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Formation of Giant Planet Satellites: Recent analyses have shown that the concluding stages of giant planet
formation are accompanied by the development of large-scale meridional flow of
gas inside the planetary Hill sphere. This circulation feeds a circumplanetary
disk that viscously expels gaseous material back into the parent nebula,
maintaining the system in a quasi-steady state. Here we investigate the
formation of natural satellites of Jupiter and Saturn within the framework of
this newly outlined picture. We begin by considering the long-term evolution of
solid material, and demonstrate that the circumplanetary disk can act as a
global dust trap, where $s_{\bullet}\sim0.1-10\,$mm grains achieve a
hydrodynamical equilibrium, facilitated by a balance between radial updraft and
aerodynamic drag. This process leads to a gradual increase in the system's
metallicity, and eventually culminates in the gravitational fragmentation of
the outer regions of the solid sub-disk into $\mathcal{R}\sim100\,$km
satellitesimals. Subsequently, satellite conglomeration ensues via pairwise
collisions, but is terminated when disk-driven orbital migration removes the
growing objects from the satellitesimal feeding zone. The resulting satellite
formation cycle can repeat multiple times, until it is brought to an end by
photo-evaporation of the parent nebula. Numerical simulations of the envisioned
formation scenario yield satisfactory agreement between our model and the known
properties of the Jovian and Saturnian moons. | astro-ph_EP |
Eccentric Companions to Kepler-448b and Kepler-693b: Clues to the
Formation of Warm Jupiters: I report the discovery of non-transiting close companions to two transiting
warm Jupiters (WJs), Kepler-448/KOI-12b (orbital period
$P=17.9\,\mathrm{days}$, radius $R_{\rm p}=1.23^{+0.06}_{-0.05}\,R_{\rm Jup}$)
and Kepler-693/KOI-824b ($P=15.4\,\mathrm{days}$, $R_{\rm
p}=0.91\pm0.05\,R_{\rm Jup}$), via dynamical modeling of their transit timing
and duration variations (TTVs and TDVs). The companions have masses of
$22^{+7}_{-5}\,M_{\rm Jup}$ (Kepler-448c) and $150^{+60}_{-40}\,M_{\rm Jup}$
(Kepler-693c), and both are on eccentric orbits ($e=0.65^{+0.13}_{-0.09}$ for
Kepler-448c and $e=0.47^{+0.11}_{-0.06}$ for Kepler-693c) with periastron
distances of $1.5\,\mathrm{au}$. Moderate eccentricities are detected for the
inner orbits as well ($e=0.34^{+0.08}_{-0.07}$ for Kepler-448b and
$e=0.2^{+0.2}_{-0.1}$ for Kepler-693b). In the Kepler-693 system, a large
mutual inclination between the inner and outer orbits
($53^{+7}_{-9}\,\mathrm{deg}$ or $134^{+11}_{-10}\,\mathrm{deg}$) is also
revealed by the TDVs. This is likely to induce a secular oscillation of the
inner WJ's eccentricity that brings its periastron close enough to the host
star for tidal star-planet interactions to be significant. In the Kepler-448
system, the mutual inclination is weakly constrained and such an eccentricity
oscillation is possible for a fraction of the solutions. Thus these WJs may be
undergoing tidal migration to become hot Jupiters (HJs), although the migration
via this process from beyond the snow line is disfavored by the close-in and
massive nature of the companions. This may indicate that WJs can be formed in
situ and could even evolve into HJs via high-eccentricity migration inside the
snow line. | astro-ph_EP |
Planet-disc interaction on a freely moving mesh: General-purpose, moving-mesh schemes for hydrodynamics have opened the
possibility of combining the accuracy of grid-based numerical methods with the
flexibility and automatic resolution adaptivity of particle-based methods. Due
to their supersonic nature, Keplerian accretion discs are in principle a very
attractive system for applying such freely moving mesh techniques. However, the
high degree of symmetry of simple accretion disc models can be difficult to
capture accurately by these methods, due to the generation of geometric grid
noise and associated numerical diffusion, which is absent in polar grids. To
explore these and other issues, in this work we study the idealized problem of
two-dimensional planet-disc interaction with the moving-mesh code AREPO. We
explore the hydrodynamic evolution of discs with planets through a series of
numerical experiments that vary the planet mass, the disc viscosity and the
mesh resolution, and compare the resulting surface density, vortensity field
and tidal torque with results from the literature. We find that the performance
of the moving-mesh code in this problem is in accordance with published
results, showing good consistency with grid codes written in polar coordinates.
We also conclude that grid noise and mesh distortions do not introduce
excessive numerical diffusion. Finally, we show how the moving-mesh approach
can naturally increase resolution in regions of high densityaround planets and
planetary wakes, while retaining the background flow at low resolution. This
provides an alternative to the difficult task of implementing adaptive mesh
refinement in conventional polar-coordinate codes. | astro-ph_EP |
Origin of Life Molecules in the Atmosphere After Big Impacts on the
Early Earth: The origin of life on Earth would benefit from a prebiotic atmosphere that
produced nitriles, like HCN, which enable ribonucleotide synthesis. However,
geochemical evidence suggests that Hadean air was relatively oxidizing with
negligible photochemical production of prebiotic molecules. These paradoxes are
resolved by iron-rich asteroid impacts that transiently reduced the entire
atmosphere, allowing nitriles to form in subsequent photochemistry. Here, we
investigate impact-generated reducing atmospheres using new time-dependent,
coupled atmospheric chemistry and climate models, which account for gas-phase
reactions and surface-catalysis. The resulting H$_2$-, CH$_4$- and NH$_3$-rich
atmospheres persist for millions of years, until hydrogen escapes to space. HCN
and HCCCN production and rainout to the surface can reach $10^9$ molecules
cm$^{-2}$ s$^{-1}$ in hazy atmospheres with a mole ratio of $\mathrm{CH_4} /
\mathrm{CO_2} > 0.1$. Smaller $\mathrm{CH_4} / \mathrm{CO_2}$ ratios produce
HCN rainout rates $< 10^5$ molecules cm$^{-2}$ s$^{-1}$, and negligible HCCCN.
The minimum impactor mass that creates atmospheric $\mathrm{CH_4} /
\mathrm{CO_2} > 0.1$ is $4 \times 10^{20}$ to $5 \times 10^{21}$ kg (570 to
1330 km diameter), depending on how efficiently iron reacts with a steam
atmosphere, the extent of atmospheric equilibration with an impact-induced melt
pond, and the surface area of nickel that catalyzes CH$_4$ production.
Alternatively, if steam permeates and deeply oxidizes crust, impactors $\sim
10^{20}$ kg could be effective. Atmospheres with copious nitriles have $> 360$
K surface temperatures, perhaps posing a challenge for RNA longevity, although
cloud albedo can produce cooler climates. Regardless, post-impact cyanide can
be stockpiled and used in prebiotic schemes after hydrogen has escaped to
space. | astro-ph_EP |
Discovery of a young subfamily of the (221)~Eos asteroid family: In this work we report the discovery of a young cluster of asteroids that
originated by the breakup of an asteroid member of the (221)Eos family. By
applying the Hierarchical clustering method to the catalog of proper elements
we have identified 26 members of this new small group of asteroids. We have
established that the statistical significance of this cluster is $>99\%$,
therefore it corresponds to a real asteroid family, named the (633)Zelima
cluster, after its lowest numbered member. The orbits of its members are
dynamically stable, a fact that enabled us to use the backward integration
method, in two variants to identify potential interlopers and estimate its age.
Applying it first on the orbits of the nominal family members we identified
three asteroids as interlopers. Then we applied it on a set of statistically
equivalent clones of each member to determine the age of the cluster, with a
result of $2.9\pm0.2$Myrs. | astro-ph_EP |
Scaling Relations for Terrestrial Exoplanet Atmospheres from Baroclinic
Criticality: The macroturbulent atmospheric circulation of Earth-like planets mediates
their equator-to-pole heat transport. For fast-rotating terrestrial planets,
baroclinic instabilities in the mid-latitudes lead to turbulent eddies that act
to transport heat poleward. In this work, we derive a scaling theory for the
equator-to-pole temperature contrast and bulk lapse rate of terrestrial
exoplanet atmospheres. This theory is built on the work of Jansen & Ferrari
(2013), and determines how unstable the atmosphere is to baroclinic instability
(the baroclinic "criticality") through a balance between the baroclinic eddy
heat flux and radiative heating/cooling. We compare our scaling theory to
General Circulation Model (GCM) simulations and find that the theoretical
predictions for equator-to-pole temperature contrast and bulk lapse rate
broadly agree with GCM experiments with varying rotation rate and surface
pressure throughout the baroclincally unstable regime. Our theoretical results
show that baroclinic instabilities are a strong control of heat transport in
the atmospheres of Earth-like exoplanets, and our scalings can be used to
estimate the equator-to-pole temperature contrast and bulk lapse rate of
terrestrial exoplanets. These scalings can be tested by spectroscopic
retrievals and full-phase light curves of terrestrial exoplanets with future
space telescopes. | astro-ph_EP |
'Oumuamua as a light sail -- evidence against artificial origin: `Oumuamua, the first detected interstellar visitor to the solar system,
exhibits non-gravitational acceleration in its trajectory. Ruling out other
means of propulsion, such as the evaporation of material via a cometary tail,
it has been argued that radiation pressure is responsible for this
acceleration. From this, the mass of the object must be approximately 40
tonnes, and given its dimensions, `Oumuamua must have a thickness of ~1 mm if
of a similar rock/iron composition as the Earth. This raises the much
publicised possibility that `Oumuamua is artificial in origin, sent
intentionally across interstellar space by an alien civilisation, This
conclusion, however, relies upon the common misapprehension that light (solar)
sails can accelerate to a considerable fraction of the speed of light,
permitting rapid interstellar travel. We show that such speeds are unattainable
for conceptual man-made sails and that, based upon its observed parameters,
`Oumuamua would require half a billion years just to travel to our solar system
from its closest likely system of origin. These cosmological time-scales make
it very unlikely that this is a probe sent by an alien civilisation. | astro-ph_EP |
Testing 2D temperature models in Bayesian retrievals of atmospheric
properties from hot Jupiter phase curves: Spectroscopic phase curves of transiting hot Jupiters are spectral
measurements at multiple orbital phases, giving a set of disc-averaged spectra
that probe multiple hemispheres. By fitting model phase curves to observations,
we can constrain the atmospheric properties of hot Jupiters such as molecular
abundance, aerosol distribution and thermal structure, which offer insights
into their dynamics, chemistry, and formation. In this work, we propose a novel
2D temperature scheme consisting of a dayside and a nightside to retrieve
information from near-infrared phase curves, and apply the scheme to phase
curves of WASP-43b observed by HST/WFC3 and Spitzer/IRAC. In our scheme,
temperature is constant on isobars on the nightside and varies with
cos$^n$(longitude/$\epsilon$) on isobars on the dayside, where $n$ and
$\epsilon$ are free parameters. We fit all orbital phases simultaneously using
the radiative transfer package NEMESISPY coupled to a Bayesian inference code.
We first validate the performance of our retrieval scheme with synthetic phase
curves generated from a GCM, and find our 2D scheme can accurately retrieve the
latitudinally-averaged thermal structure and constrain the abundance of H$_2$O
and CH$_4$. We then apply our 2D scheme to the observed phase curves of
WASP-43b and find: (1) the dayside temperature-pressure profiles do not vary
strongly with longitude and are non-inverted; (2) the retrieved nightside
temperatures are extremely low, suggesting significant nightside cloud
coverage; (3) the H$_2$O volume mixing ratio is constrained to
$5.6\times10^{-5}$--$4.0\times10^{-4}$, and we retrieve an upper bound for
CH$_4$ at $\sim$10$^{-6}$. | astro-ph_EP |
Feasibility of Passive Sounding of Uranian Moons using Uranian
Kilometric Radiation: We present a feasibility study for passive sounding of Uranian icy moons
using Uranian Kilometric Radio (UKR) emissions in the 100 - 900 kHz band. We
provide a summary description of the observation geometry, the UKR
characteristics, and estimate the sensitivity for an instrument analogous to
the Cassini Radio Plasma Wave Science (RPWS) but with a modified receiver
digitizer and signal processing chain. We show that the concept has the
potential to directly and unambiguously detect cold oceans within Uranian
satellites and provide strong constraints on the interior structure in the
presence of warm or no oceans. As part of a geophysical payload, the concept
could therefore have a key role in the detection of oceans within the Uranian
satellites. The main limitation of the concept is coherence losses attributed
to the extended source size of the UKR and dependence on the illumination
geometry. These factors represent constraints on the tour design of a future
Uranus mission in terms of flyby altitudes and encounter timing. | astro-ph_EP |
Calibration of quasi-static aberrations in exoplanet direct-imaging
instruments with a Zernike phase-mask sensor. II. Concept validation with
ZELDA on VLT/SPHERE: Warm or massive gas giant planets, brown dwarfs, and debris disks around
nearby stars are now routinely observed by dedicated high-contrast imaging
instruments on large, ground-based observatories. These facilities include
extreme adaptive optics (ExAO) and state-of-the-art coronagraphy to achieve
unprecedented sensitivities for exoplanet detection and spectral
characterization. However, differential aberrations between the ExAO sensing
path and the science path represent a critical limitation for the detection of
giant planets with a contrast lower than a few $10^{-6}$ at very small
separations (<0.3\as) from their host star. In our previous work, we proposed a
wavefront sensor based on Zernike phase contrast methods to circumvent this
issue and measure these quasi-static aberrations at a nanometric level. We
present the design, manufacturing and testing of ZELDA, a prototype that was
installed on VLT/SPHERE during its reintegration in Chile. Using the internal
light source of the instrument, we performed measurements in the presence of
Zernike or Fourier modes introduced with the deformable mirror. Our
experimental and simulation results are consistent, confirming the ability of
our sensor to measure small aberrations (<50 nm rms) with nanometric accuracy.
We then corrected the long-lived non-common path aberrations in SPHERE based on
ZELDA measurements. We estimated a contrast gain of 10 in the coronagraphic
image at 0.2\as, reaching the raw contrast limit set by the coronagraph in the
instrument. The simplicity of the design and its phase reconstruction algorithm
makes ZELDA an excellent candidate for the on-line measurements of quasi-static
aberrations during the observations. The implementation of a ZELDA-based
sensing path on the current and future facilities (ELTs, future space missions)
could ease the observation of the cold gaseous or massive rocky planets around
nearby stars. | astro-ph_EP |
The statistical reliability of 267 GHz JCMT observations of Venus: No
significant evidence for phosphine absorption: In the light of the recent announcement of the discovery of the potential
biosignature phosphine in the atmosphere of Venus I present an independent
reanalysis of the original JCMT data to assess the statistical reliability of
the detection. Two line detection methods are explored, low order polynomial
fits and higher order multiple polynomial fits. A non-parametric bootstrap
analysis reveals that neither line detection method is able to recover a
statistically significant detection. Similar to the results of other reanalyses
of ALMA Venus spectra, the polynomial fitting process results in false positive
detections in the JCMT spectrum. There is thus no significant evidence for
phosphine absorption in the JCMT Venus spectra. | astro-ph_EP |
Observational and Theoretical study of the inner region of HH 30: HH 30 is a T Tauri star. In this thesis photometric and polarimetric
observations through the source are reported. The observations were carried out
using the 84cm telescope of the National Observatory of Mexico (OAN-SPM). This
thesis also present a model of the brightness of the source using a Monte Carlo
code of Watson and Henney (2001) that solves the radiative transfer equation. I
modified this code in order to include the polarization. I used the parameters
of Wood y Whitney (1998) to calculate the polarimetric variability observed in
HH 30. | astro-ph_EP |
Dynamical Constraints on Mercury's Collisional Origin: Of the solar system's four terrestrial planets, the origin of Mercury is
perhaps the most mysterious. Modern numerical simulations designed to model the
dynamics of terrestrial planet formation systematically fail to replicate
Mercury; which possesses just 5% the mass of Earth and the highest orbital
eccentricity and inclination among the planets. However, Mercury's large
iron-rich core and low volatile inventory stand out among the inner planets,
and seem to imply a violent collisional origin. Because most algorithms used
for simulating terrestrial accretion do not consider the effects of collisional
fragmentation, it has been difficult to test these collisional hypotheses
within the larger context of planet formation. Here, we analyze a large suite
of terrestrial accretion models that account for the fragmentation of colliding
bodies. We find that planets with core mass fractions boosted as a result of
repeated hit-and-run collisions are produced in 90% of our simulations. While
many of these planets are similar to Mercury in mass, they rarely lie on
Mercury-like orbits. Furthermore, we perform an additional batch of simulations
designed to specifically test the single giant impact origin scenario. We find
less than a 1% probability of simultaneously replicating the Mercury-Venus
dynamical spacing and the terrestrial system's degree of orbital excitation
after such an event. While dynamical models have made great strides in
understanding Mars' low mass, their inability to form accurate Mercury analogs
remains a glaring problem. | astro-ph_EP |
A New M Dwarf Debris Disk Candidate in a Young Moving Group Discovered
with Disk Detective: We used the Disk Detective citizen science project and the BANYAN II Bayesian
analysis tool to identify a new candidate member of a nearby young association
with infrared excess. WISE J080822.18-644357.3, an M5.5-type debris disk system
with significant excess at both 12 and 22 $\mu$m, is a likely member ($\sim
90\%$ BANYAN II probability) of the $\sim 45$ Myr-old Carina association. Since
this would be the oldest M dwarf debris disk detected in a moving group, this
discovery could be an important constraint on our understanding of M dwarf
debris disk evolution. | astro-ph_EP |
Evidence for 9 planets in the HD 10180 system: We re-analyse the HARPS radial velocities of HD 10180 and calculate the
probabilities of models with differing numbers of periodic signals in the data.
