194 research outputs found
Planet migration: self-gravitating radiation hydrodynamical models of protoplanets with surfaces
We calculate radial migration rates of protoplanets in laminar minimum mass
solar nebula discs using three-dimensional self-gravitating radiation
hydrodynamical (RHD) models. The protoplanets are free to migrate, whereupon
their migration rates are measured. For low mass protoplanets (10-50 M_\oplus)
we find increases in the migration timescales of up to an order of magnitude
between locally-isothermal and RHD models. In the high-mass regime the
migration rates are changed very little. These results are arrived at by
calculating migration rates in locally-isothermal models, before sequentially
introducing self-gravity, and radiative transfer, allowing us to isolate the
effects of the additional physics. We find that using a locally-isothermal
equation of state, without self-gravity, we reproduce the migration rates
obtained by previous analytic and numerical models. We explore the impact of
different protoplanet models, and changes to their assumed radii, upon
migration. The introduction of self-gravity gives a slight reduction of the
migration rates, whilst the inertial mass problem, which has been proposed for
high mass protoplanets with circumplanetary discs, is reproduced. Upon
introducing radiative transfer to models of low mass protoplanets (\approx 10
M_\oplus), modelled as small radius accreting point masses, we find outward
migration with a rate of approximately twice the analytic inward rate. However,
when modelling such a protoplanet in a more realistic manner, with a surface
which enables the formation of a deep envelope, this outward migration is not
seen.Comment: 21 pages, 21 figure
Planetesimal Formation In Self-Gravitating Discs
We study particle dynamics in local two-dimensional simulations of
self-gravitating accretion discs with a simple cooling law. It is well known
that the structure which arises in the gaseous component of the disc due to a
gravitational instability can have a significant effect on the evolution of
dust particles. Previous results using global simulations indicate that spiral
density waves are highly efficient at collecting dust particles, creating
significant local over-densities which may be able to undergo gravitational
collapse. We expand on these findings, using a range of cooling times to mimic
the conditions at a large range of radii within the disc. Here we use the
Pencil Code to solve the 2D local shearing sheet equations for gas on a fixed
grid together with the equations of motion for solids coupled to the gas solely
through aerodynamic drag force. We find that spiral density waves can create
significant enhancements in the surface density of solids, equivalent to 1-10cm
sized particles in a disc following the profiles of Clarke (2009) around a
solar mass star, causing it to reach concentrations several orders of magnitude
larger than the particles mean surface density. We also study the velocity
dispersion of the particles, finding that the spiral structure can result in
the particle velocities becoming highly ordered, having a narrow velocity
dispersion. This implies low relative velocities between particles, which in
turn suggests that collisions are typically low energy, lessening the
likelihood of grain destruction. Both these findings suggest that the density
waves that arise due to gravitational instabilities in the early stages of star
formation provide excellent sites for the formation of large,
planetesimal-sized objects.Comment: 11 pages, 8 figures, accepted for publication in MNRA
On the convergence of the critical cooling timescale for the fragmentation of self-gravitating discs
We carry out simulations of gravitationally unstable discs using a Smoothed
Particle Hydrodynamics (SPH) code and a grid-based hydrodynamics code, FARGO,
to understand the previous non-convergent results reported by Meru & Bate
(2011a). We obtain evidence that convergence with increasing resolution occurs
with both SPH and FARGO and in both cases we find that the critical cooling
timescale is larger than previously thought. We show that SPH has a first-order
convergence rate while FARGO converges with a second-order rate. We show that
the convergence of the critical cooling timescale for fragmentation depends
largely on the numerical viscosity employed in both SPH and FARGO. With SPH,
particle velocity dispersion may also play a role. We show that reducing the
dissipation from the numerical viscosity leads to larger values of the critical
cooling time at a given resolution. For SPH, we find that the effect of the
dissipation due to the numerical viscosity is somewhat larger than had
previously been appreciated. In particular, we show that using a quadratic term
in the SPH artificial viscosity (beta_{SPH}) that is too low appears to lead to
excess dissipation in gravitationally unstable discs, which may affect any
results that sensitively depend on the thermodynamics, such as disc
fragmentation. We show that the two codes converge to values of the critical
cooling timescale, beta_{crit} > 20 (for a ratio of specific heats of
gamma=5/3), and perhaps even as large as beta_{crit} \approx 30. These are
approximately 3-5 times larger than has been found by most previous studies.
