrid approach enables us for
the first time to trace the full dynamical and collisional evolution of ~200
bodies throughout the whole late-stage accretion phase (several 100 Myrs). As a
first application we choose a Solar System-like architecture, with already
formed giant planets on either circular or eccentric orbits and a debris disk
spanning from 0.5 - 4 au. Realistic collision treatment leads to considerably
different results than PM, with lower mass planets and water inventories
reduced by a factor of two or more. Due to a combination of collisional losses
and considerably lengthened accretion, final water contents especially with
giant planets on circular orbits are strongly reduced to more Earth-like
values. Water delivery to potentially habitable planets is dominated by few
decisive collisions, mostly with embryo-sized or larger bodies. The high
frequency of hit-and-run, with generally low water (and mass) transfer
efficiencies, are a crucial part of this process, and of system-wide evolution
in general. | astro-ph_EP |
Detection and Characterization of Extrasolar Planets through Doppler
Spectroscopy: Over 300 extrasolar planets have been found since 1992, showing that
planetary systems are common and exhibit an outstanding variety of
characteristics. As the number of detections grows and as models of planet
formation progress to account for the existence of these new worlds,
statistical studies and confrontations of observation with theory allow to
progressively unravel the key processes underlying planet formation. In this
chapter we review the dominant contribution of Doppler spectroscopy to the
present discoveries and to our general understanding of planetary systems. We
also emphasize the synergy of Doppler spectroscopy and transit photometry in
characterizing the physical properties of transiting extrasolar planets. As we
will see, Doppler spectroscopy has not reached its limits yet and it will
undoubtly play a leading role in the detection and characterization of the
first Earth-mass planets. | astro-ph_EP |
On the dynamics of Extrasolar Planetary Systems under dissipation.
Migration of planets: We study the dynamics of planetary systems with two planets moving in the
same plane, when frictional forces act on the two planets, in addition to the
gravitational forces. The model of the general three-body problem is used.
Different laws of friction are considered. The topology of the phase space is
essential in understanding the evolution of the system. The topology is
determined by the families of stable and unstable periodic orbits, both
symmetric and non symmetric. It is along the stable families, or close to them,
that the planets migrate when dissipative forces act. At the critical points
where the stability along the family changes, there is a bifurcation of a new
family of stable periodic orbits and the migration process changes route and
follows the new stable family up to large eccentricities or to a chaotic
region. We consider both resonant and non resonant planetary systems. The 2/1,
3/1 and 3/2 resonances are studied. The migration to larger or smaller
eccentricities depends on the particular law of friction. Also, in some cases
the semimajor axes increase and in other cases they are stabilized. For
particular laws of friction and for special values of the parameters of the
frictional forces, it is possible to have partially stationary solutions, where
the eccentricities and the semimajor axes are fixed. | astro-ph_EP |
TOI-1842b: A Transiting Warm Saturn Undergoing Re-Inflation around an
Evolving Subgiant: The imminent launch of space telescopes designed to probe the atmospheres of
exoplanets has prompted new efforts to prioritise the thousands of transiting
planet candidates for follow-up characterisation. We report the detection and
confirmation of TOI-1842b, a warm Saturn identified by TESS and confirmed with
ground-based observations from Minerva-Australis, NRES, and the Las Cumbres
Observatory Global Telescope. This planet has a radius of
$1.04^{+0.06}_{-0.05}\,R_{Jup}$, a mass of $0.214^{+0.040}_{-0.038}\,M_{Jup}$,
an orbital period of $9.5739^{+0.0002}_{-0.0001}$ days, and an extremely low
density ($\rho$=0.252$\pm$0.091 g cm$^{-3}$). TOI-1842b has among the best
known combinations of large atmospheric scale height (893 km) and host-star
brightness ($J=8.747$ mag), making it an attractive target for atmospheric
characterisation. As the host star is beginning to evolve off the main
sequence, TOI-1842b presents an excellent opportunity to test models of gas
giant re-inflation. The primary transit duration of only 4.3 hours also makes
TOI-1842b an easily-schedulable target for further ground-based atmospheric
characterisation. | astro-ph_EP |
Frequencies and resonances around $L_4$ in the elliptic restricted
three-body problem: The stability of the Lagrangian point $L_4$ is investigated in the elliptic
restricted three-body problem by using Floquet's theory. Stable and unstable
domains are determined in the parameter plane of the mass parameter and the
eccentricity by computing the characteristic exponents. Frequencies of motion
around $L_4$ have been determined both in the stable and unstable domains and
fitting functions for the frequencies are derived depending on the mass
parameter and the eccentricity. Resonances between the frequencies are studied
in the whole parameter plane. It is shown that the 1:1 resonances are not
restricted only to single curves but extend to the whole unstable domain. In
the unstable domains longer escape times of the test particle from the
neighbourhood of $L_4$ are related to certain resonances, but changing the
parameters the same resonances may lead to faster escape. | astro-ph_EP |
The obliquity of Enceladus: The extraordinary activity at Enceladus' warm south pole indicates the
