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Preview of Comet C/2021 A1 (Leonard) and Its Encounter with Venus: Long period comet C/2021 A1 (Leonard) will approach Venus to within 0.029 au
on 2021 December 18 and may subsequently graze the planet with its dust trail
less than two days later. We observed C/2021 A1 with the Lowell Discovery
Telescope on 2021 January 13 and March 3, as well as with the Palomar Hale
Telescope on 2021 March 20, while the comet was inbound at heliocentric
distances of r=4.97 au, 4.46 au, and 4.28 au, respectively. Tail morphology
suggests that the dust is optically dominated by ~0.1-1 mm radius grains
produced in the prior year. Neither narrowband imaging photometry nor
spectrophotometry reveal any definitive gas emission, placing 3-sigma upper
bounds on CN production of <1e23 molec/s at both of the latter two epochs.
Trajectory analysis indicates that large (>1 mm) grains ejected at extremely
large heliocentric distances (r>30 au) are most strongly favored to reach
Venus. The flux of such meteors on Venus, and thus their potential direct or
indirect observability, is highly uncertain as the comet's dust production
history is poorly constrained at these distances, but will likely fall well
below the meteor flux from comet C/2013 A1 (Siding Spring)'s closer encounter
to Mars in 2014, and thus poses negligible risk to any spacecraft in orbit
around Venus. Dust produced in previous apparitions will not likely contribute
substantially to the meteor flux, nor will dust from any future activity apart
from an unlikely high speed (>0.5 km/s) dust outburst prior to the comet
reaching r~2 au in 2021 September. | astro-ph_EP |
Detectability of Life Using Oxygen on Pelagic Planets and Water Worlds: The search for life on exoplanets is one of the grand scientific challenges
of our time. The strategy to date has been to find (e.g., through transit
surveys like Kepler) Earth-like exoplanets in their stars habitable zone, then
use transmission spectroscopy to measure biosignature gases, especially oxygen,
in the planets atmospheres (e.g., using JWST, the James Webb Space Telescope).
Already there are more such planets than can be observed by JWST, and missions
like the Transiting Exoplanet Survey Satellite and others will find more. A
better understanding of the geochemical cycles relevant to biosignature gases
is needed, to prioritize targets for costly follow-up observations and to help
design future missions. We define a Detectability Index to quantify the
likelihood that a biosignature gas could be assigned a biological vs.
non-biological origin. We apply this index to the case of oxygen gas, O2, on
Earth-like planets with varying water contents. We demonstrate that on
Earth-like exoplanets with 0.2 weight percent (wt%) water (i.e., no exposed
continents) a reduced flux of bioessential phosphorus limits the export of
photosynthetically produced atmospheric O2 to levels indistinguishable from
geophysical production by photolysis of water plus hydrogen escape. Higher
water contents >1wt% that lead to high-pressure ice mantles further slow
phosphorus cycling. Paradoxically, the maximum water content allowing use of O2
as a biosignature, 0.2wt%, is consistent with no water based on mass and
radius. Thus, the utility of an O2 biosignature likely requires the direct
detection of both water and land on a planet. | astro-ph_EP |
The Tidal-Thermal Evolution of the Pluto-Charon System: Existence of subsurface oceans on the satellites of the giant planets and
Trans-Neptunian objects has been predicted for some time. Oceans on icy worlds
exert a considerable influence on the dynamics of the ice-ocean system and,
because of the astrobiological potential, represent an important objective for
future missions. The Pluto-Charon system is representative of an icy moon
orbiting a dwarf planet formed from the remnants of a giant impact. Evolution
of icy moons is primarily controlled by the mode and efficiency of heat
transfer through the outer ice shell, which is influenced by the presence of
impurities, by tidal dissipation in the ice shell, and the radioactive element
budget in the core. Previous studies on the evolution of the Pluto-Charon
system considered either only the thermal or the tidal evolution, and in the
cases where both were considered, the important effect of the presence of
impurities in the liquid oceans was not addressed. We consider the joint
tidal-thermal evolution of the system by combining a comprehensive tidal model
that incorporates a viscoelastic tidal response with a parameterized convection
model developed for icy worlds. This approach enables an extensive analysis of
the conditions required for formation and maintenance of subsurface liquid
oceans to the present. Our results show that because of fast circularization
and synchronization of the orbits, tidal heating is only important during the
early stages of evolution (<1 Myr). We test the sensitivity of our results to
the initial orbital and thermal parameters. In all the cases, oceans on Pluto
are always predicted to remain liquid to the present, ranging from 40 km to
150-km thick, whereas oceans on Charon have solidified. This is supported by
New Horizons observations of extensional faults on Pluto and both extensional
and compressional faults on Charon. | astro-ph_EP |
The 1998 November 14 Occultation of GSC 0622-00345 by Saturn. I.
Techniques for Ground-Based Stellar Occultations: On 1998 November 14, Saturn and its rings occulted the star GSC 0622-00345.
We observed atmospheric immersion with NSFCAM at the National Aeronautics and
Space Administration's Infrared Telescope Facility on Mauna Kea, Hawaii.
Immersion occurred at 55.5\circ S planetocentric latitude. A 2.3 {\mu}m,
methane-band filter suppressed reflected sunlight. Atmospheric emersion and
ring data were not successfully obtained. We describe our observation,
light-curve production, and timing techniques, including improvements in
aperture positioning, removal of telluric scintillation effects, and timing.
Many of these techniques are known within the occultation community, but have
not been described in the reviewed literature. We present a light curve whose
signal-to-noise ratio per scale height is 267, among the best ground-based
signals yet achieved, despite a disadvantage of up to 8 mag in the stellar flux
compared to prior work. | astro-ph_EP |
Short-term variability of comet C/2012 S1 (ISON) at 4.8 AU from the Sun: We observed comet C/2012 S1 (ISON) during six nights in February 2013 when it
was at 4.8 AU from the sun. At this distance and time the comet was not very
active and it was theoretically possible to detect photometric variations
likely due to the rotation of the cometary nucleus. The goal of this work is to
obtain differential photometry of the comet inner coma using different aperture
radii in order to derive a possible rotational period. Large field of view
images were obtained with a 4k x 4k CCD at the f/3 0.77m telescope of La Hita
Observatory in Spain. Aperture photometry was performed in order to get
relative magnitude variation versus time. Using calibrated star fields we also
obtained ISON's R-magnitudes versus time. We applied a Lomb-Scargle periodogram
analysis to get possible periodicities for the observed brightness variations,
directly related with the rotation of the cometary nucleus. The comet light
curve obtained is very shallow, with a peak-to-peak amplitude of 0.03 $\pm$
0.02 mag. A tentative synodic rotational period (single-peaked) of 14.4 $\pm$
1.2 hours for ISON's nucleus is obtained from our analysis, but there are other
possibilities. We studied the possible effect of the seeing variations in the
obtained periodicities during the same night, and from night to night. These
seeing variations had no effect on the derived periodicity. We discuss and
interpret all possible solutions for the rotational period of ISON's nucleus. | astro-ph_EP |
ALMA Observations of Asymmetric Molecular Gas Emission from a
Protoplanetary Disk in the Orion Nebula: We present Atacama Large Millimeter/submillimeter Array (ALMA) observations
of molecular line emission from d216-0939, one of the largest and most massive
protoplanetary disks in the Orion Nebula Cluster (ONC). We model the spectrally
resolved HCO$^+$ (4--3), CO (3--2), and HCN (4--3) lines observed at 0\farcs5
resolution to fit the temperature and density structure of the disk. We also
weakly detect and spectrally resolve the CS (7--6) line but do not model it.
The abundances we derive for CO and HCO$^+$ are generally consistent with
expected values from chemical modeling of protoplanetary disks, while the HCN
abundance is higher than expected. We dynamically measure the mass of the
central star to be $2.17\pm0.07\,M_\odot$ which is inconsistent with the
previously determined spectral type of K5. We also report the detection of a
spatially unresolved high-velocity blue-shifted excess emission feature with a
measurable positional offset from the central star, consistent with a Keplerian
orbit at $60\pm20\,\mathrm{au}$. Using the integrated flux of the feature in
HCO$^+$ (4--3), we estimate the total H$_2$ gas mass of this feature to be at
least $1.8-8\,M_\mathrm{Jupiter}$, depending on the assumed temperature. The
feature is due to a local temperature and/or density enhancement consistent
with either a hydrodynamic vortex or the expected signature of the envelope of
a forming protoplanet within the disk. | astro-ph_EP |
HAT-P-58b -- HAT-P-64b: Seven Planets Transiting Bright Stars: We report the discovery and characterization of 7 transiting exoplanets from
the HATNet survey. The planets, which are hot Jupiters and Saturns transiting
bright sun-like stars, include: HAT-P-58b (with mass Mp = 0.37 MJ, radius Rp =
1.33 RJ, and orbital period P = 4.0138 days), HAT-P-59b (Mp = 1.54 MJ, Rp =
1.12 RJ, P = 4.1420 days), HAT-P-60b (Mp = 0.57 MJ, Rp = 1.63 RJ, P = 4.7948
days), HAT-P-61b (Mp = 1.06 MJ, Rp = 0.90 RJ, P = 1.9023 days), HAT-P-62b (Mp =
0.76 MJ, Rp = 1.07 RJ, P = 2.6453 days), HAT-P-63b (Mp = 0.61 MJ, Rp = 1.12 RJ,
P = 3.3777 days), and HAT-P-64b (Mp = 0.58 MJ, Rp = 1.70 RJ, P = 4.0072 days).
The typical errors on these quantities are 0.06 MJ, 0.03 RJ, and 0.2seconds,
respectively. We also provide accurate stellar parameters for each of the hosts
stars. With V = 9.710+/-0.050mag, HAT-P-60 is an especially bright transiting
planet host, and an excellent target for additional follow-up observations.
With Rp = 1.703+/-0.070 RJ, HAT-P-64b is a highly inflated hot Jupiter around a
star nearing the end of its main-sequence lifetime, and is among the largest
known planets. Five of the seven systems have long-cadence observations by TESS
which are included in the analysis. Of particular note is HAT-P-59
(TOI-1826.01) which is within the Northern continuous viewing zone of the TESS
mission, and HAT-P-60, which is the TESS candidate TOI-1580.01. | astro-ph_EP |
A particle-based hybrid code for planet formation: We introduce a new particle-based hybrid code for planetary accretion. The
code uses an $N$-body routine for interactions with planetary embryos while it
can handle a large number of planetesimals using a super-particle
approximation, in which a large number of small planetesimals are represented
by a small number of tracers. Tracer-tracer interactions are handled by a
statistical routine which uses the phase-averaged stirring and collision rates.
We compare hybrid simulations with analytic predictions and pure $N$-body
simulations for various problems in detail and find good agreements for all
cases. The computational load on the portion of the statistical routine is
comparable to or less than that for the $N$-body routine. The present code
includes an option of hit-and-run bouncing but not fragmentation, which remains
for future work. | astro-ph_EP |
The Complex Rotational Light Curve of (385446) Manwë-Thorondor, a
Multi-Component Eclipsing System in the Kuiper Belt: Kuiper Belt Object (385446) Manw\"e-Thorondor is a multi-object system with
mutual events predicted to occur from 2014 to 2019. To detect the events, we
observed the system at 4 epochs (UT 2016 Aug 25 and 26, 2017 Jul 22 and 25,
2017 Nov 9, and 2018 Oct 6) in g, r, and VR bands using the 4-m SOAR and the
8.1-m Gemini South telescopes at Cerro Pach\'on, Chile and Lowell Observatory '
s 4.3-m Discovery Channel Telescope at Happy Jack, Arizona. These dates overlap
the uncertainty range (+/- 0.5 d) for four inferior events (Thorondor eclipsing
Manw\"e). We clearly observe variability for the unresolved system with a
double-peaked period 11.88190 +/- 0.00005 h and ~0.5 mag amplitude together
with much longer-term variability. Using a multi-component model, we
simultaneously fit our observations and earlier photometry measured separately
for Manw\"e and Thorondor with the Hubble Space Telescope. Our fit suggests
Manw\"e is bi-lobed, close to the barbell shape expected for a strengthless
body with density ~0.8 g/cm3 in hydrostatic equilibrium. For Manw\"e, we
thereby derive maximum width to length ratio ~0.30, surface area equivalent to
a sphere of diameter 190 km, geometric albedo 0.06, mass 1.4x1018 kg, and spin
axis oriented ~75 deg from Earth ' s line of sight. Changes in Thorondor ' s
brightness by ~0.6 mag with ~300-d period may account for the system ' s
long-term variability. Mutual events with unexpectedly shallow depth and short
duration may account for residuals to the fit. The system is complex, providing
a challenging puzzle for future modeling efforts. | astro-ph_EP |
An Ultra-Hot Neptune in the Neptune desert: About one out of 200 Sun-like stars has a planet with an orbital period
shorter than one day: an ultra-short-period planet (Sanchis-ojeda et al. 2014;
Winn et al. 2018). All of the previously known ultra-short-period planets are
either hot Jupiters, with sizes above 10 Earth radii (Re), or apparently rocky
planets smaller than 2 Re. Such lack of planets of intermediate size (the "hot
Neptune desert") has been interpreted as the inability of low-mass planets to
retain any hydrogen/helium (H/He) envelope in the face of strong stellar
irradiation. Here, we report the discovery of an ultra-short-period planet with
a radius of 4.6 Re and a mass of 29 Me, firmly in the hot Neptune desert. Data
from the Transiting Exoplanet Survey Satellite (Ricker et al. 2015) revealed
transits of the bright Sun-like star \starname\, every 0.79 days. The planet's
mean density is similar to that of Neptune, and according to thermal evolution
models, it has a H/He-rich envelope constituting 9.0^(+2.7)_(-2.9)% of the
total mass. With an equilibrium temperature around 2000 K, it is unclear how
this "ultra-hot Neptune" managed to retain such an envelope. Follow-up
observations of the planet's atmosphere to better understand its origin and
physical nature will be facilitated by the star's brightness (Vmag=9.8). | astro-ph_EP |
AcuA: the AKARI/IRC Mid-infrared Asteroid Survey: We present the results of an unbiased asteroid survey in the mid-infrared
wavelength with the Infrared Camera (IRC) onboard the Japanese infrared
satellite AKARI. About 20% of the point source events recorded in the AKARI
All-Sky Survey observations are not used for the IRC Point Source Catalog
(IRC-PSC) in its production process because of the lack of multiple detection
by position. Asteroids, which are moving objects on the celestial sphere,
remain in these "residual events". We identify asteroids out of the residual
events by matching them with the positions of known asteroids. For the
identified asteroids, we calculate the size and albedo based on the Standard
Thermal Model. Finally we have a brand-new catalog of asteroids, named the
Asteroid Catalog Using Akari (AcuA), which contains 5,120 objects, about twice
as many as the IRAS asteroid catalog. The catalog objects comprise 4,953 main
belt asteroids, 58 near Earth asteroids, and 109 Jovian Trojan asteroids. The
catalog will be publicly available via the Internet. | astro-ph_EP |
The apparent tidal decay of WASP-4 b can be explained by the Rømer
effect: Tidal orbital decay plays a vital role in the evolution of hot Jupiter
systems. As of now, this was only observationally confirmed for the WASP-12
system. There are a few other candidates, including WASP-4 b, but no conclusive
result could be obtained for these systems as of yet. In this study, we present
an analysis of new TESS data of WASP-4 b together with archival data, taking
the light-time effect (LTE), induced by the second planetary companion, into
account as well. We make use of three different Markov-Chain-Monte-Carlo
models; a circular orbit with a constant orbital period, a circular orbit with
a decaying orbit, and an elliptical orbit with apsidal precession. This
analysis is repeated for four cases. The first case features no LTE correction,
with the remaining three cases featuring three different timing correction
approaches. Comparison of these models yields no conclusive answer to the cause
of WASP-4\,b's apparent transit timing variations. A broad range of values of
the orbital decay and apsidal precession parameters are possible, depending on
the LTE correction. This work highlights the importance of continued
photometric and spectroscopic monitoring of hot Jupiters. | astro-ph_EP |
Do we need to consider electron kinetic effects to properly model a
planetary magnetosphere: the case of Mercury: The magnetosphere of Mercury is studied using an implicit full particle in
cell simulation (PIC). We use a hybrid simulation where ions are full particles
and electrons are considered as a fluid to start a full PIC simulation where
electrons are also particles and follow their distribution function. This
approach allows us to estimate the changes introduced by the electron kinetic
physics. We find that the overall macroscopic state of the magnetosphere of
Mercury is little affected but several physical processes are significantly
modified in the full PIC simulation: the foreshock region is more active with
more intense shock reformation, the Kelvin-Helmholtz rippling effects on the
nightside magnetopause are sharper, and the magnetotail current sheet becomes
thinner than those predicted by the hybrid simulation. The greatest effect of
the electron physics, comes from the processes of particle energization. Both
species, not just the electrons, are found to gain more energy when kinetic
electron processes are taken into account. The region with the most energetic
plasma is found on the dusk side of the tail where magnetic flux ropes are
formed due to reconnection. We find that the ion and electron energization is
associated with the regions of reconnection and the development of kinetic
instabilities caused by counter-streaming electron populations. The resulting
electron distributions are highly non Maxwellian, a process that neither MHD
nor hybrid models can describe. | astro-ph_EP |
Dynamical evolution and end states of active and inactive Centaurs: We numerically study the dynamical evolution of observed samples of active
and inactive Centaurs and clones that reach the Jupiter-Saturn region. Our aim
is to compare the evolution between active and inactive Centaurs, their end
states and their transfer to Jupiter family comets and Halley-type comets. We
find that the median lifetime of inactive Centaurs is about twice longer than
that for active Centaurs, suggesting that activity is related to the residence
time in the region. This view is strengthened by the observation that
high-inclination and retrograde Centaurs (Tisserand parameters with respect to
Jupiter $T_J < 2$) which have the longest median dynamical lifetime ($=1.37
\times 10^6$ yr) are all inactive. We also find that the perihelion distances
of some active, comet-like Centaurs have experienced drastic drops of a few au
in the recent past ($\sim 10^2-10^3$ yr), while such drops are not found among
inactive Centaurs. Inactive Centaurs with $T_J \lsim 2.5$ usually evolve to
Halley-type comets, whereas inactive Centaurs with $T_J \gsim 2.5$ and active
Centaurs (that also have $T_J \gsim 2.5$) evolve almost always to Jupiter
family comets and very seldom to Halley type comets. Inactive Centaurs are also
more prone to end up as sungrazers, and both inactive and active Centaurs
transit through different mean motion resonances (generally with Jupiter)
during their evolution. | astro-ph_EP |
Characterization of the KOI-94 System with Transit Timing Variation
Analysis: Implication for the Planet-Planet Eclipse: The KOI-94 system is a closely-packed, multi-transiting planetary system
discovered by the Kepler space telescope. It is known as the first system that
exhibited a rare event called a "planet-planet eclipse (PPE)," in which two
planets partially overlap with each other in their double-transit phase. In
this paper, we constrain the parameters of the KOI-94 system with an analysis
of the transit timing variations (TTVs). Such constraints are independent of
the radial velocity (RV) analysis recently performed by Weiss and coworkers,
and valuable in examining the reliability of the parameter estimate using TTVs.