We test the significance of the seven signals, corresponding to seven
exoplanets orbiting the star, in the Bayesian framework and perform comparisons
of models with up to nine periodicities. We use posterior samplings and
Bayesian model probabilities in our analyses together with suitable prior
probability densities and prior model probabilities to extract all the
significant signals from the data and to receive reliable uncertainties for the
orbital parameters of the six, possibly seven, known exoplanets in the system.
According to our results, there is evidence for up to nine planets orbiting HD
10180, which would make this this star a record holder in having more planets
in its orbits than there are in the Solar system. We revise the uncertainties
of the previously reported six planets in the system, verify the existence of
the seventh signal, and announce the detection of two additional statistically
significant signals in the data. If of planetary origin, these two additional
signals would correspond to planets with minimum masses of 5.1$^{+3.1}_{-3.2}$
and 1.9$^{+1.6}_{-1.8}$ M$_{\oplus}$ on orbits with 67.55$^{+0.68}_{-0.88}$ and
9.655$^{+0.022}_{-0.072}$ days periods (denoted using the 99% credibility
intervals), respectively. | astro-ph_EP |
Survivor bias: divergent fates of the Solar System's ejected vs.
persisting planetesimals: The orbital architecture of the Solar System is thought to have been sculpted
by a dynamical instability among the giant planets. During the instability a
primordial outer disk of planetesimals was destabilized and ended up on
planet-crossing orbits. Most planetesimals were ejected into interstellar space
but a fraction were trapped on stable orbits in the Kuiper belt and Oort cloud.
We use a suite of N-body simulations to map out the diversity of planetesimals'
dynamical pathways. We focus on two processes: tidal disruption from very close
encounters with a giant planet, and loss of surface volatiles from repeated
passages close to the Sun. We show that the rate of tidal disruption is more
than a factor of two higher for ejected planetesimals than for surviving
objects in the Kuiper belt or Oort cloud. Ejected planetesimals are
preferentially disrupted by Jupiter and surviving ones by Neptune. Given that
the gas giants contracted significantly as they cooled but the ice giants did
not, taking into account the thermal evolution of the giant planets decreases
the disruption rate of ejected planetesimals. The frequency of volatile loss
and extinction is far higher for ejected planetesimals than for surviving ones
and is not affected by the giant planets' contraction. Even if all interstellar
objects were ejected from Solar System-like systems, our analysis suggests that
their physical properties should be more diverse than those of Solar System
small bodies as a result of their divergent dynamical histories. This is
consistent with the characteristics of the two currently-known interstellar
objects. | astro-ph_EP |
Laplace-like resonances with tidal effects: We generalize the Laplace resonance among three satellites, S1, S2 , and S3,
by considering different ratios of the mean-longitude variations. These
resonances, which we call Laplace-like, are classified as first order in the
cases of the 2:1&2:1, 3:2&3:2, and 2:1&3:2 resonances, second order in the case
of the 3:1&3:1 resonance, and mixed order in the case of the 2:1&3:1 resonance.
We consider a model that includes the gravitational interaction with the
central body together with the effect due to its oblateness, the mutual
gravitational influence of the satellites S1, S2, and S3 and the secular
gravitational effect of a fourth satellite S 4 , which plays the role of
Callisto in the Galilean system. In addition, we consider the dissipative
effect due to the tidal torque between the inner satellite and the central
body. We investigate these Laplace-like resonances by studying different
aspects: (i) we study the survival of the resonances when the dissipation is
included, taking two different expressions for the dissipative effect in the
case of a fast- or a slowly rotating central body, (ii) we investigate the
behavior of the Laplace-like resonances when some parameters are varied,
specifically, the oblateness coefficient, the semimajor axes, and the
eccentricities of the satellites, (iii) we analyze the linear stability of
first-order resonances for different values of the parameters, and (iv) we also
include the full gravitational interaction with S 4 to analyze its possible
capture into resonance. The results show a marked difference between first-,
second-, and mixed-order resonances, which might find applications when the
evolutionary history of the satellites in the Solar System are studied, and
also in possible actual configurations of extrasolar planetary systems. | astro-ph_EP |
Small Bodies of the Solar System Active at Large Heliocentric Distances:
Studies with the 6-Meter Telescope of Sao Ras: A detailed study of comets active at large heliocentric distances (greater
than 4 au) which enter the Solar System for the first time and are composed of
matter in its elementary, unprocessed state, would help in our understanding of
the history and evolution of the Solar System. In particular, contemporary
giant planet formation models require the presence of accretion of volatile
elements such as neon, argon, krypton, xenon and others, which initially could
not survive at the distances where giant planets were formed. Nevertheless, the
volatile components could be effectively delivered by the Kuiper-belt and
Oort-cloud bodies, which were formed at temperatures below 30 K. This review is
dedicated to the results of a multi-year comprehensive study of small bodies of
the Solar System showing a comet-like activity at large heliocentric distances.
The data were obtained from observations with the 6-meter telescope of SAO RAS
equipped with multi-mode focal reducers SCORPIO and SCORPIO-2. | astro-ph_EP |
The effect of varying atmospheric pressure upon habitability and
biosignatures of Earth-like planets: Understanding the possible climatic conditions on rocky extrasolar planets,
and thereby their potential habitability, is one of the major subjects of
exoplanet research. Determining how the climate, as well as potential
atmospheric biosignatures, change under different conditions is a key aspect
when studying Earth-like exoplanets. One important property is the atmospheric
mass hence pressure and its influence on the climatic conditions. Therefore,
the aim of the present study is to understand the influence of atmospheric mass
on climate, hence habitability, and the spectral appearance of planets with
Earth-like, that is, N2-O2 dominated, atmospheres orbiting the Sun at 1
Astronomical Unit. This work utilizes a 1D coupled, cloud-free,
climate-photochemical atmospheric column model; varies atmospheric surface
pressure from 0.5 bar to 30 bar; and investigates temperature and key species
profiles, as well as emission and brightness temperature spectra in a range
between 2{\mu}m - 20{\mu}m. Increasing the surface pressure up to 4 bar leads
to an increase in the surface temperature due to increased greenhouse warming.
Above this point, Rayleigh scattering dominates and the surface temperature
decreases, reaching surface temperatures below 273K (approximately at ~34 bar
surface pressure). For ozone, nitrous oxide, water, methane, and carbon
dioxide, the spectral response either increases with surface temperature or
pressure depending on the species. Masking effects occur, for example, for the
bands of the biosignatures ozone and nitrous oxide by carbon dioxide, which
could be visible in low carbon dioxide atmospheres. | astro-ph_EP |
Mineralogical Characterization of Baptistina Asteroid Family:
Implications for K/T Impactor Source: Bottke et al. (2007) linked the catastrophic formation of Baptistina Asteroid
Family (BAF) to the K/T impact event. This linkage was based on dynamical and
compositional evidence, which suggested the impactor had a composition similar
to CM2 carbonaceous chondrites. However, our recent study (Reddy et al. 2009)
suggests that the composition of (298) Baptistina is similar to LL-type
ordinary chondrites rather than CM2 carbonaceous chondrites. This rules out any
possibility of it being related to the source of the K/T impactor, if the
impactor was of CM-type composition. Mineralogical study of asteroids in the
vicinity of BAF has revealed a plethora of compositional types suggesting a
complex formation and evolution environment. A detailed compositional analysis
of 16 asteroids suggests several distinct surface assemblages including
ordinary chondrites (Gaffey SIV subtype), primitive achondrites (Gaffey SIII
subtype), basaltic achondrites (Gaffey SVII subtype and V-type), and a
carbonaceous chondrite. Based on our mineralogical analysis we conclude that
(298) Baptistina is similar to ordinary chondrites (LL-type) based on olivine
and pyroxene mineralogy and moderate albedo. S-type and V-type in and around
the vicinity of BAF we characterized show mineralogical affinity to (8) Flora
and (4) Vesta and could be part of their families. Smaller BAF asteroids with
lower SNR spectra showing only a 'single' band are compositionally similar to
(298) Baptistina and L/LL chondrites. It is unclear at this point why the
silicate absorption bands in spectra of asteroids with formal family definition
seem suppressed relative to background population, despite having similar
mineralogy. | astro-ph_EP |
Exoplanets: past, present, and future: Our understanding of extra-solar planet systems is highly driven by advances
in observations in the past decade. Thanks to high precision spectrograph, we
are able to reveal unseen companions to stars with the radial velocity method.
High precision photometry from the space, especially with the Kepler mission,
enables us to detect planets when they transit their stars and dim the stellar
light by merely one percent or smaller. Ultra wide-field, high cadence,
continuous monitoring of the Galactic bulge from different sites around the
southern hemisphere provides us the opportunity to observe microlensing effects
caused by planetary systems from the solar neighborhood, all the way to the
Milky Way center. The exquisite AO imaging from ground-based large telescopes,
coupled with high-contrast coronagraph, captured the photons directly emitted
by planets around other stars. In this article, I present a concise review of
the extra-solar planet discoveries, discussing the strengths and weaknesses of
the major planetary detection methods, providing an overview of our current
understanding of planetary formation and evolution given the tremendous
observations delivered by various methods, as well as on-going and planned
observation endeavors to provide a clear picture of extra-solar planetary
systems. | astro-ph_EP |
Time-series photometry of Earth flyby asteroid 2012 DA14: Context. The object 2012 DA14 is a near-Earth asteroid with a size of several
tens of meters. It had approached closely the Earth on 15 February, 2013 UT,
providing an opportunity for precise measurements of this tiny asteroid. Aims.
The solar phase angle of 2012 DA14 had varied widely around its closest
approach but was almost constant during the following night. We performed
time-series photometric observations on those two nights to determine the
rotational properties and phase effect. Methods. The observations were carried
out using the 0.55-m telescope at Saitama University, Japan. The R-band images
were obtained continuously over a 2 hr period at the closest approach and for
about 5 hr on the next night. Results. The lightcurve data from the second
night indicates a rotational period of 11.0 +1.8/-0.6 hr and a peak-to-peak
amplitude of 1.59 +/- 0.02 mag. The brightness variation before and after the
closest approach was separated into two components that are derived from the
rotation and phase effect. We found that the phase curve slope of this asteroid
is significantly shallower than those of other L-type asteroids. Conclusions.
We suggest that 2012 DA14 is coated with a coarse surface that lacks fine
regolith particles and/or a high albedo surface. | astro-ph_EP |
Growth of calcium-aluminum-rich inclusions by coagulation and
fragmentation in a turbulent protoplanetary disk: observations and
modelisation: Whereas it is generally accepted that calcium-aluminum-rich inclusions (CAIs)
from chondritic meteorites formed in a hot environment in the solar
protoplanetary disk, the conditions of their formation remain debated. Recent
laboratory studies of CAIs have provided new kind of data: their size
distributions. We show that size distributions of CAIs measured in laboratory
from sections of carbonaceous chondrites have a power law size distribution
with cumulative size exponent between -1.7 and -1.9, which translates into
cumulative size exponent between -2.5 and -2.8 after correction for sectioning.
To explain these observations, numerical simulations were run to explore the
growth of CAIs from micrometer to centimeter sizes, in a hot and turbulent
protoplanetary disk through the competition of coagulation and fragmentation.
We show that the size distributions obtained in growth simulations are in
agreement with CAIs size distributions in meteorites. We explain the CAI sharp
cut-off of their size distribution at centimeter sizes as the direct result
from the famous fragmentation barrier, provided that CAI fragment for impact
velocities larger than 10 m/s. The growth/destruction timescales of millimeter-
and centimeter-sized CAIs is inversely proportional to the local dust/gas ratio
and is about 10 years at 1300 K and up to 104 years at 1670K. This implies that
the most refractory CAIs are expected to be smaller in size owing to their long
growth timescale compared to less refractory CAIs. Conversely, the least
refractory CAIs could have been recycled many times during the CAI production
era which may have profound consequences for their radiometric age. | astro-ph_EP |
DMPP-4: Candidate sub-Neptune mass planets orbiting a naked-eye star: We present radial velocity measurements of the very bright ($V\sim5.7$)
nearby F star, DMPP-4 (HD 184960). The anomalously low Ca II H&K emission
suggests mass loss from planets orbiting a low activity host star. Periodic
radial velocity variability with $\sim 10$ ms$^{-1}$ amplitude is found to
persist over a $>4$ year timescale. Although the non-simultaneous photometric
variability in four TESS sectors supports the view of an inactive star, we
identify periodic photometric signals and also find spectroscopic evidence for
stellar activity. We used a posterior sampling algorithm that includes the
number of Keplerian signals, $N_\textrm{p}$, as a free parameter to test and
compare (1) purely Keplerian models (2) a Keplerian model with linear activity
correlation and (3) Keplerian models with Gaussian processes. A preferred
model, with one Keplerian and quasi-periodic Gaussian process indicates a
planet with a period of $P_\textrm{b} = 3.4982^{+0.0015}_{-0.0027}$ d and
corresponding minimum mass of $m_\textrm{b}\,\textrm{sin}\,i =
12.2^{+1.8}_{-1.9}$ M$_\oplus$. Without further high time resolution
observations over a longer timescale, we cannot definitively rule out the
purely Keplerian model with 2 candidates planets with $P_\textrm{b} =
2.4570^{+0.0026}_{-0.0462}$ d, minimum mass $m_\textrm{b}\,\textrm{sin}\,i =
8.0^{+1.1}_{-1.5}$ M$_\oplus$ and $P_\textrm{c} = 5.4196^{+0.6766}_{-0.0030}$ d
and corresponding minimum mass of $m_\textrm{b}\,\textrm{sin}\,i =
12.2^{+1.4}_{-1.6}$ M$_\oplus$. The candidate planets lie in the region below
the lower-envelope of the Neptune Desert. Continued mass loss may originate
from the highly irradiated planets or from an as yet undetected body in the
system. | astro-ph_EP |
A re-assessment of the Kuiper belt size distribution for sub-kilometer
objects, revealing collisional equilibrium at small sizes: We combine several constraints provided by the crater records on Arrokoth and
the worlds of the Pluto system to compute the size-frequency distribution (SFD)
of the crater production function for craters with diameter D<10km. For this
purpose, we use a Kuiper belt objects (KBO) population model calibrated on
telescopic surveys, that describes also the evolution of the KBO population
during the early Solar System. We further calibrate this model using the crater
record on Pluto, Charon and Nix. Using this model, we compute the impact
probability on Arrokoth, integrated over the age of the Solar System. This
probability is then used together with other observational constraints to
determine the slope of the crater-production function on Arrokoth. In addition,
we use our Kuiper belt model also to compare the impact rates and velocities of
KBOs on Arrokoth with those on Charon, integrated over the crater retention
ages of their respective surfaces. This allows us to establish a relationship
between the spatial density of sub-km craters on Arrokoth and of D~20km craters
on Charon. Together, all these considerations suggest the crater production
function on these worlds has a cumulative power law slope of -1.5<q<-1.2.