presence of an internal global or local reservoir of liquid water beneath the
surface. While Tyler (2009, 2011) has suggested that the geological activity
and the large heat flow of Enceladus could result from tidal heating triggered
by a large obliquity of at least 0.05{\deg}-0.1{\deg}, theoretical models of
the Cassini state predict the obliquity to be two to three orders of magnitude
smaller for an entirely solid and rigid Enceladus. We investigate the influence
of an internal subsurface ocean and of tidal deformations of the solid layers
on the obliquity of Enceladus. Our Cassini state model takes into account the
external torque exerted by Saturn on each layer of the satellite and the
internal gravitational and pressure torques induced by the presence of the
liquid layer. As a new feature, our model also includes additional torques that
arise because of the periodic tides experienced by the satellite. We find that
the upper limit for the obliquity of a solid Enceladus is 0.00045 degrees and
is negligibly affected by elastic deformations. The presence of an internal
ocean decreases this upper limit by 13.1%, elasticity attenuating this decrease
by only 0.5%. Since the obliquity of Enceladus cannot reach Tyler's
requirement, obliquity tides are unlikely to be the source of the large heat
flow of Enceladus. More likely, the geological activity at Enceladus' south
pole results from eccentricity tides. Even in the most favorable case, the
upper limit for the obliquity of Enceladus corresponds to about two meters at
most at the surface of Enceladus. This is well below the resolution of Cassini
images. Control point calculations cannot be used to detect the obliquity of
Enceladus, let alone to constrain its interior from an obliquity measurement. | astro-ph_EP |
Distribution of solids in the rings of the HD 163296 disk: a
multiwavelength study: In this paper we analyze new observations from ALMA and VLA, at a high
angular resolution corresponding to 5 - 8 au, of the protoplanetary disk around
HD 163296 to determine the dust spatial distribution and grain properties. We
fit the spectral energy distribution as a function of the radius at five
wavelengths from 0.9 to 9\,mm, using a simple power law and a physical model
based on an analytic description of radiative transfer that includes isothermal
scattering. We considered eight dust populations and compared the models'
performance using Bayesian evidence. Our analysis shows that the moderately
high optical depth ($\tau$>1) at $\lambda \leq$ 1.3 mm in the dust rings
artificially lower the millimeter spectral index, which should therefore not be
considered as a reliable direct proxy of the dust properties and especially the
grain size. We find that the outer disk is composed of small grains on the
order of 200 $\mu$m with no significant difference between rings at 66 and 100
au and the adjacent gaps, while in the innermost 30 au, larger grains
($\geq$mm) could be present. We show that the assumptions on the dust
composition have a strong impact on the derived surface densities and grain
size. In particular, increasing the porosity of the grains to 80\% results in a
total dust mass about five times higher with respect to grains with 25\%
porosity. Finally, we find that the derived opacities as a function of
frequency deviate from a simple power law and that grains with a lower porosity
seem to better reproduce the observations of HD163296. While we do not find
evidence of differential trapping in the rings of HD163296, our overall results
are consistent with the postulated presence of giant planets affecting the dust
temperature structure and surface density, and possibly originating a
second-generation dust population of small grains. | astro-ph_EP |
First detection of orbital motion for HD 106906 b: A wide-separation
exoplanet on a Planet Nine-like orbit: HD 106906 is a 15 Myr old short-period (49 days) spectroscopic binary that
hosts a wide-separation (737 au) planetary-mass ($\sim11\,M_{\rm Jup}$) common
proper motion companion, HD 106906 b. Additionally, a circumbinary debris disk
is resolved at optical and near-infrared wavelengths that exhibits a
significant asymmetry at wide separations that may be driven by gravitational
perturbations from the planet. In this study we present the first detection of
orbital motion of HD 106906 b using Hubble Space Telescope images spanning a 14
yr period. We achieve high astrometric precision by cross-registering the
locations of background stars with the Gaia astrometric catalog, providing the
subpixel location of HD 106906 that is either saturated or obscured by
coronagraphic optical elements. We measure a statistically significant
$31.8\pm7.0$ mas eastward motion of the planet between the two most
constraining measurements taken in 2004 and 2017. This motion enables a
measurement of the inclination between the orbit of the planet and the inner
debris disk of either $36_{-14}^{+27}$ deg or $44_{-14}^{+27}$ deg, depending
on the true orientation of the orbit of the planet. There is a strong negative
correlation between periastron and mutual inclination; orbits with smaller
periastra are more misaligned with the disk plane. With a periastron of
$510_{-320}^{+480}$ au, HD 106906 b is likely detached from the planetary
region within 100 au radius, showing that a Planet Nine-like architecture can
be established very early in the evolution of a planetary system. | astro-ph_EP |
Jetting during oblique impacts of spherical impactors: During the early stages of an impact a small amount material may be jetted
and ejected at speeds exceeding the impact velocity. Jetting is an important
process for producing melt during relatively low velocity impacts. How impact
angle affects the jetting process has yet to be fully understood. Here, we
simulate jetting during oblique impacts using the iSALE shock physics code.