We numerically fit the observed TTVs of KOI-94c, KOI-94d, and KOI-94e for their
masses, eccentricities, and longitudes of periastrons, and obtain the best-fit
parameters including $m_{\rm c} = 9.4_{-2.1}^{+2.4} M_{\oplus}$, $m_{\rm d} =
52.1_{-7.1}^{+6.9} M_{\oplus}$, $m_{\rm e} = 13.0_{-2.1}^{+2.5} M_{\oplus}$,
and $e \lesssim 0.1$ for all the three planets. While these values are mostly
in agreement with the RV result, the mass of KOI-94d estimated from the TTV is
significantly smaller than the RV value $m_{\rm d} = 106 \pm 11 M_{\oplus}$. In
addition, we find that the TTV of the outermost planet KOI-94e is not well
reproduced in the current modeling. We also present analytic modeling of the
PPE and derive a simple formula to reconstruct the mutual inclination of the
two planets from the observed height, central time, and duration of the
brightening caused by the PPE. Based on this model, the implication of the
results of TTV analysis for the time of the next PPE is discussed. | astro-ph_EP |
Toroidal vortices as a solution to the dust migration problem: In an earlier letter, we reported that dust settling in protoplanetary discs
may lead to a dynamical dust-gas instability that produces global toroidal
vortices. In this letter, we investigate the evolution of a dusty
protoplanetary disc with two different dust species (1 mm and 50 cm dust
grains), under the presence of the instability. We show how toroidal vortices,
triggered by the interaction of mm grains with the gas, stop the radial
migration of metre-sized dust, potentially offering a natural and efficient
solution to the dust migration problem. | astro-ph_EP |
Failure modes and conditions of a cohesive, spherical body due to YORP
spin-up: This paper presents transition of the failure mode of a cohesive, spherical
body due to YORP spin-up. On the assumption that the distribution of materials
in the body is homogeneous, failed regions first appearing in the body at
different spin rates are predicted by comparing the yield condition of an
elastic stress in the body. It is found that as the spin rate increases, the
locations of the failed regions move from the equatorial surface to the central
region. To avoid such failure modes, the body should have higher cohesive
strength. The results by this model are consistent with those by a plastic
finite element model. Then, this model and a two-layered-cohesive model first
proposed by Hirabayashi et al. are used to classify possible evolution and
disruption of a spherical body. There are three possible pathways to
disruption. First, because of a strong structure, failure of the central region
is dominant and eventually leads to a breakup into multiple components. Second,
a weak surface and a weak interior make the body oblate. Third, a strong
internal core prevents the body from failing and only allows surface shedding.
This implies that observed failure modes may highly depend on the internal
structure of an asteroid, which could provide crucial information for giving
constraints on the physical properties. | astro-ph_EP |
DART Mission Determination of Momentum Transfer: Model of Ejecta Plume
Observations: The NASA Double Asteroid Redirection Test (DART) spacecraft will impact the
secondary member of the [65803] Didymos binary in order to perform the first
demonstration of asteroid deflection by kinetic impact. Determination of the
momentum transfer to the target body from the kinetic impact is a primary
planetary defense objective, using ground-based telescopic observations of the
orbital period change of Didymos and imaging of the DART impact ejecta plume by
the LICIACube cubesat, along with modeling and simulation of the DART impact.
LICIACube, contributed by the Italian Space Agency, will perform a flyby of
Didymos a few minutes after the DART impact, to resolve the ejecta plume
spatial structure and to study the temporal evolution. LICIACube ejecta plume
images will help determine the vector momentum transfer from the DART impact,
by determining or constraining the direction and the magnitude of the momentum
carried by ejecta. A model is developed for the impact ejecta plume optical
depth, using a point source scaling model of the DART impact. The model is
applied to expected LICIACube plume images and shows how plume images enable
characterization of the ejecta mass versus velocity distribution. The ejecta
plume structure, as it evolves over time, is determined by the amount of ejecta
that has reached a given altitude at a given time. The evolution of the plume
optical depth profiles determined from LICIACube images can distinguish between
strength-controlled and gravity-controlled impacts, by distinguishing the
respective mass versus velocity distributions. LICIACube plume images
discriminate the differences in plume structure and evolution that result from
different target physical properties, mainly strength and porosity, thereby
allowing inference of these properties to improve the determination of momentum
transfer. | astro-ph_EP |
Evidence for a lost population of close-in exoplanets: We investigate the evaporation history of known transiting exoplanets in
order to consider the origin of observed correlations between mass, surface
gravity and orbital period. We show that the survival of the known planets at
their current separations is consistent with a simple model of evaporation, but
that many of the same planets would not have survived closer to their host
stars. These putative closer-in systems represent a lost population that could
account for the observed correlations. We conclude that the relation underlying
the correlations noted by Mazeh et al. (2005) and Southworth et al. (2007) is
most likely a linear cut-off in the M^2/R^3 vs a^-2 plane, and we show that the
distribution of exoplanets in this plane is in close agreement with the
evaporation model. | astro-ph_EP |
The Effect of the Approach to Gas Disk Gravitational Instability on the
Rapid Formation of Gas Giant Planets: Observational evidence suggests that gas disk instability may be responsible
for the formation of at least some gas giant exoplanets, particularly massive
or distant gas giants. With regard to close-in gas giants, Boss (2017) used the
$\beta$ cooling approximation to calculate hydrodynamical models of inner gas
disk instability, finding that provided disks with low values of the initial
minimum Toomre stability parameter (i.e., $Q_i < 2$ inside 20 au) form,
fragmentation into self-gravitating clumps could occur even for $\beta$ as high
as 100 (i.e., extremely slow cooling). Those results implied that the evolution
of disks toward low $Q_i$ must be taken into account. This paper presents such
models: initial disk masses of 0.091 $M_\odot$ extending from 4 to 20 au around
a 1 $M_\odot$ protostar, with a range (1 to 100) of $\beta$ cooling parameters,
the same as in Boss (2017), but with all the disks starting with $Q_i = 2.7$,
i.e., gravitationally stable, and allowed to cool from their initial outer disk
temperature of 180 K to as low as 40 K. All the disks eventually fragment into
at least one dense clump. The clumps were again replaced by virtual
protoplanets (VPs) and the masses and orbits of the resulting ensemble of VPs
compare favorably with those of Boss (2017), supporting the claim that disk
instability can form gas giants rapidly inside 20 au, provided that
sufficiently massive protoplanetary disks exist. | astro-ph_EP |
Emergence of vortices at the edges of planet-driven gaps in
protoplanetary discs: Young planets embedded in protoplanetary discs (PPDs) excite spiral density
waves, which propagate, shock and deposit angular momentum in the disc. This
results in gap opening around the planetary orbit, even for low (sub-thermal)
mass planets, provided that the effective viscosity in the disc is low. The
edges of these planet-induced gaps are known to be prone to emergence of
observable vortices via the Rossby Wave Instability (RWI). We study timescales
for the development of vortices driven by low mass planets in inviscid discs.
We employ a recently developed semi-analytical theory of vortensity production
by the planet-driven shock to predict vortensity evolution near the planet,
from which we derive the radial profile of the planet-induced gap as a function
of time (this procedure can have multiple other uses, e.g. to study dust
trapping, suppression of pebble accretion, etc.). We then analyze the linear
stability of the gap edges against the RWI, obtaining the timescales for the
first appearance of unstable modes and (later) fully developed vortices at gap
edges. We present useful formulae for these timescales as functions of
planetary and disc parameters and provide their physical justification. We also
thoroughly test our semi-analytical framework against high resolution 2D
hydrodynamic simulations, confirming the accuracy of our theoretical
predictions. We discuss ways in which our semi-analytical framework can be
extended to incorporate additional physics, e.g. planetary accretion,
migration, and non-zero disc viscosity. Our results can be used to interpret
observations of PPDs and to predict emergence of vortices in simulations. | astro-ph_EP |
Heating of the Atmospheres of Short-orbit Exoplanets by Their Rapid
Orbital Motion Through an Extreme Space Environment: Exoplanets with short orbit period reside very close to their host stars.
They transition very rapidly between different sectors of the circumstellar
space environment along their orbit, leading to large variations of the
magnetic field in the vicinity of the planet on short timescales. This rapid
change of the magnetic flux through the conducting and resistive layer of the
planetary upper atmosphere may drive currents that dissipate in the form of
Joule Heating. Here, we estimate the amount of Joule Heating dissipation in the
upper atmosphere of Trappist-1e, and two hypothetical planets orbiting the Sun
in close-in orbits. We find that the rapid orbital motion could drive a
significant amount of atmospheric heating and could significantly affect the
planetary atmosphere escape rate. Thus, the process should be accounted for
when studying the long-term evolution of exoplanetary atmospheres. | astro-ph_EP |
Binary Survival in the Outer Solar System: As indicated by their special characteristics, the cold classical Kuiper belt
objects (KBOs) formed and survived at 42-47 au. Notably, they show a large
fraction of equal-size binaries whose formation is probably related to the
accretion of KBOs themselves. These binaries are uncommon in other --hot,
resonant, scattered-- populations, which are thought to have been implanted
from the massive disk below 30 au to >30 au during Neptune's migration. Here we
highlight the possibility that equal-size binaries formed in the disk but were
subsequently removed by impacts and/or dynamical effects (e.g., scattering
encounters with Neptune). We determine the dependence of these processes on the
size and separation of binary components. Our results indicate that tighter
binaries, if they formed in the massive disk, have relatively good chances of
survival (unless the disk was long-lived). In contrast, the widest binaries in
the hot population, such as 2002 VF130, have a very low survival probability
(<1%) even if the massive disk was short-lived. They may represent a trace of
lucky survivors of a much larger population of the original disk binaries, or
they formed at ~30-40~au and dodged the impact- and encounter-related
perturbations that we studied here. We find that all known satellites of the
largest KBOs would survive during the dynamical implantation of these bodies in
the Kuiper belt. The low orbital eccentricities of Pluto's small moons may have
been excited by impacts and/or encounters of the Pluto system to Neptune. | astro-ph_EP |
The Hubble Wide Field Camera 3 Test of Surfaces in the Outer Solar
System: The Compositional Classes of the Kuiper Belt: We present the first results of the Hubble Wide Field Camera 3 Test of
Surfaces in the Outer Solar System (H/WTSOSS). The purpose of this survey was
to measure the surface properties of a large number of Kuiper belt objects and
attempt to infer compositional and dynamical correlations. We find that the
Centaurs and the low-perihelion scattered disk and resonant objects exhibit
virtually identical bifurcated optical colour distributions and make up two
well defined groups of object. Both groups have highly correlated optical and
NIR colours which are well described by a pair of two component mixture models
that have different red components, but share a common neutral component. The
small, $H_{606}\gtrsim5.6$ high-perihelion excited objects are entirely
consistent with being drawn from the two branches of the mixing model
suggesting that the colour bifurcation of the Centaurs is apparent in all small
excited objects. On the other hand, objects larger than $H_{606}\sim5.6$ are
not consistent with the mixing model, suggesting some evolutionary process
avoided by the smaller objects. The existence of a bifurcation amongst all
excited populations argues that the two separate classes of object existed in
the primordial disk before the excited Kuiper belt was populated. The cold
classical objects exhibit a different type of surface which has colours that
are consistent with being drawn from the red branch of the mixing model, but
with much higher albedos. | astro-ph_EP |
Searching for Saturn's Dust Swarm: Limits on the size distribution of
Irregular Satellites from km to micron sizes: We describe a search for dust created in collisions between the Saturnian
irregular satellites using archival \emph{Spitzer} MIPS observations. Although
we detected a degree scale Saturn-centric excess that might be attributed to an
irregular satellite dust cloud, we attribute it to the far-field wings of the
PSF due to nearby Saturn. The Spitzer PSF is poorly characterised at such
radial distances, and we expect PSF characterisation to be the main issue for
future observations that aim to detect such dust. The observations place an
upper limit on the level of dust in the outer reaches of the Saturnian system,
and constrain how the size distribution extrapolates from the smallest known
(few km) size irregulars down to micron-size dust. Because the size
distribution is indicative of the strength properties of irregulars, we show
how our derived upper limit implies irregular satellite strengths more akin to
comets than asteroids. This conclusion is consistent with their presumed
capture from the outer regions of the Solar System. | astro-ph_EP |
KMT-2021-BLG-0912Lb: A microlensing super Earth around a K-type star: The light curve of the microlensing event KMT-2021-BLG-0912 exhibits a very
short anomaly relative to a single-lens single-source form. We investigate the
light curve for the purpose of identifying the origin of the anomaly. We model
the light curve under various interpretations. From this, we find four
solutions, in which three solutions are found under the assumption that the
lens is composed of two masses (2L1S models), and the other solution is found
under the assumption that the source is comprised of a binary-star system (1L2S
model). The 1L2S model is ruled out based on the contradiction that the faint
source companion is bigger than its primary, and one of the 2L1S solutions is
excluded from the combination of the relatively worse fit, blending constraint,
and lower overall probability, leaving two surviving solutions with the
planet/host mass ratios of $q\sim 2.8\times 10^{-5}$ and $\sim 1.1\times
10^{-5}$. A subtle central deviation supports the possibility of a tertiary
lens component, either a binary companion to the host with a very large or
small separation or a second planet lying near the Einstein ring, but it is
difficult to claim a secure detection due to the marginal fit improvement, lack
of consistency among different data sets, and difficulty in uniquely specifying
the nature of the tertiary component. With the observables of the event, it is
estimated that the masses of the planet and host are $\sim (6.9~M_\oplus,
0.75~M_\odot)$ according to one solution and $\sim (2.8~M_\oplus,
0.80~M_\odot)$ according to the other solution, indicating that the planet is a
super Earth around a K-type star, regardless of the solution. | astro-ph_EP |
Terrestrial planet formation in low-mass disks: dependence with initial
conditions: In general, most of the studies of terrestrial-type planet formation
typically use ad hoc initial conditions. In this work we improved the initial
conditions described in Ronco & de El\'ia (2014) starting with a
semi-analytical model wich simulates the evolution of the protoplanetary disk
during the gas phase. The results of the semi-analytical model are then used as
initial conditions for the N-body simulations. We show that the planetary
systems considered are not sensitive to the particular initial distribution of
embryos and planetesimals and thus, the results are globally similar to those
found in the previous work. | astro-ph_EP |
The PLATO 2.0 Mission: PLATO 2.0 has recently been selected for ESA's M3 launch opportunity
(2022/24). Providing accurate key planet parameters (radius, mass, density and
age) in statistical numbers, it addresses fundamental questions such as: How do
planetary systems form and evolve? Are there other systems with planets like
ours, including potentially habitable planets? The PLATO 2.0 instrument
consists of 34 small aperture telescopes (32 with 25 sec readout cadence and 2
with 2.5 sec candence) providing a wide field-of-view (2232 deg2) and a large
photometric magnitude range (4-16 mag). It focusses on bright (4-11 mag) stars
in wide fields to detect and characterize planets down to Earth-size by
photometric transits, whose masses can then be determined by ground-based
radial-velocity follow-up measurements. Asteroseismology will be performed for
these bright stars to obtain highly accurate stellar parameters, including
masses and ages. The combination of bright targets and asteroseismology results
in high accuracy for the bulk planet parameters: 2%, 4-10% and 10% for planet
radii, masses and ages, respectively. The planned baseline observing strategy
includes two long pointings (2-3 years) to detect and bulk characterize planets
reaching into the habitable zone (HZ) of solar-like stars and an additional
step-and-stare phase to cover in total about 50% of the sky. PLATO 2.0 will
observe up to 1,000,000 stars and detect and characterize hundreds of small
planets, and thousands of planets in the Neptune to gas giant regime out to the
HZ. It will therefore provide the first large-scale catalogue of bulk
characterized planets with accurate radii, masses, mean densities and ages.