Converted into a projectile SFD slope, we find -1.2<q_KBO<-1.0. These values
are close to the cumulative slope of main belt asteroids in the 0.2--2~km
range, a population in collisional equilibrium. For KBOs, however, this slope
appears to extend from ~2km down to objects a few tens of meters in diameter,
as inferred from sub-km craters on Arrokoth. From the measurement of the dust
density in the Kuiper belt made by the New Horizons mission, we predict that
the SFD of the KBOs becomes steep again below ~10-30m. All these considerations
strongly indicate that the size distribution of the KBO population is in
collisional equilibrium. | astro-ph_EP |
Cheap Space-Based Microlens Parallaxes for High-Magnification Events: We show that for high-magnification (Amax > 100) microlensing events,
accurate microlens parallaxes can be obtained from three or fewer photometric
measurements from a small telescope on a satellite in solar orbit at ~1 AU from
Earth. This is 1--2 orders of magnitude less observing resources than are
required for standard space-based parallaxes. Such microlens parallax
measurements would yield accurate mass and distance measurements to the lens
for all cases in which finite-source effects were observed from the ground over
peak. This would include virtually all high-magnification events with detected
planets and a substantial fraction of those without. Hence it would permit
accurate estimates of the Galactic distribution of planets. | astro-ph_EP |
C/O and Snowline Locations in Protoplanetary Disks: The Effect of Radial
Drift and Viscous Gas Accretion: The C/O ratio is a defining feature of both gas giant atmospheric and
protoplanetary disk chemistry. In disks, the C/O ratio is regulated by the
presence of snowlines of major volatiles at different distances from the
central star. We explore the effect of radial drift of solids and viscous gas
accretion onto the central star on the snowline locations of the main C and O
carriers in a protoplanetary disk, H2O, CO2 and CO, and their consequences for
the C/O ratio in gas and dust throughout the disk. We determine the snowline
locations for a range of fixed initial particle sizes and disk types. For our
fiducial disk model, we find that grains with sizes ~0.5 cm < s < 7 m for an
irradiated disk, and ~0.001 cm < s < 7 m for an evolving and viscous disk,
desorb at a size-dependent location in the disk, which is independent of the
particle's initial position. The snowline radius decreases for larger
particles, up to sizes of ~7 m. Compared to a static disk, we find that radial
drift and gas accretion in a viscous disk move the H2O snowline inwards by up
to 40%, the CO2 snowline by up to 60%, and the CO snowline by up to 50%. We
thus determine an inner limit on the snowline locations when radial drift and
gas accretion are accounted for. | astro-ph_EP |
The CRIRES Search for Planets Around the Lowest-Mass Stars. I.
High-Precision Near-Infrared Radial Velocities with an Ammonia Gas Cell: Radial velocities measured from near-infrared spectra are a potentially
powerful tool to search for planets around cool stars and sub-stellar objects.
However, no technique currently exists that yields near-infrared radial
velocity precision comparable to that routinely obtained in the visible. We
describe a method for measuring high-precision relative radial velocities of
these stars from K-band spectra. The method makes use of a glass cell filled
with ammonia gas to calibrate the spectrograph response similar to the "iodine
cell" technique that has been used very successfully in the visible. Stellar
spectra are obtained through the ammonia cell and modeled as the product of a
Doppler-shifted template spectrum of the object and a spectrum of the cell,
convolved with a variable instrumental profile model. A complicating factor is
that a significant number of telluric absorption lines are present in the
spectral regions containing useful stellar and ammonia lines. The telluric
lines are modeled simultaneously as well using spectrum synthesis with a
time-resolved model of the atmosphere over the observatory. The free parameters
in the complete model are the wavelength scale of the spectrum, the
instrumental profile, adjustments to the water and methane abundances in the
atmospheric model, telluric spectrum Doppler shift, and stellar Doppler shift.
Tests of the method based on the analysis of hundreds of spectra obtained for
late M dwarfs over six months demonstrate that precisions of ~5 m/s are
obtainable over long timescales, and precisions of better than 3 m/s can be
obtained over timescales up to a week. The obtained precision is comparable to
the predicted photon-limited errors, but primarily limited over long timescales
by the imperfect modeling of the telluric lines. | astro-ph_EP |
An advanced multipole model for (216) Kleopatra triple system: To interpret adaptive-optics observations of (216) Kleopatra, we need to
describe an evolution of multiple moons, orbiting an extremely irregular body
and including their mutual interactions. Such orbits are generally
non-Keplerian and orbital elements are not constants. Consequently, we use a
modified $N$-body integrator, which was significantly extended to include the
multipole expansion of the gravitational field up to the order $\ell = 10$. Its
convergence was verified against the `brute-force' algorithm. We computed the
coefficients $C_{\ell m},S_{\!\ell m}$ for Kleopatra's shape, assuming
a~constant bulk density. For solar-system applications, it was also necessary
to implement a variable distance and geometry of observations. Our $\chi^2$
metric then accounts for the absolute astrometry, the relative astrometry (2nd
moon with respect to 1st), angular velocities, and also silhouettes,
constraining the pole orientation. This allowed us to derive the orbital
elements of Kleopatra's two moons. Using both archival astrometric data and new
VLT/SPHERE observations (ESO LP 199.C-0074), we were able to identify the true
periods of the moons, $P_1 = (1.822359\pm0.004156)\,{\rm d}$, $P_2 =
(2.745820\pm0.004820)\,{\rm d}$. They orbit very close to the 3:2 mean-motion
resonance, but their osculating eccentricities are too small compared to other
perturbations (multipole, mutual), so that regular librations of the critical
argument are not present. The resulting mass of Kleopatra, $m_1 =
(1.49\pm0.16)\cdot10^{-12}\,M_\odot$ or $2.97\cdot10^{18}\,{\rm kg}$, is
significantly lower than previously thought. An implication explained in the
accompanying paper (Marchis et al.) is that (216) Kleopatra is a critically
rotating body. | astro-ph_EP |
Long Term Evolution of Planet-Induced Vortices in Protoplanetary Disks: Recent observations of large-scale asymmetric features in protoplanetary
disks suggest that large-scale vortices exist in such disks. Massive planets
are known to be able to produce deep gaps in protoplanetary disks. The gap
edges could become hydrodynamically unstable to the Rossby wave/vortex
instability and form large-scale vortices. In this study we examine the long
term evolution of these vortices by carrying out high-resolution two
dimensional hydrodynamic simulations that last more than $10^4$ orbits
(measured at the planet's orbit). We find that the disk viscosity has a strong
influence on both the emergence and lifetime of vortices. In the outer disk
region where asymmetric features are observed, our simulation results suggest
that the disk viscous $\alpha$ needs to be low $\sim 10^{-5 }$ - $10^{-4}$ to
sustain vortices to thousands and up to $10^{4}$ orbits in certain cases. The
chance of finding a vortex feature in a disk then decreases with smaller planet
orbital radius. For $\alpha \sim 10^{-3}$ or larger, even planets with masses
of 5 Jupiter-masses will have difficulty either producing or sustaining
vortices. We have also studied the effects of different disk temperatures and
planet masses. We discuss the implications of our findings on current and
future protoplanetary disk observations. | astro-ph_EP |
Constraining the Radiation and Plasma Environment of the Kepler
Circumbinary Habitable Zone Planets: The discovery of many planets using the Kepler telescope includes ten planets
orbiting eight binary stars. Three binaries, Kepler-16, Kepler-47, and
Kepler-453, have at least one planet in the circumbinary habitable-zone (BHZ).
We constrain the level of high-energy radiation and the plasma environment in
the BHZ of these systems. With this aim, BHZ limits in these Kepler binaries
are calculated as a function of time, and the habitability lifetimes are
estimated for hypothetical terrestrial planets and/or moons within the BHZ.
With the time-dependent BHZ limits established, a self-consistent model is
developed describing the evolution of stellar activity and radiation properties
as proxies for stellar aggression toward planetary atmospheres. Modeling binary
stellar rotation evolution, including the effect of tidal interaction between
stars in binaries is key to establishing the environment around these systems.
We find that Kepler-16 and its binary analogs provide a plasma environment
favorable for the survival of atmospheres of putative Mars-sized planets and
exomoons. Tides have modified the rotation of the stars in Kepler-47 making its
radiation environment less harsh in comparison to the solar system. This is a
good example of the mechanism first proposed by Mason et al. Kepler-453 has an
environment similar to that of the solar system with slightly better than Earth
radiation conditions at the inner edge of the BHZ. These results can be
reproduced and even reparametrized as stellar evolution and binary tidal models
progress, using our online tool http://bhmcalc.net. | astro-ph_EP |
Herschel celestial calibration sources: Four large main-belt asteroids
as prime flux calibrators for the far-IR/sub-mm range: Celestial standards play a major role in observational astrophysics. They are
needed to characterise the performance of instruments and are paramount for
photometric calibration. During the Herschel Calibration Asteroid Preparatory
Programme approximately 50 asteroids have been established as far-IR/sub-mm/mm
calibrators for Herschel. The selected asteroids fill the flux gap between the
sub-mm/mm calibrators Mars, Uranus and Neptune, and the mid-IR bright
calibration stars. All three Herschel instruments observed asteroids for
various calibration purposes, including pointing tests, absolute flux
calibration, relative spectral response function, observing mode validation,
and cross-calibration aspects. Here we present newly established models for the
four large and well characterized main-belt asteroids (1) Ceres, (2) Pallas,
(4) Vesta, and (21) Lutetia which can be considered as new prime flux
calibrators. The relevant object-specific properties (size, shape,
spin-properties, albedo, thermal properties) are well established. The seasonal
(distance to Sun, distance to observer, phase angle, aspect angle) and daily
variations (rotation) are included in a new thermophysical model setup for
these targets. The thermophysical model predictions agree within 5% with the
available (and independently calibrated) Herschel measurements. The four
objects cover the flux regime from just below 1,000 Jy (Ceres at mid-IR
N-/Q-band) down to fluxes below 0.1 Jy (Lutetia at the longest wavelengths).
Based on the comparison with PACS, SPIRE and HIFI measurements and pre-Herschel
experience, the validity of these new prime calibrators ranges from
mid-infrared to about 700 micron, connecting nicely the absolute stellar
reference system in the mid-IR with the planet-based calibration at sub-mm/mm
wavelengths. | astro-ph_EP |
Quadrupole and octupole order resonances in non-restricted hierarchical
planetary systems: Nonrestricted hierarchical three-body configurations are common in various
scales of astrophysical systems. Dynamical structures of the quadrupole-order
resonance (the von Zeipel-Lidov-Kozai resonance) and the octupole-order
resonance (the apsidal resonance) under the nonrestricted hierarchical
planetary systems are investigated in this work by taking advantage of
perturbative treatments. Under the quadrupole-order Hamiltonian model, the
distribution of libration and circulation regions as well as the distribution
of flipping region are analytically explored in the parameter space spanned by
the conserved quantities. The fundamental frequencies of system are produced
and then the nominal location of octupole-order resonance is identified. From
the viewpoint of perturbative theory, the quadrupole-order Hamiltonian
determines the unperturbed dynamical model and the octupole-order Hamiltonian
plays an role of perturbation to the quadrupole-order dynamics. The resonant
Hamiltonian for octupole-order resonances is formulated by means of averaging
theory, giving rise to a new constant of motion. Phase portraits are produced
to analyse dynamical structures of octupole-order resonance, including resonant
centres, saddle points, dynamical separatrices and islands of libration. By
analysing phase portraits, it is found that there are four branches of
libration centre and eight libration zones in the considered space.