Assuming both the target and impactor have the same composition (dunite), we
examine the jetted material which exceeds the impact velocity. Our results show
that oblique impacts always produce more jetted ejecta than vertical impacts,
except for grazing impacts with impact angles $< 15^{\circ}$. A 45$^{\circ}$
impact with an impact velocity of 3 km/s produces jetted material equal to
$\sim$ 7 \% of the impactor mass. This is 6 times the jetted mass produced by a
vertical impact with similar impact conditions. We also find that the origin of
jetted ejecta depends on impact angle; for impact angles less than
45$^{\circ}$, most of the jet is composed of impactor material, while at higher
impact angles the jet is dominated by target material. Our findings are
consistent with previous experimental work. In all cases, jetted materials are
preferentially distributed downrange of the impactor. | astro-ph_EP |
Dynamics of Planetary Systems Within Star Clusters: Aspects of the Solar
System's Early Evolution: Most planetary systems -- including our own -- are born within stellar
clusters, where interactions with neighboring stars can help shape the system
architecture. This paper develops an orbit-averaged formalism to characterize
the cluster's mean-field effects as well as the physics of long-period stellar
encounters. Our secular approach allows for an analytic description of the
dynamical consequences of the cluster environment on its constituent planetary
systems. We analyze special cases of the resulting Hamiltonian, corresponding
to eccentricity evolution driven by planar encounters, as well as hyperbolic
perturbations upon dissipative disks. We subsequently apply our results to the
early evolution of our solar system, where the cluster's collective potential
perturbs the solar system's plane, and stellar encounters act to increase the
velocity dispersion of the Kuiper belt. Our results are two-fold: first, we
find that cluster effects can alter the mean plane of the solar system by
$\lesssim1\deg$, and are thus insufficient to explain the $\psi\approx6\deg$
obliquity of the sun. Second, we delineate the extent to which stellar flybys
excite the orbital dispersion of the cold classical Kuiper belt, and show that
while stellar flybys may grow the cold belt's inclination by the observed
amount, the resulting distribution is incompatible with the data.
Correspondingly, our calculations place an upper limit on the product of the
stellar number density and residence time of the sun in its birth cluster,
$\eta\,\tau\lesssim2\times10^4\,$Myr/pc$^3$. | astro-ph_EP |
Chondrule size and related physical properties: a compilation and
evaluation of current data across all meteorite groups: The examination of the physical properties of chondrules has generally
received less emphasis than other properties of meteorites such as their
mineralogy, petrology, and chemical and isotopic compositions. Among the
various physical properties of chondrules, chondrule size is especially
important for the classification of chondrites into chemical groups, since each
chemical group possesses a distinct size-frequency distribution of chondrules.
Knowledge of the physical properties of chondrules is also vital for the
development of astrophysical models for chondrule formation, and for
understanding how to utilize asteroidal resources in space exploration. To
examine our current knowledge of chondrule sizes, we have compiled and provide
commentary on available chondrule dimension literature data. We include all
chondrite chemical groups as well as the acapulcoite primitive achondrites,
some of which contain relict chondrules. We also compile and review current
literature data for other astrophysically-relevant physical properties
(chondrule mass and density). Finally, we briefly examine some additional
physical aspects of chondrules such as the frequencies of compound and
'cratered' chondrules. A purpose of this compilation is to provide a useful
resource for meteoriticists and astrophysicists alike. | astro-ph_EP |
Mars Express measurements of surface albedo changes over 2004 - 2010: The pervasive Mars dust is continually transported between the surface and
the atmosphere. When on the surface, dust increases the albedo of darker
underlying rocks and regolith, which modifies climate energy balance and must
be quantified. Remote observation of surface albedo absolute value and albedo
change is however complicated by dust itself when lifted in the atmosphere.