This catalogue will include terrestrial planets at intermediate orbital
distances, where surface temperatures are moderate. Coverage of this parameter
range with statistical numbers of bulk characterized planets is unique to PLATO
2.0. | astro-ph_EP |
Asteroid Models from Multiple Data Sources: In the past decade, hundreds of asteroid shape models have been derived using
the lightcurve inversion method. At the same time, a new framework of 3-D shape
modeling based on the combined analysis of widely different data sources such
as optical lightcurves, disk-resolved images, stellar occultation timings,
mid-infrared thermal radiometry, optical interferometry, and radar
delay-Doppler data, has been developed. This multi-data approach allows the
determination of most of the physical and surface properties of asteroids in a
single, coherent inversion, with spectacular results. We review the main
results of asteroid lightcurve inversion and also recent advances in multi-data
modeling. We show that models based on remote sensing data were confirmed by
spacecraft encounters with asteroids, and we discuss how the multiplication of
highly detailed 3-D models will help to refine our general knowledge of the
asteroid population. The physical and surface properties of asteroids, i.e.,
their spin, 3-D shape, density, thermal inertia, surface roughness, are among
the least known of all asteroid properties. Apart for the albedo and diameter,
we have access to the whole picture for only a few hundreds of asteroids. These
quantities are nevertheless very important to understand as they affect the
non-gravitational Yarkovsky effect responsible for meteorite delivery to Earth,
or the bulk composition and internal structure of asteroids. | astro-ph_EP |
K2-288Bb: A Small Temperate Planet in a Low-mass Binary System
Discovered by Citizen Scientists: Observations from the Kepler and K2 missions have provided the astronomical
community with unprecedented amounts of data to search for transiting
exoplanets and other astrophysical phenomena. Here, we present K2-288, a
low-mass binary system (M2.0 +/- 1.0; M3.0 +/- 1.0) hosting a small (Rp = 1.9
REarth), temperate (Teq = 226 K) planet observed in K2 Campaign 4. The
candidate was first identified by citizen scientists using Exoplanet Explorers
hosted on the Zooniverse platform. Follow-up observations and detailed analyses
validate the planet and indicate that it likely orbits the secondary star on a
31.39-day period. This orbit places K2-288Bb in or near the habitable zone of
its low-mass host star. K2-288Bb resides in a system with a unique
architecture, as it orbits at >0.1 au from one component in a moderate
separation binary (aproj approximately 55 au), and further follow-up may
provide insight into its formation and evolution. Additionally, its estimated
size straddles the observed gap in the planet radius distribution. Planets of
this size occur less frequently and may be in a transient phase of radius
evolution. K2-288 is the third transiting planet system identified by the
Exoplanet Explorers program and its discovery exemplifies the value of citizen
science in the era of Kepler, K2, and the Transiting Exoplanet Survey
Satellite. | astro-ph_EP |
Initial mass function of planetesimals formed by the streaming
instability: The streaming instability is a mechanism to concentrate solid particles into
overdense filaments that undergo gravitational collapse and form planetesimals.
However, it remains unclear how the initial mass function of these
planetesimals depends on the box dimensions of numerical simulations. To
resolve this, we perform simulations of planetesimal formation with the largest
box dimensions to date, allowing planetesimals to form simultaneously in
multiple filaments that can only emerge within such large simulation boxes. In
our simulations, planetesimals with sizes between 80 km and several hundred
kilometers form. We find that a power law with a rather shallow exponential
cutoff at the high-mass end represents the cumulative birth mass function
better than an integrated power law. The steepness of the exponential cutoff is
largely independent of box dimensions and resolution, while the exponent of the
power law is not constrained at the resolutions we employ. Moreover, we find
that the characteristic mass scale of the exponential cutoff correlates with
the mass budget in each filament. Together with previous studies of
high-resolution simulations with small box domains, our results therefore imply
that the cumulative birth mass function of planetesimals is consistent with an
exponentially tapered power law with a power-law exponent of approximately -1.6
and a steepness of the exponential cutoff in the range of 0.3-0.4. | astro-ph_EP |
RVSPY -- Radial Velocity Survey for Planets around Young Stars. Target
characterization and high-cadence survey: We introduce our Radial Velocity Survey for Planets around Young stars
(RVSPY), characterise our target stars, and search for substellar companions at
orbital separations smaller than a few au from the host star. We use the FEROS
spectrograph to obtain high signal-to-noise spectra and time series of precise
radial velocities (RVs) of 111 stars most of which are surrounded by debris
discs. Our target stars have spectral types between early F and late K, a
median age of 400 Myr, and a median distance of 45 pc. We determine for all
target stars their basic stellar parameters and present the results of the
high-cadence RV survey and activity characterization. We achieve a median
single-measurement RV precision of 6 m/s and derive the short-term intrinsic RV
scatter of our targets (median 22 m/s), which is mostly caused by stellar
activity and decays with age from >100 m/s at <20 Myr to <20 m/s at >500 Myr.
We discover six previously unknown close companions with orbital periods
between 10 and 100 days, three of which are low-mass stars, and three are in
the brown dwarf mass regime. We detect no hot companion with an orbital period
<10 days down to a median mass limit of ~1 M_Jup for stars younger than 500
Myr, which is still compatible with the established occurrence rate of such
companions around main-sequence stars. We find significant RV periodicities
between 1.3 and 4.5 days for 14 stars, which are, however, all caused by
rotational modulation due to starspots. We also analyse the TESS photometric
time series data and find significant periodicities for most of the stars. For
11 stars, the photometric periods are also clearly detected in the RV data. We
also derive stellar rotation periods ranging from 1 to 10 days for 91 stars,
mostly from TESS data. From the intrinsic activity-related short-term RV
jitter, we derive the expected mass-detection thresholds for longer-period
companions. | astro-ph_EP |
Global simulations of protoplanetary disks with net magnetic flux: I.
Non-ideal MHD case: The planet-forming region of protoplanetary disks is cold, dense, and
therefore weakly ionized. For this reason, magnetohydrodynamic (MHD) turbulence
is thought to be mostly absent, and another mechanism has to be found to
explain gas accretion. It has been proposed that magnetized winds, launched
from the ionized disk surface, could drive accretion in the presence of a
large-scale magnetic field. The efficiency and the impact of these surface
winds on the disk structure is still highly uncertain. We present the first
global simulations of a weakly ionized disk that exhibits large-scale
magnetized winds. We also study the impact of self-organization, which was
previously demonstrated only in non-stratified models. We perform numerical
simulations of stratified disks with the PLUTO code. We compute the ionization
fraction dynamically, and account for all three non-ideal MHD effects: ohmic
and ambipolar diffusions, and the Hall drift. Simplified heating and cooling
due to non-thermal radiation is also taken into account in the disk atmosphere.
We find that disks can be accreting or not, depending on the configuration of
the large-scale magnetic field. Magnetothermal winds, driven both by magnetic
acceleration and heating of the atmosphere, are obtained in the accreting case.
In some cases, these winds are asymmetric, ejecting predominantly on one side
of the disk. The wind mass loss rate depends primarily on the average ratio of
magnetic to thermal pressure in the disk midplane. The non-accreting case is
characterized by a meridional circulation, with accretion layers at the disk
surface and decretion in the midplane. Finally, we observe self-organization,
resulting in axisymmetric rings of density and associated pressure "bumps". The
underlying mechanism and its impact on observable structures are discussed. | astro-ph_EP |
Breaking the Ice: Planetesimal Formation at the Snowline: Recently Saito & Sirono (2011) proposed that large ice aggregates which drift
in- wards in protoplanetary disks break up during sublimation, ejecting
embedded silicate particles. This would lead to a concentration of small solid
particles close to the snow- line. In view of this model we carried out
laboratory experiments where we observed freely levitating ice aggregates
sublimating. We find that frequent break up is indeed very common. Scaled to a
10 cm aggregate about 2x10^4 small silicate aggregates might result. This
supports the idea that sublimation of drifting ice aggregates might locally
increase the density of small dust (silicate) particles which might more easily
be swept up by larger dust aggregates or trigger gravitational instability.
Either way this might locally boost the formation of planetesimals at the
snowline. | astro-ph_EP |
FU Orionis outbursts, preferential recondensation of water ice, and the
formation of giant planets: Ices, including water ice, prefer to recondense onto pre-existing nuclei
rather than spontaneously forming grains from a cloud of vapor. Interestingly,
different potential recondensation nuclei have very different propensities to
actually nucleate water ice at the temperatures associated with freeze-out in
protoplanetary discs. Therefore, if a region in a disc is warmed and then
recooled, water vapor should not be expected to refreeze evenly onto all
available grains. Instead it will preferentially recondense onto the most
favorable grains. When the recooling is slow enough, only the most favorable
grains will nucleate ice, allowing them to recondense thick ice mantles. We
quantify the conditions for preferential recondensation to rapidly create
pebble-sized grains in protoplanetary discs and show that FU Orionis type
outbursts have the appropriate cooling rates to drive pebble creation in a band
about 5 astronomical units wide outside of the quiescent frost line from
approximately Jupiter's orbit to Saturn's (about 4 to 10 au). Those pebbles
could be of the appropriate size to proceed to planetesimal formation via the
Streaming Instability, or to contribute to the growth of planetesimals through
pebble accretion. We suggest that this phenomenon contributed to the formation
of the gas giants in our own Solar System. | astro-ph_EP |
Revisiting parameters for the WASP-1 planetary system: We present thirteen new transit light curves for the WASP-1 b exoplanet.
Observations were acquired with 0.5 - 1.2-m telescopes between 2007 and 2013.
Our homogeneous analysis, which also includes the literature data, results in
determining precise system parameters. New values are in agreement with those
reported in previous studies. Transit times follow a linear ephemeris with no
sign of any transit time variations. This finding is in line with the paradigm
that Jupiter-like planets on tight orbits are devoid of close planetary
companions. | astro-ph_EP |
Mapping out the parameter space for photoevaporation and core-powered
mass-loss: Understanding atmospheric escape in close-in exoplanets is critical to
interpreting their evolution. We map out the parameter space over which
photoevaporation and core-powered mass loss dominate atmospheric escape.
Generally, the transition between the two regimes is determined by the location
of the Bondi radius (i.e. the sonic point of core-powered outflow) relative to
the penetration depth of XUV photons. Photoevaporation dominates the loss when
the XUV penetration depth lies inside the Bondi radius ($R_{XUV}<R_B$) and
core-powered mass-loss when XUV radiation is absorbed higher up in the flow
($R_B<R_{XUV}$). The transition between the two regimes occurs at a roughly
constant ratio of the planet's radius to its Bondi radius, with the exact value
depending logarithmically on planetary and stellar properties. In general,
core-powered mass-loss dominates for lower-gravity planets with higher
equilibrium temperatures, and photoevaporation dominates for higher-gravity
planets with lower equilibrium temperatures. However, planets can transition
between these two mass-loss regimes during their evolution, and core-powered
mass loss can ``enhance'' photo-evaporation over a significant region of
parameter space. Interestingly, a planet that is ultimately stripped by
core-powered mass-loss has likely only ever experienced core-powered mass-loss.
In contrast a planet that is ultimately stripped by photoevaporation could have
experienced an early phase of core-powered mass-loss. Applying our results to
the observed super-Earth population suggests that it contains significant
fractions of planets where each mechanism controlled the final removal of the
H/He envelope, although photoevaporation appears to be responsible for the
final carving of the exoplanet radius-valley. | astro-ph_EP |
Comparison of space weathering spectral changes induced by solar wind
and micrometeoroid impacts using ion- and femtosecond-laser-irradiated
olivine and pyroxene: Space weathering is a process that changes the surface of airless planetary
bodies. Prime space weathering agents are solar wind irradiation and
micrometeoroid bombardment. These processes alter planetary reflectance spectra
and often modify their compositional diagnostic features. In this work we
focused on simulating and comparing the spectral changes caused by solar wind
irradiation and by micrometeoroid bombardment to gain a better understanding of
these individual space weathering processes. We used olivine and pyroxene
pellets as proxies for planetary materials. To simulate solar wind irradiation
we used hydrogen, helium, and argon ions with energies from 5 to 40 keV and
fluences of up to $10^{18}$ particles/cm$^2$. To simulate micrometeoroid
bombardment we used individual femtosecond laser pulses. We analysed the
corresponding evolution of different spectral parameters, which we determined
by applying the Modified Gaussian Model, and we also conducted principal
component analysis. The original mineralogy of the surface influences the
spectral evolution more than the weathering agent, as seen from the diverse
evolution of the spectral slope of olivine and pyroxene upon irradiation. The
spectral slope changes seen in olivine are consistent with observations of
A-type asteroids, while the moderate to no slope changes observed in pyroxene
are consistent with asteroid (4) Vesta. We also observed some differences in
the spectral effects induced by the two weathering agents. Ions simulating
solar wind have a smaller influence on longer wavelengths of the spectra than
laser irradiation simulating micrometeoroid impacts. This is most likely due to
the different penetration depths of ions and laser pulses. Our results suggest
that in some instances it might be possible to distinguish between the
contributions of the two agents on a weathered surface. | astro-ph_EP |
An Orbitrap-based laser desorption/ablation mass spectrometer designed
for spaceflight: RATIONALE: The investigation of cryogenic planetary environments as potential
harbors for extant life and/or contemporary sites of organic synthesis
represents an emerging focal point in planetary exploration. Next generation
instruments need to be capable of unambiguously determining elemental and/or
molecular stoichiometry via highly accurate mass measurements and the
separation of isobaric interferences. METHODS: An OrbitrapTM analyzer adapted
for spaceflight (referred to as the CosmOrbitrap), coupled with a commercial
pulsed UV laser source (266 nm), is shown to successfully characterize a
variety of planetary analog samples via ultrahigh resolution laser
desorption/ablation mass spectrometry. The materials analyzed in this study
include: jarosite (a hydrous sulfate detected on Mars); magnesium sulfate (a
potential component of the subsurface ocean on Europa); uracil (a nucleobase of
RNA); and a variety of amino acids. RESULTS: The instrument configuration
tested here enables: measurement of major elements and organic molecules with
ultrahigh mass resolution (m/{\Delta}m higher than 120,000, FWHM);
quantification of isotopic abundances with 1.0% (2{\sigma}) precision; and,
identification of highly accurate masses within 3.2 ppm of absolute values. The
analysis of a residue of a dilute solution of amino acids demonstrates the
capacity to detect twelve amino acids in positive ion mode at concentrations as
low as 1 pmol/mm2 while maintaining mass resolution and accuracy requirements.