Applications to orbit flips show that there are five flipping regions. | astro-ph_EP |
Planet-Mediated Precision-Reconstruction of the Evolution of the
Cataclysmic Variable HU Aquarius: Cataclysmic variables (CVs) are binaries in which a compact white dwarf
accretes material from a low-mass companion star. The discovery of two planets
in orbit around the CV HU Aquarii opens unusual opportunities for understanding
the formation and evolution of this system. In particular the orbital
parameters of the planets constrains the past and enables us to reconstruct the
evolution of the system through the common-envelope phase. During this dramatic
event the entire hydrogen envelope of the primary star is ejected, passing the
two planets on the way. The observed eccentricities and orbital separations of
the planets in HU Aqr enable us to limit the common-envelope parameter $\alpha
\lambda = 0.45\pm 0.17$ or $\gamma = 1.77\pm0.02$ and measure the rate at which
the common envelope is ejected, which turns out to be copious. The mass in the
common envelope is ejected from the binary system at a rate of ${\dot m} =
1.9\pm 0.3\,\MSun/yr$. The reconstruction of the initial conditions for HU Aqr
indicates that the primary star had a mass of $M_{\rm ZAMS} =
1.6\pm0.2$\,\MSun\, and a $m_{\rm ZAMS} = 0.47\pm 0.04$\,\MSun\, companion in a
$a=25$--160\,\RSun\, (best value $a=97$\,\RSun) binary. The two planets were
born with an orbital separation of $a_a=541\pm44$\,\RSun\, and
$a_b=750\pm72$\,\Rsun\, respectively. After the common envelope, the primary
star turns into a $0.52\pm0.01$\,\MSun\, helium white dwarf, which subsequently
accreted $\sim 0.30$\,\MSun\, from its Roche-lobe filling companion star,
grinding it down to its current observed mass of $0.18\,\MSun$. | astro-ph_EP |
Systematic KMTNet Planetary Anomaly Search. X. Complete Sample of 2017
Prime-Field Planets: We complete the analysis of planetary candidates found by the KMT
AnomalyFinder for the 2017 prime fields that cover $\sim 13\,{\rm deg}^2$. We
report 3 unambiguous planets: OGLE-2017-BLG-0640, OGLE-2017-BLG-1275, and
OGLE-2017-BLG-1237. The first two of these were not previously identified,
while the last was not previously published due to technical complications
induced by a nearby variable. We further report that a fourth anomalous event,
the previously recognized OGLE-2017-BLG-1777, is very likely to be planetary,
although its light curve requires unusually complex modeling because the lens
and source both have orbiting companions. One of the 3 unambiguous planets,
OGLE-2017-BLG-1275 is the first AnomalyFinder discovery that has a {\it
Spitzer} microlens parallax measurement, $\pi_E \sim 0.045\pm0.015$, implying
that this planetary system almost certainly lies in the Galactic bulge. In the
order listed, the four planetary events have planet-host mass ratios $q$, and
normalized projected separations $s$, of $(\log q,s)$ = $(-2.31,0.61)$,
$(-2.06,0.63/1.09)$, $(-2.10,1.04)$, and $(-2.86,0.72)$. Combined with
previously published events, the 2017 AnomalyFinder prime fields contain 11
unambiguous planets with well-measured $q$ and one very likely candidate, of
which 3 are AnomalyFinder discoveries. In addition to these 12, there are three
other unambiguous planets with large uncertainties in $q$. | astro-ph_EP |
Compositional Convection in the Deep Interior of Uranus: Uranus and Neptune share properties that are distinct from the other giant
planets in the solar system, but they are also distinct from one another,
particularly in their relative internal heat flux. Not only does Neptune emit
about ten times the amount of heat that emitted by Uranus, the relative amount
of emitted heat to the energy they absorb from the sun also differs greatly,
being comparable at Uranus and the largest of all giant planets at Neptune. As
a result, it is questionable whether thermal convection occurs within the
interior of Uranus. However, the presence of an intrinsic magnetic field
implies that interior fluid motions must exist. Here, we consider compositional
convection driven by the release of hydrogen associated with the formation of
large organic networks or diamond precipitation in the deep interior. We test
this hypotheses using a set of numerical rotating convection models where the
convective driving is varied between thermal and compositional sources and is
sufficiently vigorous to not be strongly constrained by rotation. In most
cases, we find ice-giant-like zonal flows develop, with three bands
characterized by a retrograde equatorial jet and prograde jets at higher
latitudes. Large-scale circulation cells also develop and lead to heat and mass
fluxes that tend to exhibit local maxima along the equatorial plane. This
similarity between convective flows driven by thermal and compositional
buoyancy therefore predict Uranus and Neptune to have similar interior dynamics
despite Uranus' minimal internal heat flow and may thus explain why both ice
giants have comparable magnetic fields. | astro-ph_EP |
Characterizing the WASP-4 system with TESS and radial velocity data:
Constraints on the cause of the hot Jupiter's changing orbit and evidence of
an outer planet: Orbital dynamics provide valuable insights into the evolution and diversity
of exoplanetary systems. Currently, only one hot Jupiter, WASP-12b, is
confirmed to have a decaying orbit. Another, WASP-4b, exhibits hints of a
changing orbital period that could be caused by orbital decay, apsidal
precession, or the acceleration of the system towards the Earth. We have
analyzed all data sectors from NASA's Transiting Exoplanet Survey Satellite
together with all radial velocity (RV) and transit data in the literature to
characterize WASP-4b's orbit. Our analysis shows that the full RV data set is
consistent with no acceleration towards the Earth. Instead, we find evidence of
a possible additional planet in the WASP-4 system, with an orbital period of
~7000 days and $M_{c}sin(i)$ of $5.47^{+0.44}_{-0.43} M_{Jup}$. Additionally,
we find that the transit timing variations of all of the WASP-4b transits
cannot be explained by the second planet but can be explained with either a
decaying orbit or apsidal precession, with a slight preference for orbital
decay. Assuming the decay model is correct, we find an updated period of
1.338231587$\pm$0.000000022 days, a decay rate of -7.33$\pm$0.71 msec/year, and
an orbital decay timescale of 15.77$\pm$1.57 Myr. If the observed decay results
from tidal dissipation, we derive a modified tidal quality factor of
$Q^{'}_{*}$ = 5.1$\pm$0.9$\times10^4$, which is an order of magnitude lower
than values derived for other hot Jupiter systems. However, more observations
are needed to determine conclusively the cause of WASP-4b's changing orbit and
confirm the existence of an outer companion. | astro-ph_EP |
Semi-analytical near-Earth objects propagation: the orbit history of
(35107) 1991 VH and (175706) 1996 FG3: The propagation of small bodies in the Solar system is driven by the
combination of planetary encounters that cause abrupt changes in their orbits
and secular long-term perturbations. We propose a propagation strategy that
combines both of these effects into a single framework for long-term, rapid
propagation of small bodies in the inner Solar System. The analytical secular
perturbation of Jupiter is interrupted to numerically solve planetary
encounters, which last a small fraction of the simulation time. The proposed
propagation method is compared to numerical integrations in the Solar system,
effectively capturing properties of the numerical solutions in a fraction of
the computational time. We study the orbital history of the Janus mission
targets: (35107) 1991 VH and (175706) 1996 FG3, obtaining a stochastic
representation of their long-term dynamics and frequencies of very close
encounters. Over the last million years the probability of a strongly
perturbing flyby is found to be small. | astro-ph_EP |
Benchmarking the ab initio hydrogen equations of state for the interior
structure of Jupiter: As Juno is presently measuring Jupiter's gravitational moments to
unprecedented accuracy, models for the interior structure of the planet are
putted to the test. While equations of state based on first principles or ab
initio simulations have been available and used for the two most abundant
elements constituting the envelope, hydrogen and helium, significant
discrepancies remain regarding the predictions of the inner structure of
Jupiter. The differences are severe enough to clutter the analysis of Juno's
data and even cast doubts on the usefulness of these computationally expensive
EOSs for the modeling of the interior of Jupiter and exoplanets at large. Using
our newly developed equations of state for hydrogen and helium, we asses the ab
initio equations of state currently available and establish their efficiency at
predicting the interior structure of Jupiter in a two-layers model. By
adjusting our free energy parameterization to reproduce previous ab initio EOS
behavior, we identify the source of the disagreement previously reported for
the interior structure of Jupiter. We further point to area where care should
be taken when building EOS for the modeling of giant planets. This concerns the
interpolation between the ab initio results and the physical models used to
cover the low density range as well as the interpolation of the {\sl ab initio}
simulation results at high densities. This sensitivity falls well within the
uncertainties of the ab initio simulations. This suggests that hydrogen EOS
should be carefully benchmarked using a simple planetary model before being
used in the more advanced planetary models needed to interpret the Juno data.
We finally provide an updated version of our ab initio hydrogen EOS recently
published. | astro-ph_EP |
Phase Curves of Nine Trojan Asteroids over a Wide Range of Phase Angles: We have observed well-sampled phase curves for nine Trojan asteroids in B-,
V-, and I-bands. These were constructed from 778 magnitudes taken with the
1.3-m telescope on Cerro Tololo as operated by a service observer for the
SMARTS consortium. Over our typical phase range of 0.2-10 deg, we find our
phase curves to be adequately described by a linear model, for slopes of
0.04-0.09 mag/deg with average uncertainty less than 0.02 mag/deg. (The one
exception, 51378 (2001 AT33), has a formally negative slope of -0.02 +- 0.01
mag/deg.) These slopes are too steep for the opposition surge mechanism to be
shadow hiding (SH), so we conclude that the dominant surge mechanism must be
coherent backscattering (CB). In a detailed comparison of surface properties
(including surge slope, B-R color, and albedo), we find that the Trojans have
surface properties similar to the P and C class asteroids prominent in the
outer main belt, yet they have significantly different surge properties (at a
confidence level of 99.90%). This provides an imperfect argument against the
traditional idea that the Trojans were formed around Jupiter's orbit. We also
find no overlap in Trojan properties with either the main belt asteroids or
with the small icy bodies in the outer Solar System. Importantly, we find that
the Trojans are indistinguishable from other small bodies in the outer Solar
System that have lost their surface ices (such as the gray Centaurs, gray
Scattered Disk Objects, and dead comets). Thus, we find strong support for the
idea that the Trojans originally formed as icy bodies in the outer Solar
System, were captured into their current orbits during the migration of the gas
giant planets, and subsequently lost all their surface ices. | astro-ph_EP |
Multiverse Predictions for Habitability: Fraction of Life that Develops
Intelligence: Do mass extinctions affect the development of intelligence? If so, we may
expect to be in a universe that is exceptionally placid. We consider the
effects of impacts, supervolcanoes, global glaciations, and nearby gamma ray
bursts, and how their rates depend on fundamental constants. It is interesting
that despite the very disparate nature of these processes, each occurs on
timescales of 100 Myr-Gyr. We argue that this is due to a selection effect that
favors both tranquil locales within our universe, as well as tranquil
universes. Taking gamma ray bursts to be the sole driver of mass extinctions is
disfavored in multiverse scenarios, as the rate is much lower for different
values of the fundamental constants. In contrast, geological causes of
extinction are very compatible with the multiverse. Various frameworks for the
effects of extinctions are investigated, and the intermediate disturbance
hypothesis is found to be most compatible with the multiverse. | astro-ph_EP |
A Near-coplanar Stellar Flyby of the Planet Host Star HD 106906: We present an investigation into the kinematics of HD 106906 using the newly
released Gaia DR2 catalog to search for close encounters with other members of
the Scorpius-Centaurus (Sco-Cen) association. HD 106906 is an eccentric
spectroscopic binary that hosts both a large asymmetric debris disk extending
out to at least 500 au and a directly imaged planetary-mass companion at a
projected separation of 738 au. The cause of the asymmetry in the debris disk
and the unusually wide separation of the planet is not currently known. Using a
combination of Gaia DR2 astrometry and ground-based radial velocities, we
explore the hypothesis that a close encounter with another cluster member
within the last 15 Myr is responsible for the present configuration of the
system. Out of 461 stars analyzed, we identified two candidate perturbers that
had a median closest approach (CA) distance within 1 pc of HD 106906: HIP 59716
at $D_{\rm CA}=0.65_{-0.40}^{+0.93}$ pc ($t_{\rm CA}=-3.49_{-1.76}^{+0.90}$
Myr) and HIP 59721 at $D_{\rm CA}=0.71_{-0.11}^{+0.18}$ pc ($t_{\rm
CA}=-2.18_{-1.04}^{+0.54}$ Myr), with the two stars likely forming a wide
physical binary. The trajectories of both stars relative to HD 106906 are
almost coplanar with the inner disk ($\Delta\theta = 5.4\pm1.7$ deg and
$4.2_{-1.1}^{+0.9}$ deg). These two stars are the best candidates of the
currently known members of Sco-Cen for having a dynamically important close
encounter with HD 106906, which may have stabilized the orbit of HD 106906 b in
the scenario where the planet formed in the inner system and attained high
eccentricity by interaction with the central binary. | astro-ph_EP |
The emergence of relaxation-oscillator convection on Earth and Titan: In relaxation-oscillator (RO) climate states, short-lived convective storms
with torrential rainfall form and dissipate at regular, periodic intervals. RO
states have been demonstrated in two- and three-dimensional simulations of
radiative-convective equilibrium (RCE), and it has been argued that the
existence of the RO state requires explicitly resolving moist convective
processes. However, the exact nature and emergence mechanism of the RO state
have yet to be determined. Here, we show that (1) RO states exist in
single-column-model simulations of RCE with parameterized convection, and (2)
the RO state can be understood as one that has no steady-state solutions of an
analytical model of RCE. As with model simulations with resolved convection,
these simpler, one-dimensional models of RCE clearly demonstrate RO states
emerge at high surface temperatures and/or very moist atmospheres. Emergence
occurs when atmospheric instability quantified by the convective available
potential energy can no longer support the latent heat release of deep,
entraining convective plumes. The proposed mechanism for RO emergence is
general to all moist planetary atmospheres, is agnostic of the condensing
component, and naturally leads to an understanding of Titan's bursty methane
weather. | astro-ph_EP |
A Novel Solution for Resonant Scattering Using Self-Consistent Boundary
Conditions: We present two novel additions to the semi-analytic solution of Lyman
$\alpha$ (Ly$\alpha$) radiative transfer in spherical geometry: (1)
implementation of the correct boundary condition for a steady source, and (2)
solution of the time-dependent problem for an impulsive source. For the
steady-state problem, the solution can be represented as a sum of two terms: a
previously-known analytic solution of the equation with mean intensity $J=0$ at
the surface, and a novel, semi-analytic solution which enforces the correct
boundary condition of zero-ingoing intensity at the surface. This solution is
compared to that of the Monte Carlo method, which is valid at arbitrary optical
depth. It is shown that the size of the correction is of order unity when the
spectral peaks approach the Doppler core and decreases slowly with line center
optical depth, specifically as $(a \tau_0)^{-1/3}$, which may explain
discrepancies seen in previous studies. For the impulsive problem, the time,
spatial, and frequency dependence of the solution are expressed using an
eigenfunction expansion in order to characterize the escape time distribution
and emergent spectra of photons. It is shown that the lowest-order
eigenfrequency agrees well with the decay rate found in the Monte Carlo escape
time distribution at sufficiently large line-center optical depths. The
characterization of the escape-time distribution highlights the potential for a
Monte Carlo acceleration method, which would sample photon escape properties
from distributions rather than calculating every photon scattering, thereby
reducing computational demand. | astro-ph_EP |
Thermal Wave Instability as an Origin of Gap and Ring Structures in
Protoplanetary Disks: Recent millimeter and infrared observations have shown that gap and ring-like
structures are common in both dust thermal emission and scattered-light of
protoplanetary disks. We investigate the impact of the so-called Thermal Wave
Instability (TWI) on the millimeter and infrared scattered-light images of
disks. We perform 1+1D simulations of the TWI and confirm that the TWI operates
when the disk is optically thick enough for stellar light, i.e.,
small-grain-to-gas mass ratio of $\gtrsim0.0001$. The mid-plane temperature
varies as the waves propagate and hence gap and ring structures can be seen in
both millimeter and infrared emission. The millimeter substructures can be
observed even if the disk is fully optically thick since it is induced by the
temperature variation, while density-induced substructures would disappear in
the optically thick regime. The fractional separation between TWI-induced ring
and gap is $\Delta r/r \sim$ 0.2-0.4 at $\sim$ 10-50 au, which is comparable to
those found by ALMA. Due to the temperature variation, snow lines of volatile
species move radially and multiple snow lines are observed even for a single
species. The wave propagation velocity is as fast as $\sim$ 0.6 ${\rm
au~yr^{-1}}$, which can be potentially detected with a multi-epoch observation
with a time separation of a few years. | astro-ph_EP |
The HARPS search for southern extra-solar planets XXXIV. Occurrence,
mass distribution and orbital properties of super-Earths and Neptune-mass
planets: We report on the results of an 8-year survey carried out at the La Silla
Observatory with the HARPS spectrograph to detect and characterize planets in
the super-Earth and Neptune mass regime. The size of our star sample and the
precision achieved with HARPS have allowed the detection of a sufficiently
large number of low-mass planets to study the statistical properties of their
orbital elements, the correlation of the host-star metallicity with the planet
masses, as well as the occurrence rate of planetary systems around solar-type
stars. A robust estimate of the frequency of systems shows that more than 50%
of solar-type stars harbor at least one planet of any mass and with period up
to 100 days. Different properties are observed for the population of planets
less massive than about 30M-Earth compared to the population of gaseous giant
planets. The mass distribution of Super-Earths and Neptune-mass planets (SEN)
is strongly increasing between 30 and 15M-Earth. The SEN occurence rate does
not exhibit a preference for metal rich stars. Most of the SEN planets belong
to multi-planetary systems. The orbital eccentricities of the SEN planets seems
limited to 0.45. At the opposite, the occurence rate of gaseous giant planets
is growing with the logarithm of the period, and is strongly increasing with
the host-star metallicity. About 14% of solar-type stars have a planetary
companion more massive than 50M-Earth? on an orbit with a period shorter than
10 years. Orbital eccentricities of giant planets are observed up to 0.9 and
beyond. The precision of HARPS-type spectrographs opens the possibility to
detect planets in the habitable zone of solar-type stars. Identification of a
significant number of super-Earths orbiting solar-type of the Sun vicinity is
achieved by Doppler spectroscopy. 37 newly discovered planets are announced in
the Appendix of this paper, among which 15 Super-Earths. | astro-ph_EP |
Let the Great World Spin: Revealing the Stormy, Turbulent Nature of
Young Giant Exoplanet Analogs with the Spitzer Space Telescope: We present a survey for photometric variability in young, low-mass brown
dwarfs with the Spitzer Space Telescope. The 23 objects in our sample show
robust signatures of youth and share properties with directly-imaged
exoplanets. We present three new young objects: 2MASS J03492367$+$0635078,
2MASS J09512690 $-$8023553 and 2MASS J07180871$-$6415310. We detect variability
in 13 young objects, and find that young brown dwarfs are highly likely to
display variability across the L2--T4 spectral type range. In contrast, the
field dwarf variability occurrence rate drops for spectral types $>$L9. We
examine the variability amplitudes of young objects and find an enhancement in
maximum amplitudes compared to field dwarfs. We speculate that the observed
range of amplitudes within a spectral type may be influenced by secondary
effects such as viewing inclination and/or rotation period. We combine our new
rotation periods with the literature to investigate the effects of mass on
angular momentum evolution. While high mass brown dwarfs ($>30
M_{\mathrm{Jup}}$) spin up over time, the same trend is not apparent for lower
mass objects ($<30 M_{\mathrm{Jup}}$), likely due to the small number of
measured periods for old, low-mass objects. The rotation periods of companion
brown dwarfs and planetary-mass objects are consistent with those of isolated
objects with similar ages and masses, suggesting similar angular momentum
histories. Within the AB Doradus group, we find a high variability occurrence
rate and evidence for common angular momentum evolution. The results are
encouraging for future variability searches in directly-imaged exoplanets with
facilities such as the James Webb Space Telescope and 30-meter telescopes. | astro-ph_EP |
Distant Comet C/2017 K2 and the Cohesion Bottleneck: Distant long-period comet C/2017 K2 has been outside the planetary region of
the solar system for 3 Myr, negating the possibility that heat retained from
the previous perihelion could be responsible for its activity. This inbound
comet is also too cold for water ice to sublimate and too cold for amorphous
water ice, if present, to crystallize. C/2017 K2 thus presents an ideal target
in which to investigate the mechanisms responsible for activity in distant
comets. We have used Hubble Space Telescope to study the comet in the
pre-perihelion distance range 13.8 to 15.9 AU. The coma maintains a logarithmic
surface brightness gradient $m = -1.010\pm$0.004, consistent with steady-state
mass loss. The absence of a radiation pressure swept tail indicates that the
effective particle size is large (0.1 mm) and the mass loss rate is $\sim$200
kg s$^{-1}$, remarkable for a comet still beyond the orbit of Saturn.