Here we present a method to calculate and map the bolometric solar
hemispherical albedo of the Martian surface using the 2004 - 2010 OMEGA imaging
spectrometer dataset. This method takes into account aerosols radiative
transfer, surface photometry, and instrumental issues such as registration
differences between visible and near-IR detectors. Resulting albedos are on
average 17% higher than previous estimates for bright surfaces while similar
for dark surfaces. We observed that surface albedo changes occur mostly during
the storm season due to isolated events. The main variations are observed
during the 2007 global dust storm and during the following year. A wide variety
of change timings are detected such as dust deposited and then cleaned over a
Martian year, areas modified only during successive global dust storms, and
perennial changes over decades. Both similarities and differences with previous
global dust storms are observed. While an optically thin layer of bright dust
is involved in most changes, this coating turns out to be sufficient to mask
underlying mineralogical near-IR spectral signatures. Overall, changes result
from apparently erratic events; however, a cyclic evolution emerges for some
(but not all) areas over long timescales. | astro-ph_EP |
Dust Ejection from Planetary Bodies by Temperature Gradients: Laboratory
Experiments: Laboratory experiments show that dusty bodies in a gaseous environment eject
dust particles if they are illuminated. We find that even more intense dust
eruptions occur when the light source is turned off. We attribute this to a
compression of gas by thermal creep in response to the changing temperature
gradients in the top dust layers. The effect is studied at a light flux of 13
kW/(m*m) and 1 mbar ambient pressure. The effect is applicable to
protoplanetary disks and Mars. In the inner part of protoplanetary disks,
planetesimals can be eroded especially at the terminator of a rotating body.
This leads to the production of dust which can then be transported towards the
disk edges or the outer disk regions. The generated dust might constitute a
significant fraction of the warm dust observed in extrasolar protoplanetary
disks. We estimate erosion rates of about 1 kg/s for 100 m parent bodies. The
dust might also contribute to subsequent planetary growth in different
locations or on existing protoplanets which are large enough not to be
susceptible to particle loss by light induced ejection. Due to the ejections,
planetesimals and smaller bodies will be accelerated or decelerated and drift
outward or inward, respectively. The effect might also explain the entrainment
of dust in dust devils on Mars, especially at high altitudes where gas drag
alone might not be sufficient. | astro-ph_EP |
Compact Ultra Dense Matter Impactors: We study interactions of meteorlike compact ultradense objects (CUDO), having
nuclear or greater density, with Earth and other rocky bodies in the Solar
System as a possible source of information about novel forms of matter. We
study the energy loss in CUDO puncture of the body and discuss differences
between regular matter and CUDO impacts. | astro-ph_EP |
Multiple Explanations for the Single Transit of KIC 5951458 based on
Radial Velocity Measurements Extracted with a Novel Matched-template
Technique: Planetary systems that show single-transit events are a critical pathway to
increasing the yield of long-period exoplanets from transit surveys. From the
primary Kepler mission, KIC 5951458b (Kepler-456b) was thought to be a
single-transit giant planet with an orbital period of 1310 days. However,
radial velocity (RV) observations of KIC 5951458 from the HIRES instrument on
the Keck telescope suggest that the system is far more complicated. To extract
precise RVs for this $V\approx13$ star, we develop a novel matched-template
technique that takes advantage of a broad library of template spectra acquired
with HIRES. We validate this technique and measure its noise floor to be 4 - 8
m s$^{-1}$ (in addition to internal RV error) for most stars that would be
targeted for precision RVs. For KIC 5951458, we detect a long-term RV trend
that suggests the existence of a stellar companion with an orbital period
greater than a few thousand days. We also detect an additional signal in the
RVs that is possibly caused by a planetary or brown dwarf companion with mass
in the range of 0.6 - 82 $M_{\rm J}$ and orbital period below a few thousand
days. Curiously, from just the data on hand, it is not possible to determine
which object caused the single "transit" event. We demonstrate how a modest set
of RVs allows us to update the properties of this unusual system and predict
the optimal timing for future observations. | astro-ph_EP |
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