14 research outputs found
The Importance of Disk Structure in Stalling Type I Migration
As planets form they tidally interact with their natal disks. Though the
tidal perturbation induced by Earth and super-Earth mass planets is generally
too weak to significantly modify the structure of the disk, the interaction is
potentially strong enough to cause the planets to undergo rapid type I
migration. This physical process may provide a source of short-period
super-Earths, though it may also pose a challenge to the emergence and
retention of cores on long-period orbits with sufficient mass to evolve into
gas giants. Previous numerical simulations have shown that the type I migration
rate sensitively depends upon the circumstellar disk's properties, particularly
the temperature and surface density gradients. Here, we derive these structure
parameters for 1) a self-consistent viscous-disk model based on a constant
\alpha-prescription, 2) an irradiated disk model that takes into account
heating due to the absorption of stellar photons, and 3) a layered-accretion
disk model with variable \alpha-parameter. We show that in the inner
viscously-heated regions of typical protostellar disks, the horseshoe and
corotation torques of super-Earths can exceed their differential Lindblad
torque and cause them to undergo outward migration. However, the temperature
profile due to passive stellar irradiation causes type I migration to be
inwards throughout much of the disk. For disks in which there is outwards
migration, we show that location and the mass range of the "planet traps"
depends on some uncertain assumptions adopted for these disk models. Competing
physical effects may lead to dispersion in super-Earths' mass-period
distribution.Comment: 12 pages, Submitted to Ap
Protoplanetary Disk Rings as Sites for Planetesimal Formation
Axisymmetric dust rings are a ubiquitous feature of young protoplanetary
disks. These rings are likely caused by pressure bumps in the gas profile; a
small bump can induce a traffic jam-like pattern in the dust density, while a
large bump may halt radial dust drift entirely. The resulting increase in dust
concentration may trigger planetesimal formation by the streaming instability
(SI), as the SI itself requires some initial concentration. Here we present the
first 3D simulations of planetesimal formation in the presence of a pressure
bump modeled specifically after those observed by ALMA. In particular, we place
a pressure bump at the center of a large 3D shearing box, along with an initial
solid-to-gas ratio of , and we include both particle back-reaction
and particle self-gravity. We consider both mm-sized and cm-sized particles
separately. For simulations with cm-sized particles, we find that even a small
pressure bump leads to the formation of planetesimals via the streaming
instability; a pressure bump does {\it not} need to fully halt radial particle
drift for the SI to become efficient. Furthermore, pure gravitational collapse
via concentration in pressure bumps (such as would occur at sufficiently high
concentrations and without the streaming instability) is not responsible for
planetesimal formation. For mm-sized particles, we find tentative evidence that
planetesimal formation does not occur. This result, if it holds up at higher
resolution and for a broader range of parameters, could put strong constraints
on where in protoplanetary disks planetesimals can form. Ultimately, however,
our results suggest that for cm-sized particles, planetesimal formation in
pressure bumps is an extremely robust process.Comment: accepted to ApJ; 22 pages, 16 figure
The Mass and Size Distribution of Planetesimals Formed by the Streaming Instability. II. The Effect of the Radial Gas Pressure Gradient
The streaming instability concentrates solid particles in protoplanetary
disks, leading to gravitational collapse into planetesimals. Despite its key
role in producing particle clumping and determining critical length scales in
the instability's linear regime, the influence of the disk's radial pressure
gradient on planetesimal properties has not been examined in detail. Here, we
use streaming instability simulations that include particle self-gravity to
study how the planetesimal initial mass function depends on the radial pressure
gradient. Fitting our results to a power-law, , we find a single value describes simulations in
which the pressure gradient varies by . An exponentially truncated
power-law provides a significantly better fit, with a low mass slope of
that weakly depends on the pressure gradient. The
characteristic truncation mass is found to be . We exclude the cubic dependence of the characteristic
mass with pressure gradient suggested by linear considerations, finding instead
a linear scaling. These results strengthen the case for a streaming-derived
initial mass function that depends at most weakly on the aerodynamic properties
of the disk and participating solids. A simulation initialized with zero
pressure gradient---which is {\em not} subject to the streaming
instability---also yields a top-heavy mass function but with modest evidence
for a different shape. We discuss the consistency of the theoretically
predicted mass function with observations of Kuiper Belt planetesimals, and
describe implications for models of early stage planet formation..Comment: 18 pages, 10 figures, 3 tables, accepted to Ap
The N2K Consortium. VII. Atmospheric Parameters of 1907 Metal-Rich Stars: Finding Planet-Search Targets
We report high-precision atmospheric parameters for 1907 stars in the N2K
low-resolution spectroscopic survey, designed to identify metal-rich FGK dwarfs
likely to harbor detectable planets. 284 of these stars are in the ideal
temperature range for planet searches, T_eff <= 6000K, and have a 10% or
greater probability of hosting planets based on their metallicities. The stars
in the low-resolution spectroscopic survey should eventually yield >60 new
planets, including 8-9 hot Jupiters. Short-period planets have already been
discovered orbiting the survey targets HIP 14801 and HD 149143.Comment: Accepted for publication in Astrophysical Journal Supplements. 24
pages, including 8 figures and one table. Two more tables will appear in ApJS
online editio
Grain Retention and Formation of Planetesimals near the Snow Line in MRI-driven Turbulent Protoplanetary Disks
The first challenge in the formation of both terrestrial planets and the cores of gas giants is the retention of grains in protoplanetary disks. In most regions of these disks, gas attains sub-Keplerian speeds as a consequence of a negative pressure gradient. Hydrodynamic drag leads to orbital decay and depletion of the solid material in the disk, with characteristic timescales as short as only a few hundred years for meter-sized objects at 1 AU. In this paper, we suggest a particle retention mechanism which promotes the accumulation of grains and the formation of planetesimals near the water sublimation front or “snow line.” This model is based on the assumption that, in the regions most interesting for planet formation, the viscous evolution of the disk is due to turbulence driven by the magneto-rotational instability (MRI) in the surface layers of the disk. The depth to which MRI effectively generates turbulence is a strong function of the grain size and abundance. A sharp increase in the grain-to-gas density ratio across the snow line reduces the column depth of the active layer. As the disk evolves towards a quasi-steady-state, this change in the active layer creates a local maximum in radial distribution of the gas surface density and pressure, causing the gas to rotate at super-Keplerian speed and halting the inward migration of grains. This senario presents a robust process for grain retention which may aid in the formation of proto-gas-giant cores preferentially near the snow line. Subject headings: planetary systems: protoplanetary disks – solar system: formation – turbulence 1