60 research outputs found
Triggered fragmentation in gravitationally unstable discs: forming fragments at small radii
We carry out three dimensional radiation hydrodynamical simulations of
gravitationally unstable discs using to explore the movement of mass in a disc
following its fragmentation. Compared to a more quiescent state before it
fragments, the radial velocity of the gas increases by up to a factor of ~2-3
after fragmentation. While the mass movement occurs both inwards and outwards,
the inwards motion can cause the inner spirals to be sufficiently dense that
they may become unstable and potentially fragment. Consequently, the dynamical
behaviour of fragmented discs may cause subsequent fragmentation at smaller
radii after an initial fragment has formed in the outer disc.Comment: Submitted to the conference proceedings of: Instabilities and
Structures in Proto-Planetary Disks. 5 pages; 4 figure
Non-convergence of the critical cooling timescale for fragmentation of self-gravitating discs
We carry out a resolution study on the fragmentation boundary of
self-gravitating discs. We perform three-dimensional Smoothed Particle
Hydrodynamics simulations of discs to determine whether the critical value of
the cooling timescale in units of the orbital timescale, beta_{crit}, converges
with increasing resolution. Using particle numbers ranging from 31,250 to 16
million (the highest resolution simulations to date) we do not find
convergence. Instead, fragmentation occurs for longer cooling timescales as the
resolution is increased. These results suggest that at the very least, the
critical value of the cooling timescale is longer than previously thought.
However, the absence of convergence also raises the question of whether or not
a critical value exists. In light of these results, we caution against using
cooling timescale or gravitational stress arguments to deduce whether
gravitational instability may or may not have been the formation mechanism for
observed planetary systems.Comment: Accepted for publication by MNRAS Letters. 6 pages, 3 figure
Planet migration in self-gravitating discs : survival of planets
We carry out three-dimensional SPH simulations to study whether planets can survive in self-gravitating protoplanetary discs. The discs modelled here use a cooling prescription that mimics a real disc which is only gravitationally unstable in the outer regions. We do this by modelling the cooling