70 research outputs found
The Role of the Cooling Prescription for Disk Fragmentation: Numerical Convergence & Critical Cooling Parameter in Self-Gravitating Disks
Protoplanetary disks fragment due to gravitational instability when there is
enough mass for self-gravitation, described by the Toomre parameter, and when
heat can be lost at a rate comparable to the local dynamical timescale,
described by t_c=beta Omega^-1. Simulations of self-gravitating disks show that
the cooling parameter has a rough critical value at beta_crit=3. When below
beta_crit, gas overdensities will contract under their own gravity and fragment
into bound objects while otherwise maintaining a steady state of
gravitoturbulence. However, previous studies of the critical cooling parameter
have found dependence on simulation resolution, indicating that the simulation
of self-gravitating protoplanetary disks is not so straightforward. In
particular, the simplicity of the cooling timescale t_c prevents fragments from
being disrupted by pressure support as temperatures rise. We alter the cooling
law so that the cooling timescale is dependent on local surface density
fluctuations, a means of incorporating optical depth effects into the local
cooling of an object. For lower resolution simulations, this results in a lower
critical cooling parameter and a disk more stable to gravitational stresses
suggesting the formation of large gas giants planets in large, cool disks is
generally suppressed by more realistic cooling. At our highest resolution
however, the model becomes unstable to fragmentation for cooling timescales up
to beta = 10.Comment: 10 pages, 6 figures. Accepted for publication in Ap
Filling in the Gaps: Can Gravitationally Unstable Discs Form the Seeds of Gas Giant Planets?
Circumstellar discs likely have a short window when they are self-gravitating
and prone to the effects of disc instability, but during this time the seeds of
planet formation can be sown. It has long been argued that disc fragmentation
can form large gas giant planets at wide orbital separations, but its place in
the planet formation paradigm is hindered by a tendency to form especially
large gas giants or brown dwarfs. We instead suggest that planet formation can
occur early in massive discs, through the gravitational collapse of dust which
can form the seeds of giant planets. This is different from the usual picture
of self-gravitating discs, in which planet formation is considered through the
gravitational collapse of the gas disc into a gas giant precursor. It is
familiar in the sense that the core is formed first, and gas is accreted
thereafter, as is the case in the core accretion scenario. However, by forming
a seed from the gravitational collapse of dust within a
self-gravitating disc there exists the potential to overcome traditional growth
barriers and form a planet within a few times years. The accretion of
pebbles is most efficient with centimetre-sized dust, but the accretion of
millimetre sizes can also result in formation within a Myr. Thus, if dust can
grow to these sizes, planetary seeds formed within very young, massive discs
could drastically reduce the timescale of planet formation and potentially
explain the observed ring and gap structures in young discs.Comment: MNRAS accepted. 15 pages, 12 figure
The fragmentation criteria in local vertically stratified self-gravitating disk simulations
Massive circumstellar disks are prone to gravitational instabilities, which
trigger the formation of spiral arms that can fragment into bound clumps under
the right conditions. Two dimensional simulations of self-gravitating disks are
useful starting points for studying fragmentation, allowing for high-resolution
simulations of thin disks. However, convergence issues can arise in 2D from
various sources. One of these sources is the 2D approximation of self-gravity,
which exaggerates the effect of self-gravity on small scales when the potential
is not smoothed to account for the assumed vertical extent of the disk. This
effect is enhanced by increased resolution, resulting in fragmentation at
longer cooling timescales . If true, it suggests that the 3D simulations
of disk fragmentation may not have the same convergence problem and could be
used to examine the nature of fragmentation without smoothing self-gravity on
scales similar to the disk scale height. To that end, we have carried out local
3D self-gravitating disk simulations with simple cooling with fixed
background irradiation to determine if 3D is necessary to properly describe
disk fragmentation. Above a resolution of grid cells per scale
height, we find that our simulations converge with respect to the cooling
timescale. This result converges in agreement with analytic expectations which
place a fragmentation boundary at .Comment: 11 pages, 9 figures. Accepted for publication in Ap
Formation Criteria and Initial Constraints on Objects Formed in Gravitationally Unstable Disks
Early protoplanetary disks are cool and massive and thus subject to gravitational instabilites and fragmentation of the disk into dense clumps of gas. These fragments are massive enough to become gas giant planets and brown dwarfs in the distant regions of the disks beyond 50 au where traditional planet formation scenarios have trouble creating planetary cores fast enough to explain directly observed planets. I used high-resolution three-dimensional hydrodynamic simulations to model the collapse of self-gravitating disks to constrain the formation location of these fragments and characterize their initial gas and particle masses to compare to directly observed planets and brown dwarfs. I find the traditional cooling criterion, which constrains the formation location to the outer disk, is converged in these simulations and overall masses are consistent with massive gas giants bordering on brown dwarfs. The concentration of solid material in these fragments leads to an increase of the overall metallicity of the fragment and a solid core several tens of Earth masses. To model fragmentation with full disk simulations, I have also implemented a multigrid self-gravity solver in the PLUTO code which uses adaptive mesh refinement to resolve both the disk and fragments
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