Extrapolation of the photometry indicates that activity began in 2012.1, at
25.9$\pm$0.9 AU, where the blackbody temperature is only 55 K. This large
distance and low temperature suggest that cometary activity is driven by the
sublimation of a super-volatile ice (e.g.~CO), presumably preserved by K2's
long-term residence in the Oort cloud. The mass loss rate can be sustained by
CO sublimation from an area $\lesssim 2$ km$^2$, if located near the hot
sub-solar point on the nucleus. However, while the drag force from sublimated
CO is sufficient to lift millimeter sized particles against the gravity of the
cometary nucleus, it is 10$^2$ to 10$^3$ times too small to eject these
particles against inter-particle cohesion. Our observations thus require either
a new understanding of the physics of inter-particle cohesion or the
introduction of another mechanism to drive distant cometary mass loss. We
suggest thermal fracture and electrostatic supercharging in this context. | astro-ph_EP |
A dynamical study on the habitability of terrestrial exoplanets II: The
super Earth HD 40307 g: HARPS and it Kepler results indicate that half of solar-type stars host
planets with periods P<100 d and masses M < 30 M_E. These super Earth systems
are compact and dynamically cold. Here we investigate the stability of the
super Earth system around the K-dwarf HD40307. It could host up to six planets,
with one in the habitable zone. We analyse the system's stability using
numerical simulations from initial conditions within the observational
uncertainties. The most stable solution deviates 3.1 sigma from the published
value, with planets e and f not in resonance and planets b and c apsidally
aligned. We study the habitability of the outer planet through the
yearly-averaged insolation and black-body temperature at the pole. Both undergo
large variations because of its high eccentricity and are much more intense
than on Earth. The insolation variations are precession dominated with periods
of 40 kyr and 102 kyr for precession and obliquity if the rotation period is 3
d. A rotation period of about 1.5 d could cause extreme obliquity variations
because of capture in a Cassini state. For faster rotation rates the periods
converge to 10 kyr and 20 kyr. The large uncertainty in the precession period
does not change the overall outcome. | astro-ph_EP |
Beyond Point Masses. II. Non-Keplerian Shape Effects are Detectable in
Several TNO Binaries: About 40 transneptunian binaries (TNBs) have fully determined orbits with
about 10 others being solved except for breaking the mirror ambiguity. Despite
decades of study almost all TNBs have only ever been analyzed with a model that
assumes perfect Keplerian motion (e.g., two point masses). In reality, all TNB
systems are non-Keplerian due to non-spherical shapes, possible presence of
undetected system components, and/or solar perturbations. In this work, we
focus on identifying candidates for detectable non-Keplerian motion based on
sample of 45 well-characterized binaries. We use MultiMoon, a non-Keplerian
Bayesian inference tool, to analyze published relative astrometry allowing for
non-spherical shapes of each TNB system's primary. We first reproduce the
results of previous Keplerian fitting efforts with MultiMoon, which serves as a
comparison for the non-Keplerian fits and confirms that these fits are not
biased by the assumption of a Keplerian orbit. We unambiguously detect
non-Keplerian motion in 8 TNB systems across a range of primary radii, mutual
orbit separations, and system masses. As a proof of concept for non-Keplerian
fitting, we perform detailed fits for (66652) Borasisi-Pabu, possibly revealing
a $J_2 \approx 0.44$, implying Borasisi (and/or Pabu) may be a contact binary
or an unresolved compact binary. However, full confirmation of this result will
require new observations. This work begins the next generation of TNB analyses
that go beyond the point mass assumption to provide unique and valuable
information on the physical properties of TNBs with implications for their
formation and evolution. | astro-ph_EP |
Galactic Punctuated Equilibrium: How to Undermine Carter's Anthropic
Argument in Astrobiology: We investigate a new strategy which can defeat the (in)famous Carter's
"anthropic" argument against extraterrestrial life and intelligence. In
contrast to those already considered by Wilson, Livio, and others, the present
approach is based on relaxing hidden uniformitarian assumptions, considering
instead a dynamical succession of evolutionary regimes governed by both global
(Galaxy-wide) and local (planet- or planetary system-limited) regulation
mechanisms. This is in accordance with recent developments in both astrophysics
and evolutionary biology. Notably, our increased understanding of the nature of
supernovae and gamma-ray bursts, as well as of strong coupling between the
Solar System and the Galaxy on one hand, and the theories of "punctuated
equilibria" of Eldredge and Gould and "macroevolutionary regimes" of Jablonski,
Valentine, et al. on the other, are in full accordance with the regulation-
mechanism picture. The application of this particular strategy highlights the
limits of application of Carter's argument, and indicates that in the real
universe its applicability conditions are not satisfied. We conclude that
drawing far-reaching conclusions about the scarcity of extraterrestrial
intelligence and the prospects of our efforts to detect it on the basis of this
argument is unwarranted. | astro-ph_EP |
Amino Acid Chiral Selection Via Weak Interactions in Stellar
Environments: Implications for the Origin of Life: Magnetochiral phenomena may be responsible for the selection of chiral states
of biomolecules in meteoric environments. For example, the Supernova Amino Acid
Processing (SNAAP) Model was proposed previously as a possible mode of
magnetochiral selection of amino acids by way of the weak interaction in strong
magnetic fields. In earlier work, this model was shown to produce an
enantiomeric excess (ee) as high as 0.014% for alanine. In this paper we
present the results of molecular quantum chemistry calculations from which
$ee$s are determined for the alpha-amino acids plus isovaline and norvaline,
which were found to have positive ees in meteorites. Calculations are performed
for both isolated and aqueous states. In some cases, the aqueous state was
found to produce larger $ee$s reaching values as high as a few percent under
plausible conditions. | astro-ph_EP |
Graze-and-Merge Collisions under External Perturbers: Graze-and-merge collisions (GMCs) are common multi-step mergers occurring in
low-velocity off-axis impacts between similar sized planetary bodies. The first
impact happens at somewhat faster than the mutual escape velocity; for typical
impact angles this does not result in immediate accretion, but the smaller body
is slowed down so that it loops back around and collides again, ultimately
accreting. The scenario changes in the presence of a third major body, i.e.
planets accreting around a star, or satellites around a planet. We find that
when the loop-back orbit remains inside roughly 1/3 of the Hill radius from the
target, then the overall process is not strongly affected. As the loop-back
orbit increases in radius, the return velocity and angle of the second
collision become increasingly random, with no record of the first collision's
orientation. When the loop-back orbit gets to about 3/4 of the Hill radius, the
path of smaller body is disturbed up to the point that it will usually escape
the target. | astro-ph_EP |
Mass loading at 67P/Churyumov-Gerasimenko: a case study: We study the dynamics of the interaction between the solar wind ions and a
partially ionized atmosphere around a comet, at a distance of 2.88 AU from the
sun during a period of low nucleus activity. Comparing particle data and mag-
netic field data for a case study, we highlight the prime role of the solar
wind electric field in the cometary ion dynamics. Cometary ion and solar wind
proton flow directions evolve in a correlated manner, as expected from the
theory of mass loading. We find that the main component of the acceler- ated
cometary ion flow direction is along the anti-sunward direction, and not along
the convective electric field direc- tion. This is interpreted as the effect of
an anti-sunward polarisation electric field adding up to the solar wind con-
vective electric field. | astro-ph_EP |
Cloud property trends in hot and ultra-hot giant gas planets (WASP-43b,
WASP-103b, WASP-121b, HAT-P-7b, and WASP-18b): Ultra-hot Jupiters are the hottest exoplanets discovered so far. Observations
begin to provide insight into the composition of their extended atmospheres and
their chemical day/night asymmetries. Both are strongly affected by cloud
formation. We explore trends in cloud properties for a sample of five giant gas
planets: WASP-43b, WASP-18b, HAT-P-7b, WASP-103b, and WASP-121b. This provides
a reference frame for cloud properties for the JWST targets WASP-43b and
WASP-121b. We further explore chemically inert tracers to observe geometrical
asymmetries, and if the location of inner boundary of a 3D GCM matters for the
clouds that form. The large day/night temperature differences of ultra-hot
Jupiters cause large chemical asymmetries: cloud-free days but cloudy nights,
atomic vs. molecular gases and respectively different mean molecular weights,
deep thermal ionospheres vs. low-ionised atmospheres, undepleted vs enhanced
C/O. WASP-18b, as the heaviest planet in the sample, has the lowest global C/O.
The global climate may be considered as similar amongst ultra-hot Jupiters, but
different to that of hot gas giants. The local weather, however, is individual
for each planet since the local thermodynamic conditions, and hence the local
cloud and gas properties, differ. The morning and the evening terminator of
ultra-hot Jupiters will carry signatures of their strong chemical asymmetry
such that ingress/egress asymmetries can be expected. An increased C/O ratio is
a clear sign of cloud formation, making cloud modelling a necessity when
utilizing C/O (or other mineral ratios) as tracer for planet formation. The
changing geometrical extension of the atmosphere from the day to the nightside
may be probed through chemically inert species like helium. Ultra-hot Jupiters
are likely to develop deep atmospheric ionospheres which may impact the
atmosphere dynamics through MHD processes. | astro-ph_EP |
Trapping dust particles in the outer regions of protoplanetary disks: In order to explain grain growth to mm sized particles and their retention in
outer regions of protoplanetary disks, as it is observed at sub-mm and mm
wavelengths, we investigate if strong inhomogeneities in the gas density
profiles can slow down excessive radial drift and can help dust particles to
grow. We use coagulation/fragmentation and disk-structure models, to simulate
the evolution of dust in a bumpy surface density profile which we mimic with a
sinusoidal disturbance. For different values of the amplitude and length scale
of the bumps, we investigate the ability of this model to produce and retain
large particles on million years time scales. In addition, we introduced a
comparison between the pressure inhomogeneities considered in this work and the
pressure profiles that come from magnetorotational instability. Using the
Common Astronomy Software Applications ALMA simulator, we study if there are
observational signatures of these pressure inhomogeneities that can be seen
with ALMA. We present the favorable conditions to trap dust particles and the
corresponding calculations predicting the spectral slope in the mm-wavelength
range, to compare with current observations. Finally we present simulated
images using different antenna configurations of ALMA at different frequencies,
to show that the ring structures will be detectable at the distances of the
Taurus Auriga or Ophiucus star forming regions. | astro-ph_EP |
Microbial Fuel Cells Applied to the Metabolically-Based Detection of
Extraterrestrial Life: Since the 1970's, when the Viking spacecrafts carried out experiments aimed
to the detection of microbial metabolism on the surface of Mars, the search for
nonspecific methods to detect life in situ has been one of the goals of
astrobiology. It is usually required that the methodology can detect life
independently from its composition or form, and that the chosen biological
signature points to a feature common to all living systems, as the presence of
metabolism. In this paper we evaluate the use of Microbial Fuel Cells (MFCs)
for the detection of microbial life in situ. MFCs are electrochemical devices
originally developed as power electrical sources, and can be described as fuel
cells in which the anode is submerged in a medium that contains microorganisms.
These microorganisms, as part of their metabolic process, oxidize organic
material releasing electrons that contribute to the electric current, which is
therefore proportional to metabolic and other redox processes. We show that
power and current density values measured in MFCs using microorganism cultures
or soil samples in the anode are much larger than those obtained using a medium
free of microorganisms or sterilized soil samples, respectively. In particular,
we found that this is true for extremophiles, usually proposed to live in
extraterrestrial environments. Therefore, our results show that MFCs have the
potential to be used to detect microbial life in situ. | astro-ph_EP |
New constraints on the planetary system around the young active star AU
Mic. Two transiting warm Neptunes near mean-motion resonance: AU Mic is a young, active star whose transiting planet was recently detected.
We report our analysis of its TESS data, where we modeled the BY Draconis type
quasi-periodic rotational modulation by starspots simultaneously to the flaring
activity and planetary transits. We measured a flare occurrence rate of 6.35
flares per day for flares with amplitudes in the range of $0.06\% < f_{\rm max}
< 1.5\%$ of the star flux. We employed a Bayesian MCMC analysis to model the
five transits of AU Mic b, improving the constraints on the planetary
parameters. The planet radius of $4.07\pm0.17$~R$_{\oplus}$ and a mean density
of $1.4\pm0.4$~g~cm$^{-3}$ confirms that it is a Neptune-size moderately
inflated planet. While a single feature possibly due to a second planet was
previously reported in the former TESS data, we report the detection of two
additional transit-like events in the new TESS observations of July 2020. This
represents substantial evidence for a second planet (AU Mic c) in the system.
We analyzed its three transits and obtained an orbital period of
$18.859019\pm0.000016$~d and a planetary radius of $3.24\pm0.16$~R$_{\oplus}$,
which defines it as a warm Neptune-size planet with an expected mass in the
range of 2.2~M$_{\oplus}$~$< M_{\rm c} < $25.0~M$_{\oplus}$. The two planets in
the system are in near 9:4 mean-motion resonance. We show that this
configuration is dynamically stable and should produce transit-timing
variations (TTV). Our non-detection of significant TTV in AU Mic b suggests an
upper limit for the mass of AU Mic c of $<7$~M$_{\oplus}$, indicating that this
planet is also likely to be inflated. As a young multi-planet system with at
least two transiting planets, AU Mic becomes a key system for the study of
atmospheres of infant planets and of planet-planet and planet-disk dynamics at
the early stages of planetary evolution. | astro-ph_EP |
Impacts of Dust Grains Accelerated by Supernovae on the Moon: There is evidence that ejecta from nearby supernovae have rained down on
Earth in the past. Supernovae can accelerate pre-existing dust grains in the
interstellar medium to speeds of $\sim 0.01 \mathrm{\;c}$. We investigate the
survival and impact of dust grains from supernovae on the moon, finding that
supernova dust grains can form detectable tracks with widths of $\sim 0.01 -
0.07 \mathrm{\; \mu m}$ and depths of $\sim 0.1 - 0.7 \mathrm{\; mm}$ in lunar
rocks. These tracks could potentially shed light on the timings, luminosities,
and directions of nearby supernovae. | astro-ph_EP |
UKIRT microlensing surveys as a pathfinder for $WFIRST$: The detection
of five highly extinguished low-$|b|$ events: Optical microlensing surveys are restricted from detecting events near the
Galactic plane and center, where the event rate is thought to be the highest,
due to the high optical extinction of these fields. In the near-infrared (NIR),
however, the lower extinction leads to a corresponding increase in event
detections and is a primary driver for the wavelength coverage of the $WFIRST$
microlensing survey. During the 2015 and 2016 bulge observing seasons we
conducted NIR microlensing surveys with UKIRT in conjunction with and in
support of the $Spitzer$ and $Kepler$ microlensing campaigns. Here we report on
five highly extinguished ($A_H=0.81-1.97$), low-Galactic latitude ($-0.98\le
b\le -0.36$) microlensing events discovered from our 2016 survey. Four of them
were monitored with an hourly cadence by optical surveys but were not reported
as discoveries, likely due to the high extinction. Our UKIRT surveys and
suggested future NIR surveys enable the first measurement of the microlensing
event rate in the NIR. This wavelength regime overlaps with the bandpass of the
filter in which the $WFIRST$ microlensing survey will conduct its
highest-cadence observations, making this event rate derivation critically
important for optimizing its yield. | astro-ph_EP |
Titan-Hyperion Resonance and the Tidal Q of Saturn: Lainey et al. (2012), by re-analyzing long-baseline astrometry of Saturn's
moons, have found that the moons' tidal evolution is much faster than
previously thought, implying an order of magnitude stronger tidal dissipation
within Saturn. This result is controversial and implies recent formation of at
least some of the mid-sized icy moons of Saturn. Here we show that this more
intensive tidal dissipation is in full agreement with the evolved state of the
Titan-Hyperion resonance. This resonance was previously thought to be non-tidal
in origin, as the amount of tidal evolution required for its assembly is beyond
what is possible in models that assume that all the major moons are primordial.