CONCLUSIONS: The CosmOrbitrap mass analyzer is highly sensitive and delivers
mass resolution/accuracy unmatched by any instrument sent into orbit or
launched into deep space. This prototype instrument, which maps to a
spaceflight implementation, represents a missionenabling technology capable of
advancing planetary exploration for decades to come. | astro-ph_EP |
A Sawtooth-like Timeline for the First Billion Year of Lunar Bombardment: We revisit the early evolution of the Moon's bombardment. Our work combines
modeling (based on plausible projectile sources and their dynamical decay
rates) with constraints from the lunar crater record, radiometric ages of the
youngest lunar basins, and the abundance of highly siderophile elements in the
lunar crust and mantle. We deduce that the evolution of the impact flux did not
decline exponentially over the first billion years of lunar history, but also
there was no prominent and "narrow" impact spike some 3.9 Gy ago, unlike that
typically envisioned in the lunar cataclysm scenario. Instead, we show the
timeline of the lunar bombardment has a sawtooth-like profile, with an uptick
in the impact flux near 4.1 Gy ago. The impact flux at the beginning of this
weaker cataclysm was 5-10 times higher than the immediately preceding period.
The Nectaris basin should have been one of the first basins formed at the
sawtooth. We predict the bombardment rate since about 4.1Gy ago declined slowly
and adhered relatively close to classic crater chronology models (Neukum and
Ivanov (1994)). Overall we expect that the sawtooth event accounted for about
1/4 of the total bombardment suffered by the Moon since its formation.
Consequently, considering that about 12-14 basins formed during the sawtooth
event, we expect that the net number of basins formed on the Moon was about
45-50. From our expected bombardment timeline, we derived a new and improved
lunar chronology suitable for use on Pre-Nectarian surface units. According to
this chronology, a significant portion of the oldest lunar cratered terrains
has an age of 4.38-4.42 Gyr. Moreover, the largest lunar basin, South Pole
Aitken, is older than 4.3Gy, and therefore was not produced during the lunar
cataclysm. | astro-ph_EP |
Revisiting $ε$ Eridani with NEID: Identifying New
Activity-Sensitive Lines in a Young K Dwarf Star: Recent improvements in the sensitivity and precision of the radial velocity
(RV) method for exoplanets have brought it close, but not quite to, the
threshold ($\sim$10 cm/s) required to detect Earth-mass and other potentially
habitable planets around Sun-like stars. Stellar activity-driven noise in RV
measurements remains a significant hurdle to achieving this goal. While various
efforts have been made to disentangle this noise from real planetary signals, a
greater understanding of the relationship between spectra and stellar activity
is crucial to informing stellar activity mitigation. We use a partially
automated method to analyze spectral lines in a set of observations of the
young, active star $\epsilon$ Eridani from the high-precision spectrograph
NEID, correlate their features (depth, full width at half maximum, and
integrated flux) with known activity indicators, and filter and curate for
well-behaved lines whose shape changes are sensitive to certain types of
stellar activity. We then present a list of 9 lines correlated with the S-index
in all three line features, including 4 newly-identified activity-sensitive
lines; as well as additional lines correlated with S-index in at least one
feature, and discuss the possible implications of the behavior observed in
these lines. Our line lists represent a step forward in the empirical
understanding of the complex relationships between stellar activity and
spectra, and illustrate the importance of studying the time evolution of line
morphologies with stabilized spectrographs, in the overall effort to mitigate
activity in the search for small, potentially Earth-like exoplanets. | astro-ph_EP |
Collateral Effects on Solar Nebula Oxygen Isotopes due to Injection of
26Al by a Nearby Supernova: Injection of material from a core-collapse supernova into the solar system's
already-formed disk is one proposed mechanism for producing the short-lived
radionuclides, such as 26Al and 41Ca, inferred from isotopic studies of
meteorites to have existed in the solar nebula. This hypothesis has recently
been challenged on the basis that the injection of enough supernova material to
match the meteoritic abundances of 26Al and 41Ca would produce large,
measureable, and unobserved collateral effects on oxygen isotopes. Here we
calculate again the shifts in oxygen isotopes due to injection of supernova
material in the solar nebula, using a variety of nucleosynthetic conditions of
our own progenitor explosions. Unlike previous studies of this type, we also
consider the effect of non-homogeneity in abundance distribution of the
nucleosynthesis products after the explosion. We calculate the shifts in oxygen
isotopes due to injection of sufficient supernova material to produce the
meteoritic abundances of 26Al and 41Ca, and analyze the predicted shifts in
detail for compatibility with meteoritic data. We find that the range in
possible isotopic shifts is considerable and sensitive to parameters such as
progenitor mass and anisotropy of the explosion; however, a small number of
compatible scenarios do exist. Because of the wide range of outcomes and the
sensitivity of isotopic yields to assumed conditions, it is difficult to
constrain the supernova that may have led to injection of 26Al in the solar
nebula. Conversely, we argue that the existence of viable counterexamples
demonstrates that it is premature to use oxygen isotopes to rule out injection
of 26Al and 41Ca into the solar nebula protoplanetary disk by a nearby
supernova. | astro-ph_EP |
The Chicxulub Impactor: Comet or Asteroid?: A recent paper by Siraj & Loeb (2021) entitled "Breakup of a long-period
comet as the origin of the dinosaur extinction" attempts to revive the
perennial debate about what type of body hit the Earth 66 million years ago,
triggering the end-Cretaceous extinction. Here we critique the paper and assess
the evidence it presents. To consider a comet more likely than an asteroid
requires extreme assumptions about how comets fragment, conflation of
carbonaceous chondrites with specific types of carbonaceous chondrites, and a
blind eye to the evidence of the iridium layer. | astro-ph_EP |
Growth of Jupiter: Formation in Disks of Gas and Solids and Evolution to
the Present Epoch: [Abridged] The formation of Jupiter is modeled via core-nucleated accretion,
and the planet's evolution is simulated up to the present epoch. The growth
from a small embryo until gas accretion overtakes solids' accretion was
presented by D'Angelo et al. (Icarus 2014, 241, 298). Those calculations
followed the formation for $4\times 10^{5}$ years, until the heavy-element and
H/He masses were $M_{Z}\approx 7.3$ and $M_{XY}\approx 0.15$ Earth's masses
($M_{\oplus}$), respectively, and $dM_{XY}/dt\approx dM_{Z}/dt$. The
calculation is continued through the phase when $M_{XY}=M_{Z}$, at which age,
about $2.4\times 10^{6}$ years, the planet mass is $M_{p}\approx
20\,M_{\oplus}$. About $9\times 10^{5}$ years later, $M_{p}$ is approximately
$60\,M_{\oplus}$ and $M_{Z}\approx 16\,M_{\oplus}$. Around this epoch, the
contraction of the envelope dictates gas accretion rates a few times
$10^{-3}\,M_{\oplus}$ per year, initiating the regime of disk-limited
accretion, when the planet's evolution is tied to disk's evolution. Growth is
continued by constructing simplified models of accretion disks that evolve
through viscous diffusion, winds, and accretion on the planet. Jupiter's
formation ends after $\approx 3.4$-$4.2$ Myr, when nebula gas disperses. The
young Jupiter is $4.5$-$5.5$ times as voluminous as it is presently and
thousands of times as luminous, $\sim 10^{-5}\,L_{\odot}$. The heavy-element
mass is $\approx 20\,M_{\oplus}$. The evolution proceeds through the cooling
and contraction phase, in isolation except for solar irradiation. After $4570$
Myr, radius and luminosity of the planet are within $10$% of current values.
During formation, and soon thereafter, the planet exhibits features, e.g.,
luminosity and effective temperature, that may probe aspects of the latter
stages of formation, if observable. These possibly distinctive features,
however, seem to disappear within a few tens of Myr. | astro-ph_EP |
Globally disruptive events show predictable timing patterns: Globally disruptive events include asteroid/comet impacts, large igneous
provinces and glaciations, all of which have been considered as contributors to
mass extinctions. Understanding the overall relationship between the timings of
the largest extinctions and their potential proximal causes remains one of
science's great unsolved mysteries. Cycles of about 60 million years in both
fossil diversity and environmental data suggest external drivers such as the
passage of the Solar System through the galactic plane. While cyclic phenomena
are recognised statistically, a lack of coherent mechanisms and a failure to
link key events has hampered wider acceptance of multi-million year periodicity
and its relevance to earth science and evolution. The generation of a robust
predictive model of timings, with a clear plausible primary mechanism, would
signal a paradigm shift. Here, we present a model of the timings of globally
disruptive events and a possible explanation of their ultimate cause. The
proposed model is a symmetrical pattern of 63 million-year sequences around a
central value, interpreted as the occurrence of events along, and parallel to,
the galactic midplane. The symmetry is consistent with multiple dark matter
disks, aligned parallel to the midplane. One implication of the precise pattern
of timings and the underlying physical model is the ability to predict future
events, such as a major extinction in one to two million years. | astro-ph_EP |
Information in the Reflected Light Spectra of Widely Separated Giant
Exoplanets: Giant exoplanets located >1 AU away from their parent stars have atmospheric
environments cold enough for water and/or ammonia clouds. We have developed a
new equilibrium cloud and reflected light spectrum model, ExoREL, for widely
separated giant exoplanets. The model includes the dissolution of ammonia in
liquid water cloud droplets, an effect studied for the first time for
exoplanets. While preserving the causal relationship between temperature and
cloud condensation, ExoREL is simple and fast to enable efficient exploration
of parameter space. Using the model, we find that the mixing ratio of methane
and the cloud top pressure of a giant exoplanet can be uniquely determined from
a single observation of its reflected light spectrum at wavelengths less than 1
micron if it has a cloud deck deeper than ~0.3 bars. This measurement is
enabled by the weak and strong bands of methane and requires a signal-to-noise
ratio of 20. The cloud pressure once derived, provides information about the
internal heat flux of the planet. Importantly, we find that for a low,
Uranus-like internal heat flux, the planet can have a deep liquid water cloud,
which will sequester ammonia and prevent the formation of the ammonia cloud
that would otherwise be the uppermost cloud layer. This newly identified
phenomenon causes a strong sensitivity of the cloud top pressure on the
internal heat flux. Reflected light spectroscopy from future direct-imaging
missions therefore not only measure the atmospheric abundances but also
characterize the thermal evolution of giant exoplanets. | astro-ph_EP |
Large grains can grow in circumstellar discs: We perform coagulation & fragmentation simulations to understand grain growth
in T Tauri & brown dwarf discs. We present a physically-motivated approach
using a probability distribution function for the collision velocities and
separating the deterministic & stochastic velocities. We find growth to larger
sizes compared to other models. Furthermore, if brown dwarf discs are
scaled-down versions of T Tauri discs (in terms of stellar & disc mass, and
disc radius), growth at the same location with respect to the outer edge occurs
to similar sizes in both discs. | astro-ph_EP |
Observation of metre-scale impactors by the Desert Fireball Network: The Earth is impacted by 35-40 metre-scale objects every year. These
meteoroids are the low mass end of impactors that can do damage on the ground.
Despite this they are very poorly surveyed and characterised, too infrequent
for ground based fireball bservation efforts, and too small to be efficiently
detected by NEO telescopic surveys whilst still in interplanetary space. We
want to evaluate the suitability of different instruments for characterising
metre-scale impactors and where they come from. We use data collected over the
first 3 years of operation of the continent-scale Desert Fireball Network, and
compare results with other published results as well as orbital sensors. We
find that although the orbital sensors have the advantage of using the entire
planet as collecting area, there are several serious problems with the accuracy
of the data, notably the reported velocity vector, which is key to getting an
accurate pre-impact orbit and calculating meteorite fall positions. We also
outline dynamic range issues that fireball networks face when observing large
meteoroid entries. | astro-ph_EP |
Rotation acceleration of asteroids (10115) 1992 SK, (1685) Toro, and
(1620) Geographos due to the YORP effect: The rotation state of small asteroids is affected by the
Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect, which is a net torque
caused by solar radiation directly reflected and thermally reemitted from the
surface. Due to this effect, the rotation period slowly changes, which can be
most easily measured in light curves because the shift in the rotation phase
accumulates over time quadratically. We collected archived light curves and
carried out new photometric observations for asteroids (10115) 1992 SK, (1620)
Geographos, and (1685) Toro. We applied the method of light curve inversion to
fit observations with a convex shape model. The YORP effect was modeled as a
linear change of the rotation frequency $\upsilon \equiv \mathrm{d}\omega /
\mathrm{d}t$ and optimized together with other spin and shape parameters. We
detected the acceleration $\upsilon = (8.3 \pm 0.6) \times
10^{-8}\,\mathrm{rad}\,\mathrm{d}^{-2}$ of the rotation for asteroid (10115)
1992 SK. This observed value agrees well with the theoretical value of
YORP-induced spin-up computed for our shape and spin model. For (1685) Toro, we
obtained $\upsilon = (3.3 \pm 0.3) \times
10^{-9}\,\mathrm{rad}\,\mathrm{d}^{-2}$, which confirms an earlier tentative
YORP detection. For (1620) Geographos, we confirmed the previously detected
YORP acceleration and derived an updated value of $\upsilon$ with a smaller
uncertainty. We also included the effect of solar precession into our inversion
algorithm, and we show that there are hints of this effect in Geographos' data.
The detected change of the spin rate of (10115) 1992 SK has increased the total
number of asteroids with YORP detection to ten. In all ten cases, the
$\mathrm{d}\omega / \mathrm{d}t$ value is positive, so the rotation of these
asteroids is accelerated. It is unlikely to be just a statistical fluke, but it
is probably a real feature that needs to be explained. | astro-ph_EP |
Setting the stage for the search for life with the Habitable Worlds
Observatory: Properties of 164 promising planet survey targets: The Decadal Survey on Astronomy and Astrophysics 2020 (Astro2020) has
recommended that NASA realize a large IR/O/UV space telescope optimized for
high-contrast imaging and spectroscopy of ~25 exo-Earths and transformative
general astrophysics. The NASA Exoplanet Exploration Program (ExEP) has
subsequently released a list of 164 nearby (d<25 pc) targets deemed the most
accessible to survey for potentially habitable exoplanets with the Habitable
Worlds Observatory (HWO). We present a catalog of system properties for the 164
ExEP targets, including 1744 abundance measurements for 14 elements from the
Hypatia Catalog and 924 photometry measurements spanning from 151.6 nm to 22
{\mu}m in the GALEX, Str\"omgren, Tycho, Gaia, 2MASS, and WISE bandpasses. We
independently derive stellar properties for these systems by modeling their
spectral energy distributions with Bayesian model averaging. Additionally, by
consulting the literature, we identify TESS flare rates for 46 stars, optical
variability for 78 stars, and X-ray emission for 46 stars in our sample. We
discuss our catalog in the context of planet habitability and draw attention to
key gaps in our knowledge where precursor science can help to inform HWO
mission design trade studies in the near future. Notably, only 33 of the 164
stars in our sample have reliable space-based UV measurements, and only 40 have
a mid-IR measurement. We also find that phosphorus, a bio-essential element,
has only been measured in 11 of these stars, motivating future abundance
surveys. Our catalog is publicly available and we advocate for its use in
future studies of promising HWO targets. | astro-ph_EP |
First detection of gas-phase methanol in a protoplanetary disk: The first detection of gas-phase methanol in a protoplanetary disk (TW Hya)
is presented. In addition to being one of the largest molecules detected in
disks to date, methanol is also the first disk organic molecule with an
unambiguous ice chemistry origin. The stacked methanol emission, as observed
with ALMA, is spectrally resolved and detected across six velocity channels
($>3 \sigma$), reaching a peak signal-to-noise of $5.5\sigma$, with the
kinematic pattern expected for TW~Hya. Using an appropriate disk model, a
fractional abundance of $3\times 10^{-12} - 4 \times 10^{-11}$ (with respect to
H$_2$) reproduces the stacked line profile and channel maps, with the favoured
abundance dependent upon the assumed vertical location (midplane versus
molecular layer). The peak emission is offset from the source position
suggesting that the methanol emission has a ring-like morphology: the analysis
here suggests it peaks at $\approx 30$~AU reaching a column density $\approx
3-6\times10^{12}$~cm$^{-2}$. In the case of TW Hya, the larger (up to mm-sized)
grains, residing in the inner 50~AU, may thus host the bulk of the disk ice
reservoir. The successful detection of cold gas-phase methanol in a
protoplanetary disk implies that the products of ice chemistry can be explored
in disks, opening a window to studying complex organic chemistry during
planetary system formation. | astro-ph_EP |
Direct Imaging of Exoplanets at the Era of the Extremely Large
Telescopes: Within ten years, the era of large-scale systematics surveys will decay
thanks to a complete census of exoplanetary systems within 200 pc from the Sun.
