588 research outputs found
The ARCiS framework for Exoplanet Atmospheres: The Cloud Transport Model
Understanding of clouds is instrumental in interpreting current and future
spectroscopic observations of exoplanets. Modelling clouds consistently is
complex, since it involves many facets of chemistry, nucleation theory,
condensation physics, coagulation, and particle transport. We develop a simple
physical model for cloud formation and transport, efficient and versatile
enough that it can be used in modular fashion for parameter optimization
searches of exoplanet atmosphere spectra. The transport equations are
formulated in 1D, accounting for sedimentation and diffusion. The grain size is
obtained through a moment method. For simplicity, only one cloud species is
considered and the nucleation rate is parametrized. From the resulting physical
profiles we simulate transmission spectra covering the visual to mid-IR
wavelength range. We apply our models towards KCl clouds in the atmosphere of
GJ1214 b and towards MgSiO3 clouds of a canonical hot-Jupiter. We find that
larger cloud diffusivity increases the thickness of the cloud, pushing
the surface to a lower pressure layer higher in the atmosphere. A
larger nucleation rate also increases the cloud thickness while it suppresses
the grain size. Coagulation is most important at high nuclei injection rates
() and low . We find that the investigated combinations
of and greatly affect the transmission spectra in terms
of the slope at near-IR wavelength (a proxy for grain size), the molecular
features seen at ~1\micr (which disappear for thick clouds, high in the
atmosphere), and the 10\micr silicate feature, which becomes prominent for
small grains high in the atmosphere. The result of our hybrid approach -- aimed
to provide a good balance between physical consistency and computational
efficiency -- is ideal towards interpreting (future) spectroscopic observations
of exoplanets.Comment: language and other tiny correction
Dynamical rearrangement of super-Earths during disk dispersal II. Assessment of the magnetospheric rebound model for planet formation scenarios
Context.The Kepler mission has provided a large sample to statistically
analyze the orbital properties of the super-Earth planets. We hypothesize that
these planets formed early and consider the problem of matching planet
formation theory to the current observations. Two scenarios, disk migration and
in-situ formation, have been proposed to explain their origin. In the migration
scenario planets migrate inward due to planet-disk interaction, whereas in the
in-situ scenario planets assemble locally. Therefore, planets formed by
migration are expected to end up in resonances, whereas those formed in-situ
are expected to stay in short period ratios and in non-resonant orbits. Both
predictions are at odds with observations. Aims. We investigate whether a
preferred formation scenario can be identified through a comparison between the
magnetospheric rebound model and the Kepler data. Methods. We conduct N-body
simulations of two-planet systems during the disk dispersal phase, and make a
statistical comparison between the simulations and the Kepler observations.
Results. Comparing the two scenarios, we find that magnetospheric rebound tends
to erase the difference in the orbital configuration that was initially
presented. After disk dispersal, not all planets are in resonance in the
migration scenario, whereas planets do not remain in compact configurations in
the in-situ scenario. In both scenarios, the orbits of planets increase with
the cavity expansion, and their period ratios have a wider distribution.
Conclusions. From a statistical perspective, the magnetospheric rebound model
reproduces several observed properties of Kepler planets, such as the
significant number of planets are not in resonances and planet pairs can end up
at large period ratios. The disparity in orbital configuration between the two
formation scenarios is substantially reduced after disk dispersal.Comment: 8 pages, 4 figures, accepted for publication in A&
A Lagrangian Model for Dust Evolution in Protoplanetary Disks: Formation of Wet and Dry Planetesimals at Different Stellar Masses
We introduce a new Lagrangian smooth-particle method to model the growth and
drift of pebbles in protoplanetary disks. The Lagrangian nature of the model
makes it especially suited to follow characteristics of individual (groups of)
particles, such as their composition. In this work we focus on the water
content of solid particles. Planetesimal formation via streaming instability is
taken into account, partly based on previous results on streaming instability
outside the water snowline that were presented in Schoonenberg & Ormel (2017).