We find that the survival of the Titan-Hyperion resonance is in agreement with
a past Titan-Iapetus 5:1 resonance, but not with unbroken tidal evolution of
Rhea from the rings to its current distance. | astro-ph_EP |
OGLE-2018-BLG-0567Lb and OGLE-2018-BLG-0962Lb: Two Microlensing Planets
through Planetary-Caustic Channel: We present the analyses of two microlensing events, OGLE-2018-BLG-0567 and
OGLE-2018-BLG-0962. In both events, the short-lasting anomalies were densely
and continuously covered by two high-cadence surveys. The light-curve modeling
indicates that the anomalies are generated by source crossings over the
planetary caustics induced by planetary companions to the hosts. The estimated
planet/host separation (scaled to the angular Einstein radius $\theta_{\rm E}$)
and mass ratio are $(s, q) = (1.81, 1.24\times10^{-3})$ and $(s, q) = (1.25,
2.38\times10^{-3})$, respectively. From Bayesian analyses, we estimate the host
and planet masses as $(M_{\rm h}, M_{\rm p}) =
(0.24_{-0.13}^{+0.16}\,M_{\odot}, 0.32_{-0.16}^{+0.34}\,M_{\rm J})$ and
$(M_{\rm h}, M_{\rm p}) = (0.55_{-0.29}^{+0.32}\,M_{\odot},
1.37_{-0.72}^{+0.80}\,M_{\rm J})$, respectively. These planetary systems are
located at a distance of $7.07_{-1.15}^{+0.93}\,{\rm kpc}$ for
OGLE-2018-BLG-0567 and $6.47_{-1.73}^{+1.04}\,{\rm kpc}$ for
OGLE-2018-BLG-0962, suggesting that they are likely to be near the Galactic
bulge. The two events prove the capability of current high-cadence surveys for
finding planets through the planetary-caustic channel. We find that most
published planetary-caustic planets are found in Hollywood events in which the
source size strongly contributes to the anomaly cross section relative to the
size of the caustic. | astro-ph_EP |
Asteroid 2014 OL339: yet another Earth quasi-satellite: Our planet has one permanently bound satellite -the Moon-, a likely large
number of mini-moons or transient irregular natural satellites, and three
temporary natural retrograde satellites or quasi-satellites. These quasi-moons
-(164207) 2004 GU9, (277810) 2006 FV35 and 2013 LX28- are unbound companions to
the Earth. The orbital evolution of quasi-satellites may transform them into
temporarily bound satellites of our planet. Here, we study the dynamical
evolution of the recently discovered Aten asteroid 2014 OL339 to show that it
is currently following a quasi-satellite orbit with respect to the Earth. This
episode started at least about 775 yr ago and it will end 165 yr from now. The
orbit of this object is quite chaotic and together with 164207 are the most
unstable of the known Earth quasi-satellites. This group of minor bodies is,
dynamically speaking, very heterogeneous but three of them exhibit Kozai-like
dynamics: the argument of perihelion of 164207 oscillates around -90 degrees,
the one of 277810 librates around 180 degrees and that of 2013 LX28 remains
around 0 degrees. Asteroid 2014 OL339 is not currently engaged in any
Kozai-like dynamics. | astro-ph_EP |
Long-Lived Dust Asymmetries at Dead Zone Edges in Protoplanetary Disks: A number of transition disks exhibit significant azimuthal asymmetries in
thermal dust emission. One possible origin for these asymmetries is dust
trapping in vortices formed at the edges of dead zones. We carry out
high-resolution, two-dimensional hydrodynamic simulations of this scenario,
including the effects of dust feedback. We find that, although feedback weakens
the vortices and slows down the process of dust accumulation, the dust
distribution in the disk can nonetheless remain asymmetric for many thousands
of orbits. We show that even after $10^4$ orbits, or $2.5$ Myr when scaled to
the parameters of Oph IRS 48 (a significant fraction of its age), the dust is
not dispersed into an axisymmetric ring, in contrast to the case of a vortex
formed by a planet. This is because accumulation of mass at the dead zone edge
constantly replenishes the vortex, preventing it from being fully destroyed. We
produce synthetic dust emission images using our simulation results. We find
that multiple small clumps of dust may be distributed azimuthally. These
clumps, if not resolved from one another, appear as a single large feature. A
defining characteristic of a disk with a dead zone edge is that an asymmetric
feature is accompanied by a ring of dust located about twice as far from the
central star. | astro-ph_EP |
Seasonal Evolution of Saturn's Polar Temperatures and Composition: The seasonal evolution of Saturn's polar atmospheric temperatures and
hydrocarbon composition is derived from a decade of Cassini Composite Infrared
Spectrometer (CIRS) 7-16 $\mu$m thermal infrared spectroscopy. We construct a
near-continuous record of atmospheric variability poleward of 60$^\circ$ from
northern winter/southern summer (2004, $L_s=293^\circ$) through the equinox
(2009, $L_s=0^\circ$) to northern spring/southern autumn (2014,
$L_s=56^\circ$). The hot tropospheric polar cyclones and the hexagonal shape of
the north polar belt are both persistent features throughout the decade of
observations. The hexagon vertices rotated westward by $\approx30^\circ$
longitude between March 2007 and April 2013, confirming that they are not
stationary in the Voyager-defined System III longitude system as previously
thought. The extended region of south polar stratospheric emission has cooled
dramatically poleward of the sharp temperature gradient near 75$^\circ$S,
coinciding with a depletion in the abundances of acetylene and ethane, and
suggestive of stratospheric upwelling with vertical wind speeds of
$w\approx+0.1$ mm/s. This is mirrored by a general warming of the northern
polar stratosphere and an enhancement in acetylene and ethane abundances that
appears to be most intense poleward of 75$^\circ$N, suggesting subsidence at
$w\approx-0.15$ mm/s. However, the sharp gradient in stratospheric emission
expected to form near 75$^\circ$N by northern summer solstice (2017,
$L_s=90^\circ$) has not yet been observed, so we continue to await the
development of a northern summer stratospheric vortex. North polar minima in
tropospheric and stratospheric temperatures were detected in 2008-2010 (lagging
one season, or 6-8 years, behind winter solstice); south polar maxima appear to
have occurred before the start of the Cassini observations (1-2 years after
summer solstice). [Abridged] | astro-ph_EP |
The effect of Jupiter oscillations on Juno gravity measurements: Seismology represents a unique method to probe the interiors of giant
planets. Recently, Saturn's f-modes have been indirectly observed in its rings,
and there is strong evidence for the detection of Jupiter global modes by means
of ground-based, spatially-resolved, velocimetry measurements. We propose to
exploit Juno's extremely accurate radio science data by looking at the gravity
perturbations that Jupiter's acoustic modes would produce. We evaluate the
perturbation to Jupiter's gravitational field using the oscillation spectrum of
a polytrope with index 1 and the corresponding radial eigenfunctions. We show
that Juno will be most sensitive to the fundamental mode ($n=0$), unless its
amplitude is smaller than 0.5 cm/s, i.e. 100 times weaker than the $n \sim\ 4 -
11$ modes detected by spatially-resolved velocimetry. The oscillations yield
contributions to Juno's measured gravitational coefficients similar to or
larger than those expected from shallow zonal winds (extending to depths less
than 300 km). In the case of a strong f-mode (radial velocity $\sim$ 30 cm/s),
these contributions would become of the same order as those expected from deep
zonal winds (extending to 3000 km), especially on the low degree zonal
harmonics, therefore requiring a new approach to the analysis of Juno data. | astro-ph_EP |
Episodic deluges in simulated hothouse climates: Earth's distant past and potentially its future include extremely warm
"hothouse" climate states, but little is known about how the atmosphere behaves
in such states. One distinguishing characteristic of hothouse climates is that
they feature lower-tropospheric radiative heating, rather than cooling, due to
the closing of the water vapor infrared window regions. Previous work has
suggested that this could lead to temperature inversions and significant
changes in cloud cover, but no previous modeling of the hothouse regime has
resolved convective-scale turbulent air motions and cloud cover directly, thus
leaving many questions about hothouse radiative heating unanswered. Here, we
conduct simulations that explicitly resolve convection and find that
lower-tropospheric radiative heating in hothouse climates causes the hydrologic
cycle to shift from a quasi-steady regime to a "relaxation oscillator" regime,
in which precipitation occurs in short and intense outbursts separated by
multi-day dry spells. The transition to the oscillatory regime is accompanied
by strongly enhanced local precipitation fluxes, a significant increase in
cloud cover, and a transiently positive (unstable) climate feedback parameter.
Our results indicate that hothouse climates may feature a novel form of
"temporal" convective self-organization, with implications for both cloud
coverage and erosion processes. | astro-ph_EP |
The first multi-dimensional view of mass loss from externally FUV
irradiated protoplanetary discs: Computing the flow from externally FUV irradiated protoplanetary discs
requires solving complicated and expensive photodissociation physics
iteratively in conjunction with hydrodynamics. Previous studies have therefore
been limited to 1D models of this process. In this paper we compare
2D-axisymmetric models of externally photoevaporating discs with their 1D
analogues, finding that mass loss rates are consistent to within a factor four.
The mass loss rates in 2D are higher, in part because half of the mass loss
comes from the disc surface (which 1D models neglect). 1D mass loss rates used
as the basis for disc viscous evolutionary calculations are hence expected to
be conservative. We study the anatomy of externally driven winds including the
streamline morphology, kinematic, thermal and chemical structure. A key
difference between the 1D and 2D models is in the chemical abundances. For
instance in the 2D models CO can be dissociated at smaller radial distances
from the disc outer edge than in 1D calculations because gas is
photodissociated by radiation along trajectories that are assumed infinitely
optically thick in 1D models. Multidimensional models will hence be critical
for predicting observable signatures of environmentally photoevaporating
protoplanetary discs. | astro-ph_EP |
In Search of Recent Disruption of (3200) Phaethon: Model Implication and
Hubble Space Telescope Search: Near-Earth asteroid (3200) Phaethon is notable for its association to a
strong annual meteor shower, the Geminids, indicative of one or more episodes
of mass ejection in the past. The mechanism of Phaethon's past activity is not
yet understood. Here we present a Hubble Space Telescope (HST) search of
meter-sized fragments in the vicinity of Phaethon, carried out during
Phaethon's historic approach to the Earth in mid-December of 2017. Numerical
simulation conducted to guide HST's pointing also show that the dynamical
evolution of Phaethon-originated particles is quick, as ejected materials take
no longer than $\sim250$ yr to spread to the entire orbit of Phaethon. Our
search was completed down to 4-meter-class limit (assuming Phaethon-like
albedo) and was expected to detect 0.035% particles ejected by Phaethon in the
last several decades. The negative result of our search capped the total mass
loss of Phaethon over the past few dozen orbits to be $10^{12}$ kg at $3\sigma$
level, taking the best estimates of size power-law from meteor observations and
spacecraft data. Our result also implies a millimeter-sized dust flux of
$<10^{-12} \mathrm{m^{-2} s^{-1}}$ within 0.1 au of Phaethon, suggesting that
any Phaethon-bound mission is unlikely to encounter dense dust clouds. | astro-ph_EP |
Pre-perihelion photometric behavior of comet C/2012 S1 (ISON) and its
future prospect: Comet C/2012 S1 (ISON) shows strange photometric behavior compared to many
past comets. Shortly after its discovery, majority of astronomers expected its
surviving until perihelion passage. Unprecedented evolution of its photometric
parameters nearly 2 AU from Sun, pushed this comet under survival line defined
by formula of John. E. Bortle, where ~70% comets in his analysis disintegrated.
However comparison with photometric behavior of group of past disintegrated
comets showing large differences and ISON doesn't seems to be typical member of
this group. In other hand, compared to surviving, dynamically new comets which
were observed far away from Sun and survived its perihelion passage, ISON
showing peculiar gradual decrease of its activity, probably caused by
exhaustion of active fractions of its nucleus. | astro-ph_EP |
Atmospheric Rossiter-McLaughlin effect and transmission spectroscopy of
WASP-121b with ESPRESSO: WASP-121b is one of the most studied Ultra-hot Jupiters: many recent analyses
of its atmosphere report interesting features at different wavelength ranges.
In this paper we analyze one transit of WASP-121b acquired with the
high-resolution spectrograph ESPRESSO at VLT in 1-telescope mode, and one
partial transit taken during the commissioning of the instrument in 4-telescope
mode. We investigate the anomalous in-transit radial velocity curve and study
the transmission spectrum of the planet. By analysing the in-transit radial
velocities we were able to infer the presence of the atmospheric
Rossiter-McLaughlin effect. We measured the height of the planetary atmospheric
layer that correlates with the stellar mask (mainly Fe) to be 1.052$\pm$0.015
Rp and we also confirmed the blueshift of the planetary atmosphere. By
examining the planetary absorption signal on the stellar cross-correlation
functions we confirmed the presence of a temporal variation of its blueshift
during transit, which could be investigated spectrum-by-spectrum. We detected
significant absorption in the transmission spectrum for Na, H, K, Li, CaII, Mg,
and we certified their planetary nature by using the 2D tomographic technique.