This is equivalent to a maximum gravitational stress that a disc can withstand
without fragmenting of alpha_{GI,crit} \approx 0.013-0.02, which is much
smaller than the values typically used in the literature. It is therefore
easier for self-gravitating discs to fragment than has been concluded from most
past studies.Comment: Accepted for publication by MNRAS. 26 pages, 17 figure
On the formation of hot Neptunes and super-Earths
The discovery of short-period Neptune-mass objects, now including the
remarkable system HD69830 (Lovis et al. 2006) with three Neptune analogues,
raises difficult questions about current formation models which may require a
global treatment of the protoplanetary disc. Several formation scenarios have
been proposed, where most combine the canonical oligarchic picture of core
accretion with type I migration (e.g. Terquem & Papaloizou 2007) and planetary
atmosphere physics (e.g. Alibert et al. 2006). To date, published studies have
considered only a small number of progenitors at late times. This leaves
unaddressed important questions about the global viability of the models. We
seek to determine whether the most natural model -- namely, taking the
canonical oligarchic picture of core accretion and introducing type I migration
-- can succeed in forming objects of 10 Earth masses and more in the innermost
parts of the disc. This problem is investigated using both traditional
semianalytic methods for modelling oligarchic growth as well as a new parallel
multi-zone N-body code designed specifically for treating planetary formation
problems with large dynamic range (McNeil & Nelson 2009). We find that it is
extremely difficult for oligarchic tidal migration models to reproduce the
observed distribution. Even under many variations of the typical parameters, we
form no objects of mass greater than 8 Earth masses. By comparison, it is
relatively straightforward to form icy super-Earths. We conclude that either
the initial conditions of the protoplanetary discs in short-period Neptune
systems were substantially different from the standard disc models we used, or
there is important physics yet to be understood.Comment: 19 pages, 18 figures. Final version accepted to MNRAS 30 September
200
On the dynamics and collisional growth of planetesimals in misaligned binary systems
Context. Abridged. Many stars are members of binary systems. During early
phases when the stars are surrounded by discs, the binary orbit and disc
midplane may be mutually inclined. The discs around T Tauri stars will become
mildly warped and undergo solid body precession around the angular momentum
vector of the binary system. It is unclear how planetesimals in such a disc
will evolve and affect planet formation. Aims. We investigate the dynamics of
planetesimals embedded in discs that are perturbed by a binary companion on a
circular, inclined orbit. We examine collisional velocities of the
planetesimals to determine when they can grow through accretion. We vary the
binary inclination, binary separation, D, disc mass, and planetesimal radius.
Our standard model has D=60 AU, inclination=45 deg, and a disc mass equivalent
to the MMSN. Methods. We use a 3D hydrodynamics code to model the disc.
Planetesimals are test particles which experience gas drag, the gravitational
force of the disc, the companion star gravity. Planetesimal orbit crossing
events are detected and used to estimate collisional velocities. Results. For
binary systems with modest inclination (25 deg), disc gravity prevents
planetesimal orbits from undergoing strong differential nodal precession (which
occurs in absence of the disc), and forces planetesimals to precess with the
disc on average. For bodies of different size the orbit planes become modestly
mutually inclined, leading to collisional velocities that inhibit growth. For
larger inclinations (45 degrees), the Kozai effect operates, leading to
destructively large relative velocities. Conclusions. Planet formation via
planetesimal accretion is difficult in an inclined binary system with
parameters similar to those considered in this paper. For systems in which the
Kozai mechanism operates, the prospects for forming planets are very remote.Comment: 24 pages, 16 figures, recently published in Astronomy and
Astrophysic
Planet or brown dwarf? Inferring the companion mass in HD 100546 from the wall shape using mid-infrared interferometry
On the accumulation of planetesimals near disc gaps created by protoplanets
We have performed three-dimensional two-fluid (gas-dust) hydrodynamical
models of circumstellar discs with embedded protoplanets (3 - 333 M\oplu) and
small solid bodies (radii 10cm to 10m). We find that high mass planets (\gtrsim
Saturn mass) open sufficiently deep gaps in the gas disc such that the density
maximum at the outer edge of the gap can very efficiently trap metre-sized
solid bodies. This allows the accumulation of solids at the outer edge of the
gap as solids from large radii spiral inwards to the trapping region. This
process of accumulation occurs fastest for those bodies that spiral inwards
most rapidly, typically metre-sized boulders, whilst smaller and larger objects
will not migrate sufficiently rapidly in the discs lifetime to benefit from the
process. Around a Jupiter mass planet we find that bound clumps of solid
material, as large as several Earth masses, may form, potentially collapsing
under self-gravity to form planets or planetesimals. These results are in
agreement with Lyra et al. (2009), supporting their finding that the formation
of a second generation of planetesimals or of terrestrial mass planets may be
triggered by the presence of a high mass planet.Comment: 14 pages, 10 figures. Accepted for publication in MNRA
Considerations on the accretion of Uranus and Neptune by mutual collisions of planetary embryos in the vicinity of Jupiter and Saturn
Modeling the formation of the ice giants Uranus and Neptune is a long-lasting
problem in planetary science. Due to gas-drag, collisional damping, and
resonant shepherding, the planetary embryos repel the planetesimals away from
their reach and thus they stop growing (Levison et al. 2010). This problem
persists independently of whether the accretion took place at the current
locations of the ice giants or closer to the Sun. Instead of trying to push the
runaway/oligarchic growth of planetary embryos up to 10-15 Earth masses, we
envision the possibility that the planetesimal disk could generate a system of
planetary embryos of only 1-3 Earth masses. Then we investigate whether these
embryos could have collided with each other and grown enough to reach the
masses of current Uranus and Neptune. Our results point to two major problems.
First, there is typically a large difference in mass between the first and the
second most massive core formed and retained beyond Saturn. Second, in many
simulations the final planetary system has more than two objects beyond Saturn.
The growth of a major planet from a system of embryos requires strong damping
of eccentricities and inclinations from the disk of gas. But strong damping
also favors embryos and cores to find a stable resonant configuration, so that
systems with more than two surviving objects are found. In addition to these
problems, in order to have substantial mutual accretion among embryos, it is
necessary to assume that the surface density of the gas was several times
higher than that of the minimum-mass solar nebula. However this contrasts with
the common idea that Uranus and Neptune formed in a gas-starving disk, which is
suggested by the relatively small amount of hydrogen and helium contained in
the atmospheres of these planets. Only one of our simulations "by chance"
successfully reproduced the structure of the outer Solar System.Comment: 16 pages; 15 color figures; accepted in Astronomy & Astrophysics;
shortened abstrac
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