With the first Lights foreseen between 2024 and 2028, the new generation of
extremely large telescopes and planet imagers will arrive at a propitious time
to exploit this manna of discoveries to characterize the formation, the
evolution, and the physics of giant and telluric planets with the ultimate goal
to search and discover bio-signatures. In that perspective, I will briefly
summarize the main characteristics of the direct imaging instruments of the
ELTs dedicated to the study of exoplanets, and I will review the key science
cases (from the initial conditions of planetary formation, the architecture of
planetary systems and the physics and atmospheres of giant and telluric
planets) that they will address given their predicted performances. | astro-ph_EP |
Sulfuric acid as a cryofluid and oxygen isotope reservoir of
planetesimals: The Sun exhibits a depletion in $^{17,18}$O relative to $^{16}$O by 6 %
compared to the Earth and Moon$^{1}$. The origin of such a non-mass-dependent
isotope fractionation has been extensively debated since the
three-isotope-analysis$^{2}$ became available in 1970's. Self-shielding$^{3,4}$
of CO molecules against UV photons in the solar system's parent molecular cloud
has been suggested as a source of the non-mass-dependent effect, in which a
$^{17,18}$O-enriched oxygen was trapped by ice and selectively incorporated as
water into planet-forming materials$^{5}$. The truth is that the Earth-Moon and
other planetary objects deviate positively from the Sun by ~6 % in their
isotopic compositions. A stunning exception is the magnetite/sulfide
symplectite found in Acfer 094 meteorite, which shows 24 % enrichment in
$^{17,18}$O relative to the Sun$^{6}$. Water does not explain the enrichment
this high. Here we show that the SO and SO$_2$ molecules in the molecular
cloud, ~106 % enriched in $^{17,18}$O relative to the Sun, evolved through the
protoplanetary disk and planetesimal stages to become a sulfuric acid, 24 %
enriched in $^{17,18}$O. The sulfuric acid provided a cryofluid environment in
the planetesimal and by itself reacted with ferric iron to form an amorphous
ferric-hydroxysulfate-hydrate, which eventually decomposed into the symplectite
by shock. We indicate that the Acfer-094 symplectite and its progenitor,
sulfuric acid, is strongly coupled with the material evolution in the solar
system since the days of our molecular cloud. | astro-ph_EP |
The Migration of Gap-Opening Planets is not Locked to Viscous Disk
Evolution: Most standard descriptions of Type II migration state that massive,
gap-opening planets must migrate at the viscous drift rate. This is based on
the idea that the disk is separated into an inner and outer region and gas is
considered unable to cross the gap. In fact, gas easily crosses the gap on
horseshoe orbits, nullifying this necessary premise which would set the
migration rate. In this work, it is demonstrated using highly accurate
numerical calculations that the actual migration rate is dependent on disk and
planet parameters, and can be significantly larger or smaller than the viscous
drift rate. In the limiting case of a disk much more massive than the
secondary, the migration rate saturates to a constant which is sensitive to
disk parameters and is not necessarily of order viscous rate. In the opposite
limit of a low-mass disk, the migration rate decreases linearly with disk mass.
Steady-state solutions in the low disk mass limit show no pile-up outside the
secondary's orbit, and no corresponding drainage of the inner disk. | astro-ph_EP |
Stellar wind-magnetosphere interaction at exoplanets: computations of
auroral radio powers: We present calculations of the auroral radio powers expected from exoplanets
with magnetospheres driven by an Earth-like magnetospheric interaction with the
solar wind. Specifically, we compute the twin cell-vortical ionospheric flows,
currents, and resulting radio powers resulting from a Dungey cycle process
driven by dayside and nightside magnetic reconnection, as a function of
planetary orbital distance and magnetic field strength. We include saturation
of the magnetospheric convection, as observed at the terrestrial magnetosphere,
and we present power law approximations for the convection potentials, radio
powers and spectral flux densities. We specifically consider a solar-age system
and a young (1 Gyr) system. We show that the radio power increases with
magnetic field strength for magnetospheres with saturated convection potential,
and broadly decreases with increasing orbital distance. We show that the
magnetospheric convection at hot Jupiters will be saturated, and thus unable to
dissipate the full available incident Poynting flux, such that the magnetic
Radiometric Bode's Law (RBL) presents a substantial overestimation of the radio
powers for hot Jupiters. Our radio powers for hot Jupiters are $\sim$5-1300 TW
for hot Jupiters with field strengths of 0.1-10 $B_J$ orbiting a Sun-like star,
while we find that competing effects yield essentially identical powers for hot
Jupiters orbiting a young Sun-like star. However, in particular for planets
with weaker magnetic fields our powers are higher at larger orbital distances
than given by the RBL, and there are many configurations of planet that are
expected to be detectable using SKA. | astro-ph_EP |
Line Ratios Reveal N2H+ Emission Originates Above the Midplane in TW
Hydrae: Line ratios for different transitions of the same molecule have long been
used as a probe of gas temperature. Here we use ALMA observations of the N2H+
J~=~1-0 and J~=~4-3 lines in the protoplanetary disk around TW Hya to derive
the temperature at which these lines emit. We find an averaged temperature of
39~K with a one sigma uncertainty of 2~K for the radial range 0.8-2'',
significantly warmer than the expected midplane temperature beyond 0.5'' in
this disk. We conclude that the N2H+ emission in TW Hya is not emitting from
near the midplane, but rather from higher in the disk, in a region likely
bounded by processes such as photodissociation or chemical reprocessing of CO
and N2 rather than freeze out. | astro-ph_EP |
Imaging diagnostics for transitional discs: Transitional discs are a special type of protoplanetary discs where planet
formation is thought to be taking place. These objects feature characteristic
inner cavities and/or gaps of a few tens of AUs in the sub-millimitre images of
the disc. This signature suggests a localised depletion of matter in the disc
that could be caused by planet formation processes. However, recent
observations have revealed differences in the structures imaged at different
wavelengths in some of these discs. In this paper, we aim to explain these
observational differences using self-consistent physical 2-D hydrodynamical and
dust evolution models of such objects, assuming their morphology is indeed
generated by the presence of a planet. We use these models to derive the
distribution of gas and dust in a theoretical planet-hosting disc, for various
planet masses and orbital separations. We then simulate observations of the
emitted and scattered light from these models with VLT/SPHERE ZIMPOL,
Subaru/HiCIAO, VLT/VISIR and ALMA. We do this by first computing the full
resolution images of the models at different wavelengths, and then simulating
the observations accounting for the characteristics of each particular
instrument. The presence of the planet generates pressure bumps in the gas
distribution of the disc whose characteristics strongly depend on the planet
mass and position. These bumps cause large grains to accumulate while small
grains are allowed into inner regions. This spatial differentiation of the
grain sizes explains the differences in the observations since different
wavelengths and observing techniques trace different parts of the dust size
distribution. Based on this effect, we conclude that the combination of
visible/near-infrared polarimetric and sub-mm images is the best strategy to
constrain the properties of the unseen planet responsible for the disc
structure. | astro-ph_EP |
A new pattern in Saturn's D ring created in late 2011: Images obtained by the Cassini spacecraft between 2012 and 2015 reveal a
periodic brightness variation in a region of Saturn's D ring that previously
appeared to be rather featureless. Furthermore, the intensity and radial
wavenumber of this pattern have decreased steadily with time since it was first
observed. Based on analogies with similar structures elsewhere in the D ring,
we propose that this structure was created by some event that disturbed the
orbital motions of the ring particles, giving them finite orbital
eccentricities and initially aligned pericenters. Differential orbital
precession then transformed this structure into a spiral pattern in the ring's
optical depth that became increasingly tightly wound over time. The observed
trends in the pattern's radial wavenumber are roughly consistent with this
basic model, and also indicate that the ring-disturbing event occurred in early
December 2011. Similar events in 1979 may have generated the periodic patterns
seen in this same region by the Voyager spacecraft. The 2011 event could have
been caused by debris striking the rings, or by a disturbance in the planet's
electromagnetic environment. The rapid reduction in the intensity of the
brightness variations over the course of just a few years indicates that some
process is either damping orbital eccentricities in this region or causing the
orbital pericenters of particles with the same semi-major axis to become
misaligned. | astro-ph_EP |
An optimised survey strategy for the ERIS/NIX imager: searching for
young giant exoplanets and very low mass brown dwarfs using the K-peak custom
photometric filter: We present optimal survey strategies for the upcoming NIX imager, part of the
ERIS instrument to be installed on the Very Large Telescope (VLT). We will use
a custom 2.2 micron K-peak filter to optimise the efficiency of a future
large-scale direct imaging survey, aiming to detect brown dwarfs and giant
planets around nearby stars. We use the results of previous large scale imaging
surveys (primarily SPHERE SHINE and Gemini GPIES) to inform our choice of
targets, as well as improved planet population distributions. We present four
possible approaches to optimise survey target lists for the highest yield of
detections: i) targeting objects with anomalous proper motion trends, ii) a
follow-up survey of dense fields from SPHERE SHINE and Gemini GPIES iii)
surveying nearby star-forming regions and iv) targeting newly discovered
members of nearby young moving groups. We also compare the predicted
performance of NIX to other state-of-the-art direct imaging instruments. | astro-ph_EP |
Eleven Multi-planet Systems from K2 Campaigns 1 & 2 and the Masses of
Two Hot Super-Earths: We present a catalog of 11 multi-planet systems from Campaigns 1 and 2 of the
K2 mission. We report the sizes and orbits of 26 planets split between seven
2-planet systems and four 3-planet systems. These planets stem from a
systematic search of the K2 photometry for all dwarf stars observed by K2 in
these fields. We precisely characterized the host stars with adaptive optics
imaging and analysis of high-resolution optical spectra from Keck/HIRES and
medium-resolution spectra from IRTF/SpeX. We confirm two planet candidates by
mass detection and validate the remaining 24 candidates to $>99\%$ confidence.
Thirteen planets were previously validated or confirmed by other studies and 24
were previously identified as planet candidates. The planets are mostly smaller
than Neptune (21/26 planets) as in the Kepler mission and all have short
periods ($P < 50$ d) due to the duration of the K2 photometry. The host stars
are relatively bright (most have $Kp < 12.5$ mag) and are amenable to follow-up
characterization. For K2-38, we measured precise radial velocities using
Keck/HIRES and provide initial estimates of the planet masses. K2-38b is a
short-period super-Earth with a radius of $1.55 \pm 0.16~R_\oplus$, a mass of
$12.0 \pm 2.9~M_\oplus$, and a high density consistent with an iron-rich
composition. The outer planet K2-38c is a lower density sub-Neptune-size planet
with a radius of $2.42 \pm 0.29~R_\oplus$ and a mass of $9.9 \pm 4.6~M_\oplus$
that likely has a substantial envelope. This new planet sample demonstrates the
capability of K2 to discover numerous planetary systems around bright stars. | astro-ph_EP |
Photometry of 10 Million Stars from the First Two Years of TESS Full
Frame Images: The Transiting Exoplanet Survey Satellite (TESS) is the first high-precision
full-sky photometry survey in space. We present light curves from a magnitude
limited set of stars and other stationary luminous objects from the TESS Full
Frame Images, as reduced by the MIT Quick Look Pipeline (QLP). Our light curves
cover the full two-year TESS Primary Mission and include $\sim$ 14,770,000 and
$\sim$ 9,600,000 individual light curve segments in the Southern and Northern
ecliptic hemispheres, respectively. We describe the photometry and detrending
techniques we used to create the light curves, and compare the noise properties
with theoretical expectations. All of the QLP light curves are available at
MAST as a High Level Science Product via doi.org/10.17909/t9-r086-e880
(https://archive.stsci.edu/hlsp/qlp). This is the largest collection of TESS
photometry available to the public to date. | astro-ph_EP |
Outwards migration for planets in stellar irradiated 3D discs: For the very first time we present 3D simulations of planets embedded in
stellar irradiated discs. It is well known that thermal effects could reverse
the direction of planetary migration from inwards to outwards, potentially
saving planets in the inner, optically thick parts of the protoplanetary disc.
When considering stellar irradiation in addition to viscous friction as a
source of heating, the outer disc changes from a shadowed to a flared
structure. Using a suited analytical formula it has been shown that in the
flared part of the disc the migration is inwards; planets can migrate outwards
only in shadowed regions of the disc, { because the radial gradient of entropy
is stronger there}. In order to confirm this result numerically, we have
computed the total torque acting on planets held on fixed orbits embedded in
stellar irradiated 3D discs using the hydrodynamical code FARGOCA. We find
qualitatively good agreement between the total torque obtained with numerical
simulations and the one predicted by the analytical formula. For large masses
(>20 Earth masses) we find quantitative agreement, and we obtain outwards
migration regions for planets up to 60 Earth masses in the early stages of
accretional discs. We find nevertheless that the agreement with the analytic
formula is quite fortuitous because the formula underestimates the size of the
horseshoe region; this error is compensated by imperfect estimates of other
terms, most likely the cooling rate and saturation. | astro-ph_EP |
Mineral dust increases the habitability of terrestrial planets but
confounds biomarker detection: Identification of habitable planets beyond our solar system is a key goal of
current and future space missions. Yet habitability depends not only on the
stellar irradiance, but equally on constituent parts of the planetary
atmosphere. Here we show, for the first time, that radiatively active mineral
dust will have a significant impact on the habitability of Earth-like
exoplanets. On tidally-locked planets, dust cools the day-side and warms the
night-side, significantly widening the habitable zone. Independent of orbital
configuration, we suggest that airborne dust can postpone planetary water loss
at the inner edge of the habitable zone, through a feedback involving
decreasing ocean coverage and increased dust loading. The inclusion of dust
significantly obscures key biomarker gases (e.g. ozone, methane) in simulated
transmission spectra, implying an important influence on the interpretation of
observations. We demonstrate that future observational and theoretical studies
of terrestrial exoplanets must consider the effect of dust. | astro-ph_EP |
Stellar Winds and Dust Avalanches in the AU Mic Debris Disk: We explain the fast-moving, ripple-like features in the edge-on debris disk
orbiting the young M dwarf AU Mic. The bright features are clouds of sub-micron
dust repelled by the host star's wind. The clouds are produced by avalanches:
radial outflows of dust that gain exponentially more mass as they shatter
background disk particles in collisional chain reactions. The avalanches are
triggered from a region a few AU across -- the "avalanche zone" -- located on
AU Mic's primary "birth" ring, at a true distance of $\sim$35 AU from the star
but at a projected distance more than a factor of 10 smaller: the avalanche
zone sits directly along the line of sight to the star, on the side of the ring
nearest Earth, launching clouds that disk rotation sends wholly to the
southeast, as observed. The avalanche zone marks where the primary ring
intersects a secondary ring of debris left by the catastrophic disruption of a
progenitor up to Varuna in size, less than tens of thousands of years ago. Only
where the rings intersect are particle collisions sufficiently violent to spawn
the sub-micron dust needed to seed the avalanches. We show that this picture
works quantitatively, reproducing the masses, sizes, and velocities of the
observed escaping clouds. The Lorentz force exerted by the wind's magnetic
field, whose polarity reverses periodically according to the stellar magnetic
cycle, promises to explain the observed vertical undulations. The timescale
between avalanches, about 10 yr, might be set by time variability of the wind
mass-loss rate or, more speculatively, by some self-regulating limit cycle. | astro-ph_EP |
Thermal and orbital evolution of low-mass exoplanets: Thermal, orbital, and rotational dynamics of tidally loaded exoplanets are
interconnected by intricate feedback. The rheological structure of the planet
determines its susceptibility to tidal deformation and, as a consequence,
participates in shaping its orbit. The orbital parameters and the spin state,
conversely, control the rate of tidal dissipation and may lead to substantial
changes of the interior. We investigate the coupled thermal-orbital evolution
of differentiated rocky exoplanets governed by the Andrade viscoelastic
rheology. The coupled evolution is treated by a semi-analytical model, 1d
parametrized heat transfer and self-consistently calculated tidal dissipation.