Particularly remarkable is the detection of Li, with a line contrast of
$\sim$0.2% detected at the 6$\sigma$ level. With the cross-correlation
technique we confirmed the presence of FeI, FeII, CrI and VI. H$\alpha$ and
CaII are present up to very high altitudes in the atmosphere ($\sim$1.44 Rp and
$\sim$2 Rp, respectively), and also extend beyond the transit-equivalent Roche
lobe radius of the planet. These layers of the atmosphere have a large line
broadening that is not compatible with being caused by the tidally-locked
rotation of the planet alone, and could arise from vertical winds or
high-altitude jets in the evaporating atmosphere. | astro-ph_EP |
An extensive radial velocity survey toward NGC 6253: The old and metal rich open cluster NGC 6253 was observed with the FLAMES
multi-object spectrograph during an extensive radial velocity campaign
monitoring 317 stars with a median of 15 epochs per object. All the targeted
stars are located along the upper main sequence of the cluster between 14.8 $<$
V $<$ 16.5. Fifty nine stars are confirmed cluster members both by radial
velocities and proper motions and do not show evidence of variability. We
detected 45 variable stars among which 25 belong to NGC 6253. We were able to
derive an orbital solution for 4 cluster members (and for 2 field stars)
yielding minimum masses in between $\sim$90 M$\rm_J$ and $\sim$460 M$\rm_J$ and
periods between 3 and 220 days. Simulations demonstrated that this survey was
sensitive to objects down to 30 M$\rm_J$ at 10 days orbital periods with a
detection efficiency equal to 50%. On the basis of these results we concluded
that the observed frequency of binaries down to the hydrogen burning limit and
up to 20 days orbital period is around (1.5$\pm$1.3)% in NGC 6253. The overall
observed frequency of binaries around the sample of cluster stars is
(13$\pm$3)%. The median radial velocity precision achieved by the GIRAFFE
spectrograph in this magnitude range was around $\sim$240m$\rm\,s^{-1}$
($\sim$180 m$\rm\,s^{-1}$ for UVES). Based on a limited follow-up analysis of 7
stars in our sample with the HARPS spectrograph we determined that a precision
of 35 m $\rm s^{-1}$ can be reached in this magnitude range, offering the
possibility to further extend the variability analysis into the substellar
domain. Prospects are even more favourable once considering the upcoming
ESPRESSO spectrograph at VLT. | astro-ph_EP |
Gaia Data Release 3: Reflectance spectra of Solar System small bodies: The Gaia mission of the European Space Agency (ESA) has been routinely
observing Solar System objects (SSOs) since the beginning of its operations in
August 2014. The Gaia data release three (DR3) includes, for the first time,
the mean reflectance spectra of a selected sample of 60 518 SSOs, primarily
asteroids, observed between August 5, 2014, and May 28, 2017. Each reflectance
spectrum was derived from measurements obtained by means of the Blue and Red
photometers (BP/RP), which were binned in 16 discrete wavelength bands. We
describe the processing of the Gaia spectral data of SSOs, explaining both the
criteria used to select the subset of asteroid spectra published in Gaia DR3,
and the different steps of our internal validation procedures. In order to
further assess the quality of Gaia SSO reflectance spectra, we carried out
external validation against SSO reflectance spectra obtained from ground-based
and space-borne telescopes and available in the literature. For each selected
SSO, an epoch reflectance was computed by dividing the calibrated spectrum
observed by the BP/RP at each transit on the focal plane by the mean spectrum
of a solar analogue. The latter was obtained by averaging the Gaia spectral
measurements of a selected sample of stars known to have very similar spectra
to that of the Sun. Finally, a mean of the epoch reflectance spectra was
calculated in 16 spectral bands for each SSO. The agreement between Gaia mean
reflectance spectra and those available in the literature is good for bright
SSOs, regardless of their taxonomic spectral class. We identify an increase in
the spectral slope of S-type SSOs with increasing phase angle. Moreover, we
show that the spectral slope increases and the depth of the 1 um absorption
band decreases for increasing ages of S-type asteroid families. | astro-ph_EP |
On possible life-dispersal patterns beyond the Earth: We model hypothetical bio-dispersal within a single Galactic region using the
stochastic infection dynamics process, which is inspired by these local
properties of life dispersal on Earth. We split the population of stellar
systems into different categories regarding habitability and evolved them
through time using probabilistic cellular automata rules analogous to the
model. As a dynamic effect, we include the existence of natural dispersal
vectors (e.g., dust, asteroids) in a way that avoids assumptions about their
agency. By assuming that dispersal vectors have a finite velocity and range,
the model includes the parameter of 'optical depth of life spreading'. The
effect of the oscillatory infection rate on the long-term behavior of the
dispersal flux, which adds a diffusive component to its progression, is also
taken into account. We found that phase space is separated into subregions of
long-lasting transmission, rapidly terminated transmission, and a transition
region between the two. We observed that depending on the amplitude of the
oscillatory life spreading rate, life-transmission in the Galactic patch might
take on different geometrical shapes. Even if some host systems are
uninhabited, life transmission has a certain threshold, allowing a patch to be
saturated with viable material over a long period. Although stochastic
fluctuations in the local density of habitable systems allow for clusters that
can continuously infect one another, the spatial pattern disappears when life
transmission is below the observed threshold, so that transmission process is
not permanent in time. Both findings suggest that a habitable planet in a
densely populated region may remain uninfected. | astro-ph_EP |
The Shape of Saturn's Huygens Ringlet Viewed by Cassini ISS: A new model for the shape of the prominent eccentric ringlet in the gap
exterior to Saturn's B-ring is developed based on Cassini imaging observations
taken over about 8 years. Unlike previous treatments, the new model treats each
edge of the ringlet separately. The Keplerian component of the model is
consistent with results derived from Voyager observations, and $m=2$ modes
forced by the nearby Mimas 2:1 Lindblad resonance are seen. Additionally, a
free $m=2$ mode is seen on the outer edge of the ringlet. Significant irregular
structure that cannot be described using normal-mode analysis is seen on the
ringlet edges as well. Particularly on the inner edge, that structure remains
coherent over multi-year intervals, moving at the local Keplerian rate. We
interpret the irregular structure as the signature of embedded massive bodies.
The long coherence time suggests the responsible bodies are concentrated near
the edge of the ringlet. Long wake-like structures originate from two locations
on the inner edge of the ringlet, revealing the locations of the two most
massive embedded bodies in that region. As with the Voyager observations, the
Cassini data sets showed no correlation between the width and the radius of the
ringlet as would be expected for a self-gravitating configuration, except for a
brief interval during late 2006, when the width-radius relation was similar to
those seen in most other narrow eccentric ringlets in the Solar System. | astro-ph_EP |
The Physical Origin of the Venus Low Atmosphere Chemical Gradient: Venus shares many similarities with the Earth, but concomitantly, some of its
features are extremely original. This is especially true for its atmosphere,
where high pressures and temperatures are found at the ground level. In these
conditions, carbon dioxide, the main component of Venus' atmosphere, is a
supercritical fluid. The analysis of VeGa-2 probe data has revealed the high
instability of the region located in the last few kilometers above the ground
level. Recent works have suggested an explanation based on the existence of a
vertical gradient of molecular nitrogen abundances, around 5 ppm per meter. Our
goal was then to identify which physical processes could lead to the
establishment of this intriguing nitrogen gradient, in the deep atmosphere of
Venus. Using an appropriate equation of state for the binary mixture CO2-N2
under supercritical conditions, and also molecular dynamics simulations, we
have investigated the separation processes of N2 and CO2 in the Venusian
context. Our results show that molecular diffusion is strongly inefficient, and
potential phase separation is an unlikely mechanism. We have compared the
quantity of CO2 required to form the proposed gradient with what could be
released by a diffuse degassing from a low volcanic activity. The needed fluxes
of CO2 are not so different from what can be measured over some terrestrial
volcanic systems, suggesting a similar effect at work on Venus. | astro-ph_EP |
Climate Modeling of a Potential ExoVenus: The planetary mass and radius sensitivity of exoplanet discovery capabilities
has reached into the terrestrial regime. The focus of such investigations is to
search within the Habitable Zone where a modern Earth-like atmosphere may be a
viable comparison. However, the detection bias of the transit and radial
velocity methods lies close to the host star where the received flux at the
planet may push the atmosphere into a runaway greenhouse state. One such
exoplanet discovery, Kepler-1649b, receives a similar flux from its star as
modern Venus does from the Sun, and so was categorized as a possible exoVenus.
Here we discuss the planetary parameters of Kepler-1649b with relation to Venus
to establish its potential as a Venus analog. We utilize the general
circulation model ROCKE-3D to simulate the evolution of the surface temperature
of Kepler-1649b under various assumptions, including relative atmospheric
abundances. We show that in all our simulations the atmospheric model rapidly
diverges from temperate surface conditions towards a runaway greenhouse with
rapidly escalating surface temperatures. We calculate transmission spectra for
the evolved atmosphere and discuss these spectra within the context of the
James Webb Space Telescope (JWST) Near-Infrared Spectrograph (NIRSpec)
capabilities. We thus demonstrate the detectability of the key atmospheric
signatures of possible runaway greenhouse transition states and outline the
future prospects of characterizing potential Venus analogs. | astro-ph_EP |
An interstellar origin for Jupiter's retrograde co-orbital asteroid: Asteroid (514107) 2015 BZ509 was discovered recently in Jupiter's co-orbital
region with a retrograde motion around the Sun. The known chaotic dynamics of
the outer Solar System have so far precluded the identification of its origin.
Here, we perform a high-resolution statistical search for stable orbits and
show that asteroid (514107) 2015 BZ509 has been in its current orbital state
since the formation of the Solar System. This result indicates that (514107)
2015 BZ509 was captured from the interstellar medium 4.5 billion years in the
past as planet formation models cannot produce such a primordial
large-inclination orbit with the planets on nearly-coplanar orbits interacting
with a coplanar debris disk that must produce the low-inclination small-body
reservoirs of the Solar System such as the asteroid and Kuiper belts. This
result also implies that more extrasolar asteroids are currently present in the
Solar System on nearly-polar orbits. | astro-ph_EP |
Detection of Rotational Variability in Floofy Objects at Optical
Wavelengths: Phase resolved observations of planetary bodies allow us to understand the
longitudinal and latitudinal variations that make each one unique. Rotational
variations have been detected in several types of astronomical bodies beyond
those of planetary mass, including asteroids, brown dwarfs, and stars.
Unexpected rotational variations, such as those presented in this work, reminds
us that the universe can be complicated, with more mysteries to uncover. In
this work we present evidence for a new class of astronomical objects we
identify as "floofy" with observational distinctions between several sub-types
of these poorly understood objects. Using optical observations contributed by
the community, we have identified rotational variation in several of these
floofy objects, which suggests that they may have strong differences between
their hemispheres, likely caused by differing reflectivity off their surfaces.
Additional sub-types show no rotational variability suggesting a uniform
distribution of reflective elements on the floofy object. While the work here
is a promising step towards the categorization of floofy objects, further
observations with more strictly defined limits on background light,
illumination angles, and companion objects are necessary to develop a better
understanding of the many remaining mysteries of these astronomical objects. | astro-ph_EP |
Physical Characterization of ~2-meter Diameter Near-Earth Asteroid 2015
TC25: A possible boulder from E-type Asteroid (44) Nysa: Small near-Earth asteroids (>20 meters) are interesting because they are
progenitors for meteorites in our terrestrial collection. Crucial to our
understanding of the effectiveness of our atmosphere in filtering low-strength
impactors is the physical characteristics of these small near-Earth asteroids
(NEAs). In the past, characterization of small NEAs has been a challenge
because of the difficulty in detecting them prior to close Earth flyby. In this
study we physically characterized the 2-meter diameter near-Earth asteroid 2015
TC25 using ground-based optical, near-infrared and radar assets during a close
flyby of the Earth (distance 69,000 miles) in Oct. 2015. Our observations
suggest that its surface composition is similar to aubrites, a rare class of
high albedo differentiated meteorites. Aubrites make up only 0.14 % of all know
meteorites in our terrestrial meteorite collection. 2015 TC25 is also a very
fast rotator with a rotation period of 133 seconds. We compared spectral and
dynamical properties of 2015 TC25 and found the best candidate source body in
the inner main belt to be the 70-km diameter E-type asteroid (44) Nysa. We
attribute difference in spectral slope between the two objects to the lack of
regolith on the surface of 2015 TC25. Using the albedo of E-type asteroids
(50-60%) we refine the diameter of 2015 TC25 to 2-meters making it one of the
smallest NEA ever to be characterized. | astro-ph_EP |
Super stellar abundances of alkali metals suggest significant migration
for Hot Jupiters: We investigate the origin of the measured over-abundance of alkali metals in
the atmospheres of hot gas giants, relative to both their host stars and their
atmospheric water abundances. We show that formation exterior to the water snow
line followed by inward disc-driven migration results in excess accretion of
oxygen-poor, refractory-rich material from within the snow-line. This naturally
leads to enrichment of alkali metals in the planetary atmosphere relative to
the bulk composition of its host star but relative abundances of water that are
similar to the stellar host. These relative abundances cannot be explained by
in situ formation which places the refractory elements in the planetary deep
interior rather than the atmosphere. We therefore suggest that the measured
compositions of the atmospheres of hot Jupiters are consistent with significant
migration for at least a subset of hot gas giants. Our model makes robust
predictions about atmospheric composition that can be confirmed with future
data from JWST and Ariel. | astro-ph_EP |
The Evolving Photometric Lightcurve of Comet 1P/Halley's Coma During the
1985/86 Apparition: We present new analyses of the photometric lightcurve of Comet 1P/Halley
during its 1985/86 apparition. As part of a world-wide campaign coordinated by
the International Halley Watch (IHW), narrowband photometry was obtained with
telescopes at 18 observatories. Following submissions to and basic reductions
by the Photometry and Polarimetry Network of the IHW, we computed production
rates and created composite lightcurves for each species. These were used to
measure how the apparent rotational period (~7.35 day), along with its shape,
evolved with time during the apparition. The lightcurve shape systematically
varied from double-peaked to triple-peaked and back again every 8-9 weeks, due
to Halley's non-principal axis (complex) rotation and the associated component
periods. Unexpectedly, we found a phase shift of one-half cycle also took place
during this interval, and therefore the actual beat frequency between the
component periods is twice this interval or 16-18 weeks. Preliminary modeling
suggests that a single source might produce the entire post-perihelion
lightcurve variability and associated evolution. The detailed evolution of the
apparent period varied in a non-smooth manner between 7.2 and 7.6 day, likely
due to a combination of synodic effects and the interaction of solar
illumination with isolated source regions on a body in complex rotation. The
need to simultaneously reproduce each of these characteristics will provide
very strong additional constraints on Halley's component periods associated
with its complex rotation. To assist in these and future analyses, we created a
synthetic lightcurve based directly on the measured data. We unexpectedly
discovered a strong correlation of ion tail disconnection event start times
with minima in the comet's gas production, implying that a decrease in
outgassing is another cause of these events. | astro-ph_EP |
Electric charging of dust aggregates and its effect on dust coagulation
in protoplanetary disks: Mutual sticking of dust aggregates is the first step toward planetesimal
formation in protoplanetary disks. In spite that the electric charging of dust
particles is well recognized in some contexts, it has been largely ignored in
the current modeling of dust coagulation. In this study, we present a general
analysis of the dust charge state in protoplanetary disks, and then demonstrate
how the electric charging could dramatically change the currently accepted
scenario of dust coagulation. First, we describe a new semianalytical method to
calculate the dust charge state and gas ionization state self-consistently.
This method is far more efficient than previous numerical methods, and provides
a general and clear description of the charge state of gas-dust mixture.
Second, we apply this analysis to early evolutionary stages where the dust has
been thought to grow into fractal ($D \sim 2$) aggregates with a
quasi-monodisperse (i.e., narrow) size distribution. We find that, for a wide
range of model parameters, the fractal growth is strongly inhibited by the
electric repulsion between colliding aggregates and eventually "freezes out" on
its way to the subsequent growth stage involving collisional compression.
