3,193 research outputs found
The fate of planetesimals in turbulent disks with dead zones. II. Limits on the viability of runaway accretion
A critical phase in the standard model for planet formation is the runaway
growth phase. During runaway growth bodies in the 0.1--100 km size range
(planetesimals) quickly produce a number of much larger seeds. The runaway
growth phase is essential for planet formation as the emergent planetary
embryos can accrete the leftover planetesimals at large gravitational focusing
factors. However, torques resulting from turbulence-induced density
fluctuations may violate the criterion for the onset of runaway growth, which
is that the magnitude of the planetesimals' random (eccentric) motions are less
than their escape velocity. This condition represents a more stringent
constraint than the condition that planetesimals survive their mutual
collisions. To investigate the effects of MRI turbulence on the viability of
the runaway growth scenario, we apply our semi-analytical recipes of Paper I,
which we augment by a coagulation/fragmentation model for the dust component.
We find that the surface area-equivalent abundance of 0.1 micron particles is
reduced by factors 10^2--10^3, which tends to render the dust irrelevant to the
turbulence. We express the turbulent activity in the midplane regions in terms
of a size s_run above which planetesimals will experience runaway growth. We
find that s_run is mainly determined by the strength of the vertical net field
that threads the disks and the disk radius. At disk radii beyond 5 AU, s_run
becomes larger than ~100 km and the collision times among these bodies longer
than the duration of the nebula phase. Our findings imply that the classical,
planetesimal-dominated, model for planet formation is not viable in the outer
regions of a turbulent disk.Comment: ApJ accepte
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
Rapid Formation of Saturn after Jupiter Completion
We have investigated Saturn's core formation at a radial pressure maximum in
a protoplanetary disk, which is created by gap opening by Jupiter. A core
formed via planetesimal accretion induces the fragmentation of surrounding
planetesimals, which generally inhibits further growth of the core by removal
of the resulting fragments due to radial drift caused by gas drag. However, the
emergence of the pressure maximum halts the drift of the fragments, while their
orbital eccentricities and inclinations are efficiently damped by gas drag. As
a result, the core of Saturn rapidly grows via accretion of the fragments near
the pressure maximum. We have found that in the minimum-mass solar nebula,
kilometer sized planetesimals can produce a core exceeding 10 Earth masses
within two million years. Since Jupiter may not have undergone significant type
II inward migration, it is likely that Jupiter's formation was completed when
the local disk mass has already decayed to a value comparable to or less than
Jovian mass. The expected rapid growth of Saturn's core on a timescale
comparable to or shorter than observationally inferred disk lifetime enables
Saturn to acquire the current amount of envelope gas before the disk gas is
completely depleted. The high heat energy release rate onto the core surface
due to the rapid accretion of the fragments delays onset of runaway gas
accretion until the core mass becomes somewhat larger than that of Jupiter,
which is consistent with the estimate based on interior modeling. Therefore,
the rapid formation of Saturn induced by gap opening of Jupiter can account for
the formation of multiple gas giants (Jupiter and Saturn) without significant
inward migration and larger core mass of Saturn than that of Jupiter.Comment: Accepted for publication in Ap
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&
Closed-form expressions for particle relative velocities induced by turbulence
In this note we present complete, closed-form expressions for random relative
velocities between colliding particles of arbitrary size in nebula turbulence.
These results are exact for very small particles (those with stopping times
much shorter than the large eddy overturn time) and are also surprisingly
accurate in complete generality (that is, also apply for particles with
stopping times comparable to, or much longer than, the large eddy overturn
time). We note that some previous studies may have adopted previous simple
expressions, which we find to be in error regarding the size dependence in the
large particle regime.Comment: 8 pages, accepted as Research Note by A&
Formation of TRAPPIST-1 and other compact systems
TRAPPIST-1 is a nearby 0.08 M M-star, which was recently found to harbor a
planetary system of at least seven Earth-mass planets, all within 0.1 au. The
configuration confounds theorists as the planets are not easily explained by
either in situ or migration models. In this Paper we present a scenario for the
formation and orbital architecture of the TRAPPIST-1 system. In our model,
planet formation starts at the H2O iceline, where pebble-size particles --
whose origin is the outer disk -- concentrate to trigger streaming
instabilities. After their formation, planetary embryos quickly mature by
pebble accretion. Planet growth stalls at Earth masses, where the planet's
gravitational feedback on the disk keeps pebbles at bay. Planets are
transported by Type I migration to the inner disk, where they stall at the
magnetospheric cavity and end up in mean motion resonances. During disk
dispersal, the cavity radius expands and the inner-most planets escape
resonance. We argue that the model outlined here can also be applied to other
compact systems and that the many close-in super-Earth systems are a scaled-up
version of TRAPPIST-1. We also hypothesize that few close-in compact systems
harbor giant planets at large distances, since they would have stopped the
pebble flux from the outer disk.Comment: 8 pages, accepted for publication in A&
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