First, we conduct several parametric studies, exploring the effect of the
rheological properties, the planet's size, and the orbital eccentricity on the
tidal locking and dissipation. These tests show that the role of tidal locking
into high spin-orbit resonances is most prominent on low eccentric orbits,
where it results in substantially higher tidal heating than the synchronous
rotation. Second, we calculate the long-term evolution of three currently known
low-mass exoplanets with nonzero orbital eccentricity and absent or yet unknown
eccentricity forcing (namely GJ 625 b, GJ 411 b, and Proxima Centauri b). The
tidal model incorporates the formation of a stable magma ocean and a
consistently evolving spin rate. We find that the thermal state is strongly
affected by the evolution of eccentricity and spin state and proceeds as a
sequence of thermal equilibria. Final despinning into synchronous rotation
slows down the orbital evolution and helps to maintain long-term stable orbital
eccentricity. | astro-ph_EP |
The Effect of Multiple Heat Sources on Exomoon Habitable Zones: With dozens of Jovian and super-Jovian exoplanets known to orbit their host
stars in or near the stellar habitable zones, it has recently been suggested
that moons the size of Mars could offer abundant surface habitats beyond the
solar system. Several searches for such exomoons are now underway, and the
exquisite astronomical data quality of upcoming space missions and ground-based
extremely large telescopes could make the detection and characterization of
exomoons possible in the near future. Here we explore the effects of tidal
heating on the potential of Mars- to Earth-sized satellites to host liquid
surface water, and we compare the tidal heating rates predicted by tidal
equilibrium model and a viscoelastic model. In addition to tidal heating, we
consider stellar radiation, planetary illumination and thermal heat from the
planet. However, the effects of a possible moon atmosphere are neglected. We
map the circumplanetary habitable zone for different stellar distances in
specific star-planet-satellite configurations, and determine those regions
where tidal heating dominates over stellar radiation. We find that the
`thermostat effect' of the viscoelastic model is significant not just at large
distances from the star, but also in the stellar habitable zone, where stellar
radiation is prevalent. We also find that tidal heating of Mars-sized moons
with eccentricities between 0.001 and 0.01 is the dominant energy source beyond
3--5 AU from a Sun-like star and beyond 0.4--0.6 AU from an M3 dwarf star. The
latter would be easier to detect (if they exist), but their orbital stability
might be under jeopardy due to the gravitational perturbations from the star. | astro-ph_EP |
Stellar orbit evolution in close circumstellar disc encounters: The formation and early evolution of circumstellar discs often occurs within
dense, newborn stellar clusters. For the first time, we apply the moving-mesh
code AREPO, to circumstellar discs in 3-D, focusing on disc-disc interactions
that result from stellar fly-bys. Although a small fraction of stars are
expected to undergo close approaches, the outcomes of the most violent
encounters might leave an imprint on the discs and host stars that will
influence both their orbits and their ability to form planets. We first
construct well-behaved 3-D models of self-gravitating discs, and then create a
suite of numerical experiments of parabolic encounters, exploring the effects
of pericenter separation r_p, disc orientation and disc-star mass ratio
(M_d/M_*) on the orbital evolution of the host stars. Close encounters (2r_p<~
disc radius) can truncate discs on very short time scales. If discs are
massive, close encounters facilitate enough orbital angular momentum extraction
to induce stellar capture. We find that for realistic primordial disc masses
M_d<~0.1M_*, non-colliding encounters induce minor orbital changes, which is
consistent with analytic calculations of encounters in the linear regime. The
same disc masses produce entirely different results for grazing/colliding
encounters. In the latter case, rapidly cooling discs lose orbital energy by
radiating away the energy excess of the shock-heated gas, thus causing capture
of the host stars into a bound orbit. In rare cases, a tight binary with a
circumbinary disc forms as a result of this encounter. | astro-ph_EP |
True Polar Wander on Dynamic Planets: Approximative Methods vs. Full
Solution: Almost three decades ago, the problem of long term polar wander on a dynamic
planet was formulated and simplified within the framework of normal mode
theory. The underlying simplifications have been debated ever since, recently
in a series of papers by Hu et al. 2017a, 2017b, and 2019, who clarify the role
of neglecting short-term relaxation modes of the body. However, the authors
still do not solve the governing equations in full, because they make
approximations to the Liouville equation (LE). In this paper I use a time
domain approach and for previously studied test loads I solve both the
relaxation of the body and the LE in full. I also compute the energy balance of
true polar wander (TPW) in order to analyze the existing LE approximations. For
fast rotating bodies such as the Earth, I show that the rotation axis is always
aligned with the maximum principal axis of inertia (w||MMOI) once free
oscillations are damped. The w||MMOI assumption is also re-derived
theoretically. Contrary to previous beliefs, I demonstrate that it is not
necessarily linked to the quasi-fluid simplification of the body's viscoelastic
response to loading and rotation, but that it is an expression of neglecting
the Coriolis and Euler forces in the equation of motion. For slowly rotating
bodies such as Venus, the full LE together with energy analysis indicate that
previous estimates of TPW in the normal direction need to be revisited. The
numerical code LIOUSHELL is made freely available. | astro-ph_EP |
The Rossiter-McLaughlin effect and exoplanet transits: A delicate
association at medium and low spectral resolution: The characterization of exoplanetary atmospheres via transit spectroscopy is
based on the comparison between the stellar spectrum filtered through the
atmosphere and the unadulterated spectrum from the occulted stellar region. The
disk-integrated spectrum is often used as a proxy for the occulted spectrum,
yet they differ along the transit chord depending on stellar type and
rotational velocity. This is refereed to as the Rossiter-McLaughlin (RM)
effect, which is known to bias transmission spectra at high spectral resolution
when calculated with the disk-integrated stellar spectrum. Recently, it was
shown that the first claimed atmospheric signal from an exoplanet cannot arise
from absorption in the core of the sodium doublet, because the features
observed at high resolution are well reproduced by the RM effect. However, it
remains unclear as to whether the detection made at medium spectral resolution
with the HST arises from the smoothed RM signature or from the wings of the
planetary absorption line. More generally, the impact of the RM effect at
medium and low spectral resolution remains poorly explored. To address this
question, we simulated realistic transmission spectra in a variety of systems
using the EVaporating Exoplanets code. We find that the RM effect should not
bias broadband atmospheric features, such as hazes or molecular absorption,
measured with the JWST/NIRSPEC (prism mode) at low resolution. However,
absorption signatures from metastable helium or sodium measured at medium
resolution with the JWST/NIRSPEC (G140H mode) or HST/STIS can be biased,
especially for planets on misaligned orbits across fast rotators. In contrast,
we show that the Na signature originally reported in HD209458b, an aligned
system, cannot be explained by the RM effect, supporting a planetary origin. | astro-ph_EP |
From exo-Earths to exo-Venuses -- Flux and Polarization Signatures of
Reflected Light: Terrestrial exoplanets in habitable zones are ubiquitous. It is, however,
unknown which have Earth-like or Venus-like climates. Distinguishing different
planet-types is crucial for determining whether a planet could be habitable. We
investigate the potential of polarimetry for distinguishing exo-Earths from
exo-Venuses. We present computed fluxes and polarisation of starlight that is
reflected by exoplanets with atmospheres in evolutionary states from current
Earth to current Venus, with cloud compositions ranging from pure water to 0.75
sulfuric acid solution, for wavelengths between 0.3 and 2.5 microns. The
polarisation of the reflected light shows larger variations with the planetary
phase angle than the total flux. Across the visible, the largest polarisation
is reached for an Earth-like atmosphere with water clouds, due to Rayleigh
scattering above the clouds and the rainbow near 40 deg phase angle. In the
near-infrared, the planet with a Venus-like CO2 atmosphere and thin water
clouds shows the most prominent polarisation features due to scattering by the
small cloud droplets. A planet around Alpha Centauri A would leave temporal
variations on the order of 10E-13 W/m3 in the reflected flux and 10E-11 in the
degree of polarisation along the planet's orbit for a spatially unresolved
star-planet system. Star-planet contrasts are on the order of 10E-10. Current
polarimeters cannot distinguish between the possible evolutionary phases of
spatially unresolved terrestrial exoplanets, as a sensitivity near 10E-10 is
required to discern the planet signal on the background of unpolarised
starlight. Telescopes capable of reaching planet-star contrasts lower than
10E-9 should be able to observe the variation of the planet's resolved degree
of polarisation as a function of its phase angle and thus to discern an
exo-Earth from an exo-Venus based on its clouds' unique polarisation
signatures. | astro-ph_EP |
HELIOS: An Open-source, GPU-accelerated Radiative Transfer Code For
Self-consistent Exoplanetary Atmospheres: We present the open-source radiative transfer code named HELIOS, which is
constructed for studying exoplanetary atmospheres. In its initial version, the
model atmospheres of HELIOS are one-dimensional and plane-parallel, and the
equation of radiative transfer is solved in the two-stream approximation with
non-isotropic scattering. A small set of the main infrared absorbers is
employed, computed with the opacity calculator HELIOS-K and combined using a
correlated-$k$ approximation. The molecular abundances originate from validated
analytical formulae for equilibrium chemistry. We compare HELIOS with the work
of Miller-Ricci & Fortney using a model of GJ 1214b, and perform several tests,
where we find: model atmospheres with single-temperature layers struggle to
converge to radiative equilibrium; $k$-distribution tables constructed with
$\gtrsim 0.01$ cm$^{-1}$ resolution in the opacity function ($ \lesssim 10^3$
points per wavenumber bin) may result in errors $\gtrsim 1$-10 % in the
synthetic spectra; and a diffusivity factor of 2 approximates well the exact
radiative transfer solution in the limit of pure absorption. We construct
"null-hypothesis" models (chemical equilibrium, radiative equilibrium and solar
element abundances) for 6 hot Jupiters. We find that the dayside emission
spectra of HD 189733b and WASP-43b are consistent with the null hypothesis,
while it consistently under-predicts the observed fluxes of WASP-8b, WASP-12b,
WASP-14b and WASP-33b. We demonstrate that our results are somewhat insensitive
to the choice of stellar models (blackbody, Kurucz or PHOENIX) and metallicity,
but are strongly affected by higher carbon-to-oxygen ratios. The code is
publicly available as part of the Exoclimes Simulation Platform (ESP;
exoclime.net). | astro-ph_EP |
Photodissociation and induced chemical asymmetries on ultra-hot gas
giants. A case study of HCN on WASP-76 b: Recent observations have resulted in the detection of chemical gradients on
ultra-hot gas giants. Notwithstanding their high temperature, chemical
reactions in ultra-hot atmospheres may occur in disequilibrium, due to vigorous
day-night circulation and intense UV radiation from their stellar hosts. The
goal of this work is to explore whether photochemistry is affecting the
composition of ultra-hot giant planets, and if it can introduce horizontal
chemical gradients. In particular, we focus on hydrogen cyanide (HCN) on
WASP-76 b, as it is a photochemically active molecule with a reported detection
on only one side of this planet. We use a pseudo-2D chemical kinetics code to
model the chemical composition of WASP-76 b along its equator. Our approach
improves on chemical equilibrium models by computing vertical mixing,
horizontal advection, and photochemistry. We find that production of HCN is
initiated through thermal and photochemical dissociation of CO and N$_2$ on the
day side of WASP-76 b. The resulting radicals are subsequently transported to
the night side via the equatorial jet stream, where they recombine into
different molecules. This process results in an HCN gradient with a maximal
abundance on the planet's morning limb. We verified that photochemical
dissociation is a necessary condition for this mechanism, as thermal
dissociation alone proves insufficient. Other species produced via night-side
disequilibrium chemistry are SO$_2$ and S$_2$. Our model acts as a proof of
concept for chemical gradients on ultra-hot exoplanets. We demonstrate that
even ultra-hot planets can exhibit disequilibrium chemistry and recommend that
future studies do not neglect photochemistry in their analyses of ultra-hot
planets. | astro-ph_EP |
The Beam Balance -- Measuring Binary Systems via Relativistic Beaming
Signals from Stars and their Companions: In this paper I show that the concept of relativistic beaming -- the process
by which light emitted by a fast moving sources is lensed towards the direction
of motion -- can be easily extended to model the signal from both the star and
any secondary companions. Most companions will be cooler and less massive than
their host star. Their lower mass leads to faster orbital velocities, and thus
a potentially larger beaming effect. The lower temperature will mean that most
of their light is emitted at longer wavelengths, where the relative photometric
dominance of the primary is reduced. Thus for some systems, the secondary
companion can be the main contributor to observed relativistic beaming signals
at long wavelengths. Furthermore, if the system is observed over a range of
wavelengths we can independently constrain the temperature of the companion,
and the mass and radius ratio of the binary. To conclude I discuss the current
and future observational prospects of this signal, using the properties of
known exoplanets to show that such a signal may be observable by upcoming
surveys. | astro-ph_EP |
The Role of Early Giant Planet Instability in the Terrestrial Planet
Formation: The terrestrial planets are believed to have formed by violent collisions of
tens of lunar- to Mars-size protoplanets at time t<200 Myr after the
protoplanetary gas disk dispersal (t_0). The solar system giant planets rapidly
formed during the protoplanetary disk stage and, after t_0, radially migrated
by interacting with outer disk planetesimals. An early (t<100 Myr) dynamical
instability is thought to have occurred with Jupiter having gravitational
encounters with a planetary-size body, jumping inward by ~0.2-0.5 au, and
landing on its current, mildly eccentric orbit. Here we investigate how the
giant planet instability affected formation of the terrestrial planets. We
study several instability cases that were previously shown to match many solar
system constraints. We find that resonances with the giant planets help to
remove solids available for accretion near ~1.5 au, thus stalling the growth of
Mars. It does not matter, however, whether the giant planets are placed on
their current orbits at t_0 or whether they realistically evolve in one of our
instability models; the results are practically the same. The tight orbital
spacing of Venus and Earth is difficult to reproduce in our simulations,
including cases where bodies grow from a narrow annulus at 0.7-1 au, because
protoplanets tend to radially spread during accretion. The best results are
obtained in the narrow-annulus model when protoplanets emerging from the
dispersing gas nebula are assumed to have (at least) the Mars mass. This
suggests efficient accretion of the terrestrial protoplanets during the first
~10 Myr of the solar system. | astro-ph_EP |
Oscillations in the Habitable Zone around Alpha Centauri B: The Alpha Centauri AB system is an attractive one for radial velocity
observations to detect potential exoplanets. The high metallicity of both Alpha
Centauri A and B suggest that they could have possessed circumstellar discs
capable of forming planets. As the closest star system to the Sun, with well
over a century of accurate astrometric measurements (and Alpha Centauri B
exhibiting low chromospheric activity) high precision surveys of Alpha Centauri
B's potential exoplanetary system are possible with relatively cheap
instrumentation. Authors studying habitability in this system typically adopt
habitable zones (HZs) based on global radiative balance models that neglect the
radiative perturbations of Alpha Centauri A.
We investigate the habitability of planets around Alpha Centauri B using 1D
latitudinal energy balance models (LEBMs), which fully incorporate the presence
of Alpha Centauri A as a means of astronomically forcing terrestrial planet
climates. We find that the extent of the HZ is relatively unchanged by the
presence of Alpha Centauri A, but there are variations in fractional
habitability for planets orbiting at the boundaries of the zone due to Alpha
Centauri A, even in the case of zero eccentricity. Temperature oscillations of
a few K can be observed at all planetary orbits, the strength of which varies
with the planet's ocean fraction and obliquity. | astro-ph_EP |
Identification of asteroids using the Virtual Observatory: the WFCAM
Transit Survey: The nature and physical properties of asteroids, in particular those orbiting
in the near-Earth space, are of scientific interest and practical importance.