Strong disk turbulence would help the aggregates to overcome this growth
barrier, but then it would cause catastrophic collisional fragmentation in
later growth stages. These facts suggest that the combination of electric
repulsion and collisional fragmentation would impose a serious limitation on
dust growth in protoplanetary disks. We propose a possible scenario of dust
evolution after the freeze-out. Finally, we point out that the fractal growth
of dust aggregates tends to maintain a low ionization degree and, as a result,
a large magnetorotationally stable region in the disk. | 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 |
TERMS Photometry of Known Transiting Exoplanets: The Transit Ephemeris Refinement and Monitoring Survey (TERMS) conducts
radial velocity and photometric monitoring of known exoplanets in order to
refine planetary orbits and predictions of possible transit times. This effort
is primarily directed towards planets not known to transit, but a small sample
of our targets consist of known transiting systems. Here we present precision
photometry for 6 WASP planets acquired during their transit windows. We perform
a Markov Chain Monte Carlo (MCMC) analysis for each planet and combine these
data with previous measurements to redetermine the period and ephemerides for
these planets. These observations provide recent mid-transit times which are
useful for scheduling future observations. Our results improve the ephemerides
of WASP-4b, WASP-5b and WASP-6b and reduce the uncertainties on the mid-transit
time for WASP-29b. We also confirm the orbital, stellar and planetary
parameters of all 6 systems. | astro-ph_EP |
H-Atmospheres of Icy Super-Earths Formed in situ in the Outer Solar
System: An Application to a Possible Planet Nine: We examine the possibility that icy super-Earth mass planets, formed over
long time scales (0.1--1~Gyr) at large distances ($\sim$ 200--1000~AU) from
their host stars, will develop massive H-rich atmospheres. Within the interior
of these planets, high pressure converts CH$_4$ into ethane, butane, or diamond
and releases H$_2$. Using simplified models which capture the basic physics of
the internal structure, we show that the physical properties of the atmosphere
depend on the outflux of H$_2$ from the mantle. When this outflux is $\lesssim
10^{10}$ [molec cm$^{-2}$ s$^{-1}$], the outgassed atmosphere has base pressure
$\lesssim$ 1 bar. Larger outflows result in a substantial atmosphere where the
base pressure may approach $10^3 - 10^4$ bar. For any pressure, the mean
density of these planets, 2.4--3 [g cm$^{-3}$], is much larger than the mean
density of Uranus and Neptune, 1.3--1.6 [g cm$^{-3}$]. Thus, observations can
distinguish between a Planet Nine with a primordial H/He-rich atmosphere
accreted from the protosolar nebula and one with an atmosphere outgassed from
the core. | astro-ph_EP |
Properties and occurrence rates of $Kepler$ exoplanet candidates as a
function of host star metallicity from the DR25 catalog: Correlations between the occurrence rate of exoplanets and their host star
properties provide important clues about the planet formation processes. We
studied the dependence of the observed properties of exoplanets (radius, mass,
and orbital period) as a function of their host star metallicity. We analyzed
the planetary radii and orbital periods of over 2800 $Kepler$ candidates from
the latest $Kepler$ data release DR25 (Q1-Q17) with revised planetary radii
based on $Gaia$~DR2 as a function of host star metallicity (from the Q1-Q17
(DR25) stellar and planet catalog). With a much larger sample and improved
radius measurements, we are able to reconfirm previous results in the
literature. We show that the average metallicity of the host star increases as
the radius of the planet increases. We demonstrate this by first calculating
the average host star metallicity for different radius bins and then
supplementing these results by calculating the occurrence rate as a function of
planetary radius and host star metallicity. We find a similar trend between
host star metallicity and planet mass: the average host star metallicity
increases with increasing planet mass. This trend, however, reverses for masses
$> 4.0\, M_\mathrm{J}$: host star metallicity drops with increasing planetary
mass. We further examined the correlation between the host star metallicity and
the orbital period of the planet. We find that for planets with orbital periods
less than 10 days, the average metallicity of the host star is higher than that
for planets with periods greater than 10 days. | astro-ph_EP |
The time evolution of $M_{\mathrm{d}}/\dot M$ in protoplanetary discs as
a way to disentangle between viscosity and MHD winds: As the classic viscous paradigm for protoplanetary disk accretion is
challenged by the observational evidence of low turbulence, the alternative
scenario of MHD disk winds is being explored as potentially able to reproduce
the same observed features traditionally explained with viscosity. Although the
two models lead to different disk properties, none of them has been ruled out
by observations - mainly due to instrumental limitations. In this work, we
present a viable method to distinguish between the viscous and MHD framework
based on the different evolution of the distribution in the disk mass
($M_{\mathrm{d}}$) - accretion rate ($\dot M$) plane of a disk population. With
a synergy of analytical calculations and 1D numerical simulations, performed
with the population synthesis code \texttt{Diskpop}, we find that both
mechanisms predict the spread of the observed ratio $M_{\mathrm{d}}/\dot M$ in
a disk population to decrease over time; however, this effect is much less
pronounced in MHD-dominated populations as compared to purely viscous
populations. Furthermore, we demonstrate that this difference is detectable
with the current observational facilities: we show that convolving the
intrinsic spread with the observational uncertainties does not affect our
result, as the observed spread in the MHD case remains significantly larger
than in the viscous scenario. While the most recent data available show a
better agreement with the wind model, ongoing and future efforts to obtain
direct gas mass measurements with ALMA and ngVLA will cause a reassessment of
this comparison in the near future. | astro-ph_EP |
Segregation on small rubble bodies due to impact-induced seismic shaking: We present a framework to study regolith segregation on rubble-pile asteroids
(self-gravitating granular aggregates) due to seismic shaking induced by
impacts sustained during their lifetimes. We first relate the amplitude and
frequency of surface vibrations to the location and severity of an impact, and
the rubble body's geometry and bulk properties. For clarity, the body is taken
to be an ellipsoid with size and spin close to that of Itokawa, although other
asteroids are also easily incorporated. We then model the body's collisional
history stochastically given the variability in the impact activity on an
asteroid. Finally, we utilize discrete element simulations to investigate the
regolith's response to impacts. In these simulations, in any sample collisional
history, every time an impact occurs, a bin filled with a grain mixture and
located at the region of interest on the asteroid is vibrated at that impact's
associated amplitude and frequency. Utilizing this framework we find that
impact-driven seismicity is sufficient to drive size segregation on small
rubble-piles, but the segregation quality depends on several aspects, e.g.
total impact energy supplied, placement of the region of interest, bulk wave
speed, and seismic diffusivity. | astro-ph_EP |
Shellspec39 -- a tool for modelling the spectra, light curves, and
images of interacting binaries and exoplanets: Program SHELLSPEC is designed to calculate light curves, spectra, and images
of interacting binaries and extra-solar planets immersed in a moving gaseous or
dusty circumstellar matter. It solves simple radiative transfer along the line
of sight in 3D moving media. The Roche model and synthetic spectra from the
stellar atmosphere models such as TLUSTY from Ivan Hubeny can be used as a
boundary condition for the radiative transfer. The latest publicly available
version is Shellspec39. The code has been combined with other methods such as
Doppler tomography and interferometry and used to analyze spectroscopic,
photometric, and interferometric observations of binary stars and transiting
exoplanets. A few examples are briefly mentioned. | astro-ph_EP |
Gas and multi-species dust dynamics in viscous protoplanetary discs: the
importance of the dust back-reaction: We study the dynamics of a viscous protoplanetary disc hosting a population
of dust grains with a range of sizes. We compute steady-state solutions, and
show that the radial motion of both the gas and the dust can deviate
substantially from those for a single-size dust population. Although the
aerodynamic drag from the dust on the gas is weaker than in the case where all
grains are optimally coupled to the gas, the cumulative "back-reaction" of the
dust particles can still alter the gas dynamics significantly. In typical
protoplanetary discs, the net effect of the dust back-reaction decreases the
gas accretion flow compared to the dust-free (viscous) case, even for
dust-to-gas ratios of order $1\%$. In the outer disc, where dust grains are
typically less strongly coupled to the gas and settle towards the midplane, the
dust back-reaction can even drive outward gas flow. Moreover, the radial inward
drift of large grains is reduced below the gas motion in the inner disc
regions, while small dust grains follow the gas dynamics over all the disc
extent. The resulting dust and gas dynamics can give rise to observable
structures, such as gas and dust cavities. Our results show that the dust
back-reaction can play a major role in both the dynamics and observational
appearance of protoplanetary discs, and cannot be ignored in models of
protoplanetary disc evolution. | astro-ph_EP |
Multi-wavelength aperture polarimetry of debris disc host stars: Debris discs around main sequence stars have been extensively characterised
from infrared to millimetre wavelengths through imaging, spectroscopic, and
total intensity (scattered light and/or thermal emission) measurements.
Polarimetric observations have only been used sparingly to interpret the
composition, structure, and size of dust grains in these discs. Here we present
new multi-wavelength aperture polarisation observations with parts-per-million
sensitivity of a sample of twelve bright debris discs, spanning a broad range
of host star spectral types, and disc properties. These measurements were
mostly taken with the HIgh Precision Polarimetric Instrument on the
Anglo-Australian Telescope. We combine these polarisation observations with the
known disc architectures and geometries of the discs to interpret the
measurements. We detect significant polarisation attributable to circumstellar
dust from HD 377 and HD 39060, and find tentative evidence for HD 188228 and HD
202628. | astro-ph_EP |
Multibeam Blind Search of Targeted SETI Observations toward 33 Exoplanet
Systems with FAST: The search for extraterrestrial intelligence (SETI) is to search for
technosignatures associated with extraterrestrial life, such as engineered
radio signals. In this paper, we apply the multibeam coincidence matching
(MBCM) strategy, and propose a new search mode based on the MBCM which we call
MBCM blind search mode. In our recent targeted SETI research, 33 exoplanet
systems are observed by the Five-hundred-meter Aperture Spherical radio
Telescope (FAST). With this blind search mode, we search for narrowband
drifting signals across $1.05-1.45$ GHz in two orthogonal linear polarization
directions separately. There are two special signals, one of which can only be
detected by the blind search mode while the other can be found by both blind
and targeted search modes. This result reveals huge advantages of the new blind
search mode. However, we eliminate the possibility of the special signals being
ETI signals based on much evidence, such as the polarization, drift, frequency
and beam coverage characteristics. Our observations achieve an unprecedented
sensitivity and our work provides a deeper understanding to the polarization
analysis of extraterrestrial signals. | astro-ph_EP |
Frequency of Close Companions among Kepler Planets - a TTV study: A transiting planet exhibits sinusoidal transit-time-variations (TTVs) if
perturbed by a companion near a mean-motion-resonance (MMR). We search for
sinusoidal TTVs in more than 2600 Kepler candidates, using the publicly
available Kepler light-curves (Q0-Q12). We find that the TTV fractions rise
strikingly with the transit multiplicity. Systems where four or more planets
transit enjoy roughly five times higher TTV fraction than those where a single
planet transits, and about twice higher than those for doubles and triples. In
contrast, models in which all transiting planets arise from similar dynamical
configurations predict comparable TTV fractions among these different systems.
One simple explanation for our results is that there are at least two different
classes of Kepler systems, one closely packed and one more sparsely populated. | astro-ph_EP |
Planet formation in small separation binaries: not so excited after all: Existence of planets is binaries with relatively small separations (around 20
AU), such as \alpha Centauri or \gamma Cephei poses severe challenges to
standard planet formation theories. The problem lies in the vigorous secular
excitation of planetesimal eccentricities at separations of several AU, where
some of the planets are found, by the massive, eccentric stellar companions.
High relative velocities of planetesimals preclude their growth in mutual
collisions for a wide range of sizes, from below 1 km up to several hundred km,
resulting in fragmentation barrier to planet formation. Here we show that rapid
apsidal precession of planetesimal orbits, caused by the gravity of the
circumstellar protoplanetary disk, acts to strongly reduce eccentricity
excitation, lowering planetesimal velocities by an order of magnitude or even
more at 1 AU. By examining the details of planetesimal dynamics we demonstrate
that this effect eliminates fragmentation barrier for in-situ growth of
planetesimals as small as < 10 km even at separations as wide as 2.6 AU
(semi-major axis of the giant planet in HD 196885), provided that the
circumstellar protoplanetary disk is relatively massive, ~0.1 M_Sun. | astro-ph_EP |
Formation of recurring slope lineae on Mars by rarefied gas-triggered
granular flows: Active dark flows known as recurring slope lineae have been observed on the
warmest slopes of equatorial Mars. The morphology, composition and seasonality
of the lineae suggest a role of liquid water in their formation. However,
internal and atmospheric sources of water appear to be insufficient to sustain
the observed slope activity. Experimental evidence suggests that under the low
atmospheric pressure at the surface of Mars, gas can flow upwards through
porous Martian soil due to thermal creep under surface regions heated by the
Sun, and disturb small particles. Here we present numerical simulations to
demonstrate that such a dry process involving the pumping of rarefied gas in
the Martian soil due to temperature contrasts can explain the formation of the
recurring slope lineae. In our simulations, solar irradiation followed by
shadow significantly reduces the angle of repose due to the resulting temporary
temperature gradients over shaded terrain, and leads to flow at intermediate
slope angles. The simulated flow locations are consistent with observed
recurring slope lineae that initiate in rough and bouldered terrains with local
shadows over the soil. We suggest that this dry avalanche process can explain
the formation of the recurring slope lineae on Mars without requiring liquid
water or CO2 frost activity. | astro-ph_EP |
OGLE-2017-BLG-0173Lb: Low Mass-Ratio Planet in a "Hollywood"
Microlensing Event: We present microlensing planet OGLE-2017-BLG-0173Lb, with planet-host mass
ratio either $q\simeq 2.5\times 10^{-5}$ or $q\simeq 6.5\times 10^{-5}$, the
lowest or among the lowest ever detected. The planetary perturbation is
strongly detected, $\Delta\chi^2\sim 10,000$, because it arises from a bright
(therefore, large) source passing over and enveloping the planetary caustic: a
so-called "Hollywood" event. The factor $\sim 2.5$ offset in $q$ arises because
of a previously unrecognized discrete degeneracy between Hollywood events in
which the caustic is fully enveloped and those in which only one flank is
enveloped, which we dub "Cannae" and "von Schlieffen", respectively. This
degeneracy is "accidental" in that it arises from gaps in the data.
Nevertheless, the fact that it appears in a $\Delta\chi^2=10,000$ planetary
anomaly is striking. We present a simple formalism to estimate the sensitivity
of other Hollywood events to planets and show that they can lead to detections
close to, but perhaps not quite reaching, the Earth/Sun mass ratio of $3\times
10^{-6}$. This formalism also enables an analytic understanding of the factor
$\sim 2.5$ offset in $q$ between the Cannae and von Schlieffen solutions. The
Bayesian estimates for the host-mass, system distance, and planet-host
projected separation are $M=0.39^{+0.40}_{-0.24}\,M_\odot$,
$D_L=4.8^{+1.5}_{-1.8}\,\kpc$, and $a_\perp=3.8\pm 1.6\,\au$. The two estimates
of the planet mass are $m_p=3.3^{+3.8}_{-2.1}\,M_\oplus$ and
$m_p=8^{+11}_{-6}\,M_\oplus$. The measured lens-source relative proper motion
$\mu=6\,\masyr$ will permit imaging of the lens in about 15 years or at first
light on adaptive-optics imagers on next-generation telescopes. These will
allow to measure the host mass but probably cannot resolve the planet-host
mass-ratio degeneracy. | astro-ph_EP |
Planetary Entry Probe Dataset: Analysis and Rules of Thumb for Future
Missions: Since the beginning of robotic interplanetary exploration nearly six decades
ago, successful atmospheric entry has been accomplished at Venus, Earth, Mars,
Jupiter, and Titan. More entry probe missions are planned to Venus, Titan, and
Uranus in the next decade. Atmospheric entry subjects the vehicle to rapid
deceleration and aerothermal loads which the vehicle must be designed for, to
deliver the robotic instruments inside the atmosphere. The design of planetary
probes and their mission architecture is complex, and involves various
engineering constraints such as peak deceleration, heating rate, heating load,
and communications which must be satisfied within the budget and schedule of
cost constrained mission opportunities. Engineering design data from previous
entry probe missions serve as a valuable reference for designing future
missions. The present study compiles an augmented version of the blue book
entry probe dataset, performs a comparative analysis of the entry conditions,
and provides engineering rules of thumb for design of future missions. Using
the dataset, the present study proposes a new empirical correlation which aims
to more accurately predict the thermal protection system mass fraction for high
heat load conditions during entry and aerocapture at Uranus and Neptune. | astro-ph_EP |
On the Habitable Lifetime of Terrestrial Worlds with High Radionuclide
Abundances: The presence of a liquid solvent is widely regarded as an essential
prerequisite for habitability. We investigate the conditions under which worlds
outside the habitable zones of stars are capable of supporting liquid solvents
on their surface over geologically significant timescales via combined
radiogenic and primordial heat. Our analysis suggests that super-Earths with
radionuclide abundances that are $\gtrsim 10^3$ times higher than Earth can
host long-lived water oceans. In contrast, the requirements for long-lived
ethane oceans, which have been explored in the context of alternative
biochemistries, are less restrictive: relative radionuclide abundances of
$\gtrsim 10^2$ could be sufficient. We find that this class of worlds might be
detectable ($10\sigma$ detection over $\sim 10$ days integration time at $12.8$
$\mu$m) in principle by the James Webb Space Telescope at distances of $\sim
10$ pc if their ages are $\lesssim 1$ Gyr. | astro-ph_EP |
Solar XUV and ENA-driven water loss from early Venus' steam atmosphere: The influence of the hydrogen hydrodynamic upper atmosphere escape, driven by
the solar soft X-ray and extreme ultraviolet radiation (XUV) flux, on an
expected magma ocean outgassed steam atmosphere of early Venus is studied. By
assuming that the young Sun was either a weak or moderate active young G star,
we estimated the water loss from a hydrogen dominated thermosphere due to the
absorption of the solar XUV flux and the precipitation of solar wind produced
energetic hydrogen atoms (ENAs). The production of ENAs and their interaction
with the hydrodynamic extended upper atmosphere, including collision-related
feedback processes, have been calculated by means of Monte Carlo models. ENAs
that collide in the upper atmosphere deposit their energy and heat the
surrounding gas mainly above the main XUV energy deposition layer. It is shown
that precipitating ENAs modify the thermal structure of the upper atmosphere,
but the enhancement of the thermal escape rates caused by these energetic
hydrogen atoms is negligible. Our results also indicate that the majority of
oxygen arising from dissociated H$_2$O molecules is left behind during the
first 100 Myr. It is thus suggested that the main part of the remaining oxygen
has been absorbed by crustal oxidation. | astro-ph_EP |
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