We validate our model by reproducing earlier results from the literature and
apply our model to steady-state viscous gas disks (with constant gas accretion
rate) around stars with different masses. We also present various other models
where we explore the effects of pebble accretion, the fragmentation velocity
threshold, the global metallicity of the disk, and a time-dependent gas
accretion rate. We find that planetesimals preferentially form in a local
annulus outside the water snowline, at early times in the lifetime of the disk
(), when the pebble mass fluxes are high enough to
trigger the streaming instability. During this first phase in the planet
formation process, the snowline location hardly changes due to slow viscous
evolution, and we conclude that assuming a constant gas accretion rate is
justified in this first stage. The efficiency of converting the solids
reservoir of the disk to planetesimals depends on the location of the water
snowline. Cooler disks with a closer-in water snowline are more efficient at
producing planetesimals than hotter disks where the water snowline is located
further away from the star. Therefore, low-mass stars tend to form
planetesimals more efficiently, but any correlation may be overshadowed by the
spread in disk properties.Comment: 18 pages, 15 figures, accepted for publication in A&
What pebbles are made of: Interpretation of the V883 Ori disk
Recently, an Atacama Large Millimeter/submillimeter Array (ALMA) observation
of the water snow line in the protoplanetary disk around the FU Orionis star
V883 Ori was reported. The radial variation of the spectral index at
mm-wavelengths around the snow line was interpreted as being due to a pileup of
particles interior to the snow line. However, radial transport of solids in the
outer disk operates on timescales much longer than the typical timescale of an
FU Ori outburst (-- yr). Consequently, a steady-state pileup is
unlikely. We argue that it is only necessary to consider water evaporation and
re-coagulation of silicates to explain the recent ALMA observation of V883 Ori
because these processes are short enough to have had their impact since the
outburst. Our model requires the inner disk to have already been optically
thick before the outburst, and our results suggest that the carbon content of
pebbles is low.Comment: Accepted to A&A Letter
On the filtering and processing of dust by planetesimals 1. Derivation of collision probabilities for non-drifting planetesimals
Context. Circumstellar disks are known to contain a significant mass in dust
ranging from micron to centimeter size. Meteorites are evidence that individual
grains of those sizes were collected and assembled into planetesimals in the
young solar system. Aims. We assess the efficiency of dust collection of a
swarm of non-drifting planetesimals {\rev with radii ranging from 1 to
\,km and beyond. Methods. We calculate the collision probability of dust
drifting in the disk due to gas drag by planetesimal accounting for several
regimes depending on the size of the planetesimal, dust, and orbital distance:
the geometric, Safronov, settling, and three-body regimes. We also include a
hydrodynamical regime to account for the fact that small grains tend to be
carried by the gas flow around planetesimals. Results. We provide expressions
for the collision probability of dust by planetesimals and for the filtering
efficiency by a swarm of planetesimals. For standard turbulence conditions
(i.e., a turbulence parameter ), filtering is found to be
inefficient, meaning that when crossing a minimum-mass solar nebula (MMSN) belt
of planetesimals extending between 0.1 AU and 35 AU most dust particles are
eventually accreted by the central star rather than colliding with
planetesimals. However, if the disk is weakly turbulent ()
filtering becomes efficient in two regimes: (i) when planetesimals are all
smaller than about 10 km in size, in which case collisions mostly take place in
the geometric regime; and (ii) when planetary embryos larger than about 1000 km
in size dominate the distribution, have a scale height smaller than one tenth
of the gas scale height, and dust is of millimeter size or larger in which case
most collisions take place in the settling regime. These two regimes have very
different properties: we find that the local filtering efficiency
scales with (where is the orbital distance) in
the geometric regime, but with to in the settling regime.
This implies that the filtering of dust by small planetesimals should occur
close to the central star and with a short spread in orbital distances. On the
other hand, the filtering by embryos in the settling regime is expected to be
more gradual and determined by the extent of the disk of embryos. Dust
particles much smaller than millimeter size tend only to be captured by the
smallest planetesimals because they otherwise move on gas streamlines and their
collisions take place in the hydrodynamical regime. Conclusions. Our results
hint at an inside-out formation of planetesimals in the infant solar system
because small planetesimals in the geometrical limit can filter dust much more
efficiently close to the central star. However, even a fully-formed belt of
planetesimals such as the MMSN only marginally captures inward-drifting dust
and this seems to imply that dust in the protosolar disk has been filtered by
planetesimals even smaller than 1 km (not included in this study) or that it
has been assembled into planetesimals by other mechanisms (e.g., orderly
growth, capture into vortexes). Further refinement of our work concerns, among
other things: a quantitative description of the transition region between the
hydro and settling regimes; an assessment of the role of disk turbulence for
collisions, in particular in the hydro regime; and the coupling of our model to
a planetesimal formation model.Comment: Accepted for publication in A\&A. 31 pages, 29 figures. (Version
corrected by the A\&A Language Editor
Effect of Core Cooling on the Radius of Sub-Neptune Planets
Sub-Neptune planets are very common in our galaxy and show a large diversity
in their mass-radius relation. In sub-Neptunes most of the planet mass is in
the rocky part (hereafter core) which is surrounded by a modest hydrogen-helium
envelope. As a result, the total initial heat content of such a planet is
dominated by that of the core. Nonetheless, most studies contend that the core
cooling will only have a minor effect on the radius evolution of the gaseous
envelope, because the core's cooling is in sync with the envelope, i.e., most
of the initial heat is released early on timescales of about 10-100 Myr. In
this Letter we examine the importance of the core cooling rate for the thermal
evolution of the envelope. Thus, we relax the early core cooling assumption and
present a model where the core is characterized by two parameters: the initial
temperature and the cooling time. We find that core cooling can significantly
enhance the radius of the planet when it operates on a timescale similar to the
observed age, i.e. several Gyr. Consequently, the interpretation of
sub-Neptunes' mass-radius observations depends on the assumed core thermal
properties and the uncertainty therein. The degeneracy of composition and core
thermal properties can be reduced by obtaining better estimates of the planet
ages (in addition to their radii and masses) as envisioned by future
observations.Comment: Accepted for publication in A&A Letter
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