Exoplanet surveys can be excellent resources to detect asteroids, both already
known and new objects. This is due their similar observing requirements: large
fields of view, long sequences, and short cadence. If the targeted fields are
not located far from the ecliptic, many asteroids will cross occasionally the
field of view. We present two complementary methodologies to identify asteroids
serendipitously observed in large-area astronomical surveys. One methodology
focuses on detecting already known asteroids using the Virtual Observatory tool
SkyBoT, which predicts their positions and motions in the sky at a specific
epoch. The other methodology applies the ssos pipeline, which is able to
identify known and new asteroids based on their apparent motion. The
application of these methods to the 6.4 deg 2 of the sky covered by the
Wide-Field CAMera Transit Survey in the J-band is described. We identified 15
661 positions of 1 821 different asteroids. Of them, 182 are potential new
discoveries. A publicly accessible online, Virtual Observatory compliant
catalogue was created. We obtained the shapes and periods for five of our
asteroids from their light-curves built with additional photometry taken from
external archives. We demonstrated that our methodologies are robust and
reliable approaches to find, at zero cost of observing time, asteroids observed
by chance in astronomical surveys. Our future goal is to apply them to other
surveys with adequate temporal coverage. | astro-ph_EP |
Mercury's geochronology revised by applying Model Production Functions
to Mariner 10 data: geological implications: Model Production Function chronology uses dynamic models of the Main Belt
Asteroids (MBAs) and Near Earth Objects (NEOs) to derive the impactor flux to a
target body. This is converted into the crater size-frequency-distribution for
a specific planetary surface, and calibrated using the radiometric ages of
different regions of the Moon's surface. This new approach has been applied to
the crater counts on Mariner 10 images of the highlands and of several large
impact basins on Mercury. MPF estimates for the plains show younger ages than
those of previous chronologies. Assuming a variable uppermost layering of the
Hermean crust, the age of the Caloris interior plains may be as young as 3.59
Ga, in agreement with MESSENGER results that imply that long-term volcanism
overcame contractional tectonics. The MPF chronology also suggests a variable
projectile flux through time, coherent with the MBAs for ancient periods and
then gradually comparable also to the NEOs. | astro-ph_EP |
The Carbon-Deficient Evolution of TRAPPIST-1c: Transiting planets orbiting M dwarfs provide the best opportunity to study
the atmospheres of rocky planets with current facilities. As JWST enters its
second year of science operations, an important initial endeavor is to
determine whether these rocky planets have atmospheres at all. M dwarf host
stars are thought to pose a major threat to planetary atmospheres due to their
high magnetic activity over several billion-year timescales, and might
completely strip atmospheres. Several Cycle 1 and 2 GO and GTO programs are
testing this hypothesis, observing a series of rocky planets to determine
whether they retained their atmospheres. A key case-study is TRAPPIST-1c, which
receives almost the same bolometric flux as Venus. We might, therefore, expect
TRAPPIST-1c to possess a thick, $\mathrm{CO}_2$-dominated atmosphere. Instead,
Zieba et al. (2023) show that TRAPPIST-1c has little to no CO$_2$ in its
atmosphere. To interpret these results, we run coupled time-dependent
simulations of planetary outgassing and atmospheric escape to model the
evolution of TRAPPIST-1c's atmosphere. We find that the stellar wind stripping
that is expected to occur on TRAPPIST-1c over its lifetime can only remove up
to $\sim 16$ bar of $\mathrm{CO}_2$, less than the modern $\mathrm{CO}_2$
inventory of either Earth or Venus. Therefore, we infer that TRAPPIST-1c either
formed volatile-poor, as compared to Earth and Venus, or lost a substantial
amount of $\mathrm{CO}_2$ during an early phase of hydrodynamic hydrogen
escape. Finally, we scale our results for the other TRAPPIST-1 planets, finding
that the more distant TRAPPIST-1 planets may readily retain atmospheres. | astro-ph_EP |
On the Outer Edges of Protoplanetary Dust Disks: The expectation that aerodynamic drag will force the solids in a gas-rich
protoplanetary disk to spiral in toward the host star on short timescales is
one of the fundamental problems in planet formation theory. The nominal
efficiency of this radial drift process is in conflict with observations,
suggesting that an empirical calibration of solid transport mechanisms in a
disk is highly desirable. However, the fact that both radial drift and grain
growth produce a similar particle size segregation in a disk (such that larger
particles are preferentially concentrated closer to the star) makes it
difficult to disentangle a clear signature of drift alone. We highlight a new
approach, by showing that radial drift leaves a distinctive "fingerprint" in
the dust surface density profile that is directly accessible to current
observational facilities. Using an analytical framework for dust evolution, we
demonstrate that the combined effects of drift and (viscous) gas drag naturally
produce a sharp outer edge in the dust distribution (or, equivalently, a sharp
decrease in the dust-to-gas mass ratio). This edge feature forms during the
earliest phase in the evolution of disk solids, before grain growth in the
outer disk has made much progress, and is preserved over longer timescales when
both growth and transport effects are more substantial. The key features of
these analytical models are reproduced in detailed numerical simulations, and
are qualitatively consistent with recent millimeter-wave observations that find
gas/dust size discrepancies and steep declines in dust continuum emission in
the outer regions of protoplanetary disks. | astro-ph_EP |
TESS Photometric Mapping of a Terrestrial Planet in the Habitable Zone:
Detection of Clouds, Oceans, and Continents: To date, a handful of exoplanets have been photometrically mapped using
phase-modulated reflection or emission from their surfaces, but the small
amplitudes of such signals have limited previous maps almost exclusively to
coarse dipolar features on hot giant planets. In this work, we uncover a signal
using recently released data from the Transiting Exoplanet Survey Satellite
(TESS), which we show corresponds to time-variable reflection from a
terrestrial planet with a rotation period of 0.9972696 days. Using a spherical
harmonic-based reflection model developed as an extension of the STARRY
package, we are able to reconstruct the surface features of this rocky world.
We recover a time-variable albedo map of the planet including persistent
regions which we interpret as oceans and cloud banks indicative of continental
features. We argue that this planet represents the most promising detection of
a habitable world to date, although the potential intelligence of any life on
it is yet to be determined. | astro-ph_EP |
Is the hot, dense sub-Neptune TOI-824b an exposed Neptune mantle?
Spitzer detection of the hot day side and reanalysis of the interior
composition: The Kepler and TESS missions revealed a remarkable abundance of sub-Neptune
exoplanets. Despite this abundance, our understanding of the nature and
compositional diversity of sub-Neptunes remains limited, to a large part
because atmospheric studies via transmission spectroscopy almost exclusively
aimed for low-density sub-Neptunes and even those were often affected by
high-altitude clouds. The recent TESS discovery of the hot, dense TOI-824b
($2.93\,R_\oplus$ and $18.47\,M_\oplus$) opens a new window into sub-Neptune
science by enabling the study of a dense sub-Neptune via secondary eclipses.
Here, we present the detection of TOI-824b's hot day side via Spitzer secondary
eclipse observations in the $3.6$ and $4.5\,\mathrm{\mu m}$ channels, combined
with a reanalysis of its interior composition. The measured eclipse depths
(142$^{+57}_{-52}$ and 245$^{+75}_{-77}$ ppm) and brightness temperatures
(1463$^{+183}_{-196}$ and 1484$^{+180}_{-202}$ K) indicate a poor heat
redistribution ($f>$ 0.49) and a low Bond albedo (A$_{B}<$ 0.26). We conclude
that TOI-824b could be an "exposed Neptune mantle": a planet with a
Neptune-like water-rich interior that never accreted a hydrogen envelope or
that subsequently lost it. The hot day-side temperature is then naturally
explained by a high-metallicity envelope re-emitting the bulk of the incoming
radiation from the day side. TOI-824b's density is also consistent with a
massive rocky core that accreted up to 1% of hydrogen, but the observed eclipse
depths favor our high-metallicity GCM simulation to a solar-metallicity GCM
simulation with a likelihood ratio of 7:1. The new insights into TOI-824b's
nature suggest that the sub-Neptune population may be more diverse than
previously thought, with some of the dense hot sub-Neptunes potentially not
hosting a hydrogen-rich envelope as generally assumed for sub-Neptunes. | astro-ph_EP |
Transformation of Trojans into Quasi-Satellites During Planetary
Migration and Their Subsequent Close-Encounters with the Host Planet: We use numerical integrations to investigate the dynamical evolution of
resonant Trojan and quasi-satellite companions during the late stages of
migration of the giant planets Jupiter, Saturn, Uranus, and Neptune. Our
migration simulations begin with Jupiter and Saturn on orbits already well
separated from their mutual 2:1 mean-motion resonance. Neptune and Uranus are
decoupled from each other and have orbital eccentricities damped to near their
current values. From this point we adopt a planet migration model in which the
migration speed decreases exponentially with a characteristic timescale tau
(the e-folding time). We perform a series of numerical simulations, each
involving the migrating giant planets plus test particle Trojans and
quasi-satellites. We find that the libration frequencies of Trojans are similar
to those of quasi-satellites. This similarity enables a dynamical exchange of
objects back and forth between the Trojan and quasi-satellite resonances during
planetary migration. Furthermore, under the influence of these secondary
resonances quasi-satellites can have their libration amplitudes enlarged until
they undergo a close-encounter with their host planet and escape from the
resonance. High-resolution simulations of this escape process reveal that ~80%
of Jovian quasi-satellites experience one or more close-encounters within
Jupiter's Hill radius (R_H) as they are forced out of the quasi-satellite
resonance. As many as ~20% come within R_H/4 and ~2.5% come within R_H/10.
Close-encounters of escaping quasi-satellites occur near or even below the
2-body escape velocity from the host planet. | astro-ph_EP |
Explosion of Comet 17P/Holmes as revealed by the Spitzer Space Telescope: An explosion on comet 17P/Holmes occurred on 2007 Oct 23, projecting
particulate debris of a wide range of sizes into the interplanetary medium. We
observed the comet using the Spitzer spectrograph on 2007 Nov 10 and 2008 Feb
27, and the photometer, on 2008 Mar 13. The fresh ejecta have detailed
mineralogical features from small crystalline silicate grains. The 2008 Feb 27
spectra, and the central core of the 2007 Nov 10 spectral map, reveal nearly
featureless spectra, due to much larger grains that were ejected from the
nucleus more slowly. We break the infrared image into three components (size,
speed) that also explain the temporal evolution of the mm-wave flux. Optical
images were obtained on multiple dates spanning 2007 Oct 27 to 2008 Mar 10 at
the Holloway Comet Observatory and 1.5-m telescope at Palomar Observatory. The
orientation of the leading edge of the ejecta shell and the ejecta blob,
relative to the nucleus, do not change as the orientation of the Sun changes;
instead, the configuration was imprinted by the orientation of the initial
explosion. The kinetic energy of the ejecta >1e21 erg is greater than the
gravitational binding energy of the nucleus. We model the explosion as being
due to crystallization and release of volatiles from interior amorphous ice
within a subsurface cavity; once the pressure in the cavity exceeded the
surface strength, the material above the cavity was propelled from the comet.
The size of the cavity and the tensile strength of the upper layer of the
nucleus are constrained by the observed properties of the ejecta; tensile
strengths on >10 m scale must be greater than 10 kPa. The appearance of the
2007 outburst is similar to that witnessed in 1892, but the 1892 explosion was
less energetic by a factor of about 20. | astro-ph_EP |
Collisions of small ice particles under microgravity conditions (II):
Does the chemical composition of the ice change the collisional properties?: Context: Understanding the collisional properties of ice is important for
understanding both the early stages of planet formation and the evolution of
planetary ring systems. Simple chemicals such as methanol and formic acid are
known to be present in cold protostellar regions alongside the dominant water
ice; they are also likely to be incorporated into planets which form in
protoplanetary disks, and planetary ring systems. However, the effect of the
chemical composition of the ice on its collisional properties has not yet been
studied. Aims: Collisions of 1.5 cm ice spheres composed of pure crystalline
water ice, water with 5% methanol, and water with 5% formic acid were
investigated to determine the effect of the ice composition on the collisional
outcomes. Methods: The collisions were conducted in a dedicated experimental
instrument, operated under microgravity conditions, at relative particle impact
velocities between 0.01 and 0.19 m s^-1, temperatures between 131 and 160 K and
a pressure of around 10^-5 mbar. Results: A range of coefficients of
restitution were found, with no correlation between this and the chemical
composition, relative impact velocity, or temperature. Conclusions: We conclude
that the chemical composition of the ice (at the level of 95% water ice and 5%
methanol or formic acid) does not affect the collisional properties at these
temperatures and pressures due to the inability of surface wetting to take
place. At a level of 5% methanol or formic acid, the structure is likely to be
dominated by crystalline water ice, leading to no change in collisional
properties. The surface roughness of the particles is the dominant factor in
explaining the range of coefficients of restitution. | astro-ph_EP |
Long lived dust rings around HD169142: Recent ALMA observations of the protoplanetary disc around HD~169142 reveal a
peculiar structure made of concentric dusty rings: a main ring at $\sim$20 au,
a triple system of rings at $\sim 55-75$ au in millimetric continuum emission
and a perturbed gas surface density from the $^{12}$CO,$^{13}$CO and
C$^{18}$O$(J=2-1)$ surface brightness profile. In this Letter, we perform
three-dimensional numerical simulations and radiative transfer modeling
exploring the possibility that two giant planets interacting with the disc and
orbiting in resonant locking can be responsible for the origin of the observed
dust inner rings structure. We find that in this configuration the dust
structure is actually long lived while the gas mass of the disc is accreted
onto the star and the giant planets, emptying the inner region. In addition, we
also find that the innermost planet is located at the inner edge of the dust
ring, and can accrete mass from the disc, generating a signature in the dust
ring shape that can be observed in mm ALMA observations. | astro-ph_EP |
Aggregate dust particles at comet 67P/Churyumov-Gerasimenko: Comets are thought to preserve almost pristine dust particles, thus providing
a unique sample of the properties of the early solar nebula. The microscopic
properties of this dust played a key part in particle aggregation during the
formation of the Solar System. Cometary dust was previously considered to
comprise irregular, fluffy agglomerates on the basis of interpretations of
remote observations in the visible and infrared and the study of chondritic
porous interplanetary dust particles that were thought, but not proved, to
originate in comets. Although the dust returned by an earlier mission has
provided detailed mineralogy of particles from comet 81P/Wild, the fine-grained
aggregate component was strongly modified during collection. Here we report in
situ measurements of dust particles at comet 67P/Churyumov-Gerasimenko. The
particles are aggregates of smaller, elongated grains, with structures at
distinct sizes indicating hierarchical aggregation. Topographic images of
selected dust particles with sizes of one micrometre to a few tens of
micrometres show a variety of morphologies, including compact single grains and
large porous aggregate particles, similar to chondritic porous interplanetary
dust particles. The measured grain elongations are similar to the value
inferred for interstellar dust and support the idea that such grains could
represent a fraction of the building blocks of comets. In the subsequent growth
phase, hierarchical agglomeration could be a dominant process and would produce
aggregates that stick more easily at higher masses and velocities than
homogeneous dust particles. The presence of hierarchical dust aggregates in the
near-surface of the nucleus of comet 67P also provides a mechanism for lowering
the tensile strength of the dust layer and aiding dust release. | astro-ph_EP |
Revisiting the distributions of Jupiter's irregular moons: II. orbital
characteristics: This paper statistically describes the orbital distribution laws of Jupiter's
irregular moons, most of which are members of the Ananke, Carme and Pasiphae
groups. By comparing 19 known continuous distributions, it is verified that
suitable distribution functions exist to describe the orbital distributions of
these natural satellites. For each distribution type, interval estimation is
used to estimate the corresponding parameter values. At a given significance
level, a one-sample Kolmogorov-Smirnov non-parametric test is applied to verify
the specified distribution, and we often select the one with the largest
$p$-value. The results show that the semi-major axis, mean inclination and
orbital period of the moons in the Ananke group and Carme group obey Stable
distributions. In addition, according to Kepler's third law of planetary motion
and by comparing the theoretically calculated best-fitting cumulative
distribution function (CDF) with the observed CDF, we demonstrate that the
theoretical distribution is in good agreement with the empirical distribution.
