1,111 research outputs found
The Two Modes of Gas Giant Planet Formation
I argue for two modes of gas giant planet formation and discuss the
conditions under which each mode operates. Gas giant planets at disk radii
AU are likely to form in situ by disk instability, while core accretion
plus gas capture remains the dominant formation mechanism for AU.
During the mass accretion phase, mass loading can push disks toward
fragmentation conditions at large . Massive, extended disks can fragment
into clumps of a few to tens of Jupiter masses. This is confirmed by radiation
hydrodynamics simulations. The two modes of gas giant formation should lead to
a bimodal distribution of gas giant semi-major axes. Because core accretion is
expected to be less efficient in low-metallicity systems, the ratio of gas
giants at large to planets at small should increase with decreasing
metallicity.Comment: Submitted to ApJL after addressing referee's comment
On The Possibility of Enrichment and Differentiation in Gas Giants During Birth by Disk Instability
We investigate the coupling between rock-size solids and gas during the
formation of gas giant planets by disk fragmentation in the outer regions of
massive disks. In this study, we use three-dimensional radiative hydrodynamics
simulations and model solids as a spatial distribution of particles. We assume
that half of the total solid fraction is in small grains and half in large
solids. The former are perfectly entrained with the gas and set the opacity in
the disk, while the latter are allowed to respond to gas drag forces, with the
back reaction on the gas taken into account. To explore the maximum effects of
gas-solid interactions, we first consider 10cm-size particles. We then compare
these results to a simulation with 1 km-size particles, which explores the
low-drag regime. We show that (1) disk instability planets have the potential
to form large cores due to aerodynamic capturing of rock-size solids in spiral
arms before fragmentation; (2) that temporary clumps can concentrate tens of
of solids in very localized regions before clump disruption; (3)
that the formation of permanent clumps, even in the outer disk, is dependent on
the grain-size distribution, i.e., the opacity; (4) that nonaxisymmetric
structure in the disk can create disk regions that have a solids-to-gas ratio
greater than unity; (5) that the solid distribution may affect the
fragmentation process; (6) that proto-gas giants and proto-brown dwarfs can
start as differentiated objects prior to the H collapse phase; (7) that
spiral arms in a gravitationally unstable disk are able to stop the inward
drift of rock-size solids, even redistributing them to larger radii; and, (8)
that large solids can form spiral arms that are offset from the gaseous spiral
arms. We conclude that planet embryo formation can be strongly affected by the
growth of solids during the earliest stages of disk accretion.Comment: Accepted by ApJ. 55 pages including 24 figures. In response to
comments from the referee, we have included a new simulation with km-size
objects and have revised some discussions and interpretations. Major
conclusions remain unchanged, and new conclusions have been added in response
to the new ru
The Heavy Element Composition of Disk Instability Planets Can Range From Sub- to Super-Nebular
Transit surveys combined with Doppler data have revealed a class of gas giant
planets that are massive and highly enriched in heavy elements (e.g.,
HD149026b, GJ436b, and HAT-P-20b). It is tempting to consider these planets as
validation of core accretion plus gas capture because it is often assumed that
disk instability planets should be of nebular composition. We show in this
paper, to the contrary, that gas giants that form by disk instability can have
a variety of heavy element compositions, ranging from sub- to super-nebular
values. High levels of enrichment can be achieved through one or multiple
mechanisms, including enrichment at birth, planetesimal capture, and
differentiation plus tidal stripping. As a result, the metallicity of an
individual gas giant cannot be used to discriminate between gas giant formation
modes.Comment: Accepted by Ap
High-Temperature Processing of Solids Through Solar Nebular Bow Shocks: 3D Radiation Hydrodynamics Simulations with Particles
A fundamental, unsolved problem in Solar System formation is explaining the
melting and crystallization of chondrules found in chondritic meteorites.
Theoretical models of chondrule melting in nebular shocks has been shown to be
consistent with many aspects of thermal histories inferred for chondrules from
laboratory experiments; but, the mechanism driving these shocks is unknown.
Planetesimals and planetary embryos on eccentric orbits can produce bow shocks
as they move supersonically through the disk gas, and are one possible source
of chondrule-melting shocks. We investigate chondrule formation in bow shocks
around planetoids through 3D radiation hydrodynamics simulations. A new
radiation transport algorithm that combines elements of flux-limited diffusion
and Monte Carlo methods is used to capture the complexity of radiative
transport around bow shocks. An equation of state that includes the rotational,
vibrational, and dissociation modes of H is also used. Solids are followed
directly in the simulations and their thermal histories are recorded. Adiabatic
expansion creates rapid cooling of the gas, and tail shocks behind the embryo
can cause secondary heating events. Radiative transport is efficient, and bow
shocks around planetoids can have luminosities few
L. While barred and radial chondrule textures could be produced in
the radiative shocks explored here, porphyritic chondrules may only be possible
in the adiabatic limit. We present a series of predicted cooling curves that
merit investigation in laboratory experiments to determine whether the solids
produced by bow shocks are represented in the meteoritic record by chondrules
or other solids.Comment: Accepted for publication in ApJ. Images have been resized to conform
to arXiv limits, but are all readable upon adjusting the zoom. Changes from
v1: Corrected typos discovered in proofs. Most changes are in the appendi
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