Therefore, these characteristics of Jupiter's irregular moons are indeed very
likely to follow some specific distribution laws, and it will be possible to
use these laws to help study certain features of poorly investigated moons or
even predict undiscovered ones. | astro-ph_EP |
Dynamical Evolution of the Earth-Moon Progenitors - Whence Theia?: We present integrations of a model Solar System with five terrestrial planets
(beginning ~30-50 Myr after the formation of primitive Solar System bodies) in
order to determine the preferred regions of parameter space leading to a giant
impact that resulted in the formation of the Moon. Our results indicate which
choices of semimajor axes and eccentricities for Theia (the proto-Moon) at this
epoch can produce a late Giant Impact, assuming that Mercury, Venus, and Mars
are near the current orbits. We find that the likely semimajor axis of Theia,
at the epoch when our simulations begin, depends on the assumed mass ratio of
Earth-Moon progenitors (8/1, 4/1, or 1/1). The low eccentricities of the
terrestrial planets are most commonly produced when the progenitors have
similar semimajor axes at the epoch when our integrations commence.
Additionally, we show that mean motion resonances among the terrestrial planets
and perturbations from the giant planets can affect the dynamical evolution of
the system leading to a late Giant Impact. | astro-ph_EP |
WASP-131 b with ESPRESSO I: A bloated sub-Saturn on a polar orbit around
a differentially rotating solar-type star: In this paper, we present observations of two high-resolution transit
datasets obtained with ESPRESSO of the bloated sub-Saturn planet WASP-131~b. We
have simultaneous photometric observations with NGTS and EulerCam. In addition,
we utilised photometric lightcurves from {\tess}, WASP, EulerCam and TRAPPIST
of multiple transits to fit for the planetary parameters and update the
ephemeris. We spatially resolve the stellar surface of WASP-131 utilising the
Reloaded Rossiter McLaughlin technique to search for centre-to-limb convective
variations, stellar differential rotation, and to determine the star-planet
obliquity for the first time. We find WASP-131 is misaligned on a nearly
retrograde orbit with a projected obliquity of $\lambda = 162.4\substack{+1.3
\\ -1.2}^{\circ}$. In addition, we determined a stellar differential rotation
shear of $\alpha = 0.61 \pm 0.06$ and disentangled the stellar inclination
($i_* = 40.9\substack{+13.3 \\ -8.5}^{\circ}$) from the projected rotational
velocity, resulting in an equatorial velocity of $v_{\rm{eq}} =
7.7\substack{+1.5 \\ -1.3}$~km s$^{-1}$. In turn, we determined the true 3D
obliquity of $\psi = 123.7\substack{+12.8 \\ -8.0}^{\circ}$, meaning the planet
is on a perpendicular/polar orbit. Therefore, we explored possible mechanisms
for the planetary system's formation and evolution. Finally, we searched for
centre-to-limb convective variations where there was a null detection,
indicating that centre-to-limb convective variations are not prominent in this
star or are hidden within red noise. | astro-ph_EP |
Physical Characterisation of Interstellar Comet 2I/2019 Q4 (Borisov): We present a study of interstellar comet 2I/2019 Q4 (Borisov) using both
preperihelion and postperihelion observations spanning late September 2019
through late January 2020. The intrinsic brightness of the comet was observed
to continuously decline throughout the timespan, likely due to the decreasing
effective scattering cross-section as a result of volatile sublimation with a
slope of $-0.43 \pm 0.02$ km$^{2}$ d$^{-1}$. We witnessed no significant change
in the slightly reddish colour of the comet, with mean values of $\left \langle
g - r \right \rangle = 0.68 \pm 0.04$, $\left \langle r - i \right \rangle =
0.23 \pm 0.03$, and the normalised reflectivity gradient across the $g$ and $i$
bands $\overline{S'} \left(g,i\right) = \left(10.6 \pm 1.4\right)$ % per $10^3$
\AA, all unremarkable in the context of solar system comets. Using the
available astrometric observations, we confidently detect the nongravitational
acceleration of the comet following a shallow heliocentric distance dependency
of $r_{\rm H}^{-1 \pm 1}$. Accordingly, we estimate that the nucleus is most
likely $\lesssim$0.4 km in radius, and that a fraction of $\gtrsim$0.2% of the
total nucleus in mass has been eroded due to the sublimation activity since the
earliest observation of the comet in 2018 December by the time of perihelion.
Our morphology simulation suggests that the dust ejection speed increased from
$\sim$4 m s$^{-1}$ in September 2019 to $\sim$7 m s$^{-1}$ around perihelion
for the optically dominant dust grains of $\beta \sim 0.01$, and that the
observable dust grains are no smaller than micron size. | astro-ph_EP |
CCD polarimetry of distant comets C/2010 S1 (LINEAR) and C/2010 R1
(LINEAR) at the 6-m telescope of the SAO RAS: We present first measurements of the degree of linear polarization of distant
comets C/2010 S1 (LINEAR) and C/2010 R1 (LINEAR) at heliocentric distances r=
5.9 - 7.0 AU. Observations were carried out with the SCORPIO-2 focal reducer at
the 6-m telescope of the SAO RAS. Both comets showed considerable level of
activity beyond a zone where water ice sublimation is negligible (up to 5 AU).
Significant spatial variations both in the intensity and polarization are found
in both comets. The slope of radial profiles of intensity changes gradually
with the distance from the photocenter: from - 0.7 near the nucleus up to about
- 1.3 for larger distances (up to 100000 km). The variation in polarization
profiles indicates the non uniformity in the polarization distribution over the
coma. The polarization degree over the coma gradually increases (in absolute
value) with increasing the photocentric distance from of about - 1.9% up to -
3% for comet C/2010 S1 (LINEAR), and from of about - 2.5% up to - 3.5% for
comet C/2010 R1 (LINEAR). These polarization values are significantly higher
than typical value of the whole coma polarization (-1.5%) for comets at
heliocentric distances less than 5 AU. The obtained photometric and
polarimetric data are compared with those derived early for other comets at
smaller heliocentric distances. Numerical modeling of light scattering
characteristics was performed for media composed of particles with different
refractive index, shape, and size. The computations were made by using the
superposition T-matrix method. We obtained that for comet C/2010 S1 (LINEAR),
the dust in the form of aggregates of overall radius R ~ 1.3 {\mu}m composed of
N = 1000 spherical monomers with radius a = 0.1 {\mu}m, refractive index m =
1.65 + i 0.05, allows to obtain a satisfactory agreement between the results of
polarimetric observations of comet C/2010 S1 and computations. | astro-ph_EP |
The impact of faculae on the radius determination of exoplanets: The
case of the M-star GJ1214: Precise measurements of exoplanets radii are of key importance for our
understanding of the origin and nature of these objects. Measurement of the
planet radii using the transit method have reached a precision that the effects
of stellar surface features have to be taken into account. While the effects
from spots have already been studied in detail, our knowledge of the effects
caused by faculae is still limited. This is particularly the case for M-stars.
Faculae can pose a problem if they are inhomogeneously distributed on the
stellar surface. Using the eclipse mapping method, we study the distribution of
the faculae on the surface of GJ1214 using the CaIIH&K lines as tracers. In
order to assess the homogeneity of the distribution in a quantitative way, we
introduce the inhomogeneity factor IHF. IHF is 0% if the distribution is
homogeneous, positive, if the plage regions are preferentially located along
the path of the planet, and negative, if they are preferentially located
outside the path of the planet. For GJ1214, we derive a rather small value of
IHF=7.7-7.7+12.0%. We discuss the relevance of this result in the context of
the PLATO and ARIEL missions. | astro-ph_EP |
Disentangling Atmospheric Compositions of K2-18 b with Next Generation
Facilities: Recent analysis of the planet K2-18b has shown the presence of water vapour
in its atmosphere. While the H2O detection is significant, the Hubble Space
Telescope (HST) WFC3 spectrum suggests three possible solutions of very
different nature which can equally match the data. The three solutions are a
primary cloudy atmosphere with traces of water vapour (cloudy sub-Neptune), a
secondary atmosphere with a substantial amount (up to 50% Volume Mixing Ratio)
of H2O (icy/water world) and/or an undetectable gas such as N2 (super-Earth).
Additionally, the atmospheric pressure and the possible presence of a
liquid/solid surface cannot be investigated with currently available
observations.
In this paper we used the best fit parameters from Tsiaras et al. (2019) to
build James Webb Space Telescope (JWST) and Ariel simulations of the three
scenarios. We have investigated 18 retrieval cases, which encompass the three
scenarios and different observational strategies with the two observatories.
Retrieval results show that twenty combined transits should be enough for the
Ariel mission to disentangle the three scenarios, while JWST would require only
two transits if combining NIRISS and NIRSpec data. This makes K2-18b an ideal
target for atmospheric follow-ups by both facilities and highlights the
capabilities of the next generation of space-based infrared observatories to
provide a complete picture of low mass planets. | astro-ph_EP |
The HARPS search for southern extrasolar planets. XXIII. 8 planetary
companions to low-activity solar-type stars: In this paper, we present our HARPS radial-velocity data for eight
low-activity solar-type stars belonging to the HARPS volume-limited sample:
HD6718, HD8535, HD28254, HD290327, HD43197, HD44219, HD148156, and HD156411.
Keplerian fits to these data reveal the presence of low-mass companions around
these targets. With minimum masses ranging from 0.58 to 2.54 MJup, these
companions are in the planetary mass domain. The orbital periods of these
planets range from slightly less than one to almost seven years. The eight
orbits presented in this paper exhibit a wide variety of eccentricities: from
0.08 to above 0.8. | astro-ph_EP |
Effect of Finite Larmor Radius on the Cosmic Ray Penetration into an
Interplanetary Magnetic Flux Rope: We discuss a mechanism for cosmic ray penetration into an interplanetary
magnetic flux rope, particularly the effect of the finite Larmor radius and
magnetic field irregularities. First, we derive analytical solutions for cosmic
ray behavior inside a magnetic flux rope, on the basis of the Newton-Lorentz
equation of a particle, to investigate how cosmic rays penetrate magnetic flux
ropes under an assumption of there being no scattering by small-scale magnetic
field irregularities. Next, we perform a numerical simulation of a cosmic ray
penetration into an interplanetary magnetic flux rope by adding small-scale
magnetic field irregularities. This simulation shows that a cosmic ray density
distribution is greatly different from that deduced from a guiding center
approximation because of the effect of the finite Larmor radius and magnetic
field irregularities for the case of a moderate to large Larmor radius compared
to the flux rope radius. | astro-ph_EP |
Rotation periods of late-type dwarf stars from time-series
high-resolution spectroscopy of chromospheric indicators: We determine rotation periods of a sample of 48 late F-type to mid-M dwarf
stars using time-series high-resolution spectroscopy of the Ca II H&K and
H-alpha chromospheric activity indicators. We find good agreement between the
rotation periods obtained from each of these two indicators. An empirical
relationship between the level of chromospheric emission measured by log (R'HK)
and the spectroscopic rotation periods is reported. This relation is largely
independent of the spectral type and the metallicity of the stars and can be
used to make a reliable prediction of rotation periods for late K to mid-M
dwarfs with low levels of activity. For some stars in the sample, the measured
spectroscopic rotation periods coincide, or are very close, to the orbital
periods of postulated planets. In such cases, further studies are needed to
clarify whether the associated periodic radial velocity signals reveal the
existence of planets or are due to magnetic activity. | astro-ph_EP |
Is Extraterrestrial Life Suppressed on Subsurface Ocean Worlds due to
the Paucity of Bioessential Elements?: The availability of bioessential elements for "life as we know it", such as
phosphorus (P) or possibly molybdenum (Mo), is expected to restrict the
biological productivity of extraterrestrial biospheres. Here, we consider
worlds with subsurface oceans and model the dissolved concentrations of
bioessential elements. In particular, we focus on the sources and sinks of P
(available as phosphates), and find that the average steady-state oceanic
concentration of P is likely to be lower than the corresponding value on Earth
by a few orders of magnitude, provided that the oceans are alkaline and possess
hydrothermal activity. While our result does not eliminate the prospects of
life on subsurface worlds like Enceladus, it suggests that the putative
biospheres might be oligotrophic, and perhaps harder to detect. Along these
lines, potential biospheres in the clouds of Venus may end up being limited by
the availability of Mo. We also point out the possibility that stellar
spectroscopy can be used to deduce potential constraints on the availability of
bioessential elements on planets and moons. | astro-ph_EP |
The Next Generation Transit Survey (NGTS): We describe the Next Generation Transit Survey (NGTS), which is a
ground-based project searching for transiting exoplanets orbiting bright stars.
NGTS builds on the legacy of previous surveys, most notably WASP, and is
designed to achieve higher photometric precision and hence find smaller planets
than have previously been detected from the ground. It also operates in red
light, maximising sensitivity to late K and early M dwarf stars. The survey
specifications call for photometric precision of 0.1 per cent in red light over
an instantaneous field of view of 100 square degrees, enabling the detection of
Neptune-sized exoplanets around Sun-like stars and super-Earths around M
dwarfs. The survey is carried out with a purpose-built facility at Cerro
Paranal, Chile, which is the premier site of the European Southern Observatory
(ESO). An array of twelve 20cm f/2.8 telescopes fitted with back-illuminated
deep-depletion CCD cameras are used to survey fields intensively at
intermediate Galactic latitudes. The instrument is also ideally suited to
ground-based photometric follow-up of exoplanet candidates from space
telescopes such as TESS, Gaia and PLATO. We present observations that combine
precise autoguiding and the superb observing conditions at Paranal to provide
routine photometric precision of 0.1 per cent in 1 hour for stars with I-band
magnitudes brighter than 13. We describe the instrument and data analysis
methods as well as the status of the survey, which achieved first light in 2015
and began full survey operations in 2016. NGTS data will be made publicly
available through the ESO archive. | astro-ph_EP |
Potential Backup Targets for Comet Interceptor: Comet Interceptor is an ESA F-class mission expected to launch in 2028 on the
same launcher as ESA's ARIEL mission. Comet Interceptor's science payload
consists of three spacecraft, a primary spacecraft that will carry two smaller
probes to be released at the target. The three spacecraft will fly-by the
target along different chords, providing multiple simultaneous perspectives of
the comet nucleus and its environment. Each of the spacecraft will be equipped
with different but complementary instrument suites designed to study the far
and near coma environment and surface of a comet or interstellar object (ISO).
The primary spacecraft will perform a fly-by at ~1000 km from the target. The
two smaller probes will travel deeper into the coma, closer to the nucleus. The
mission is being designed and launched without a specific comet designated as
its main target. Comet Interceptor will travel to the Sun-Earth L2 Lagrangian
point with ARIEL and wait in hibernation until a suitable long-period comet
(LPC) is found that will come close enough to the Sun for the spacecraft to
maneuver to an encounter trajectory. To prepare for all eventualities, the
science team has assembled a preliminary set of backup targets from the known
Jupiter family comets, where a suitable fly-by trajectory can be achieved
during the nominal mission timeline (including the possibility of some launch
delay). To better prioritize this list, we are releasing our potential backup
targets in order to solicit the planetary community's help with observations of
these objects over future apparitions and to encourage publication of archival
data on these objects. | astro-ph_EP |
An Analytic Method to determine Habitable Zones for S-Type Planetary
Orbits in Binary Star Systems: With more and more extrasolar planets discovered in and around binary star
systems, questions concerning the determination of the classical Habitable Zone
arise. Do the radiative and gravitational perturbations of the second star
influence the extent of the Habitable Zone significantly, or is it sufficient
to consider the host-star only? In this article we investigate the implications
of stellar companions with different spectral types on the insolation a
terrestrial planet receives orbiting a Sun-like primary. We present time
independent analytical estimates and compare these to insolation statistics
gained via high precision numerical orbit calculations. Results suggest a
strong dependence of permanent habitability on the binary's eccentricity, as
well as a possible extension of Habitable Zones towards the secondary in close
binary systems. | astro-ph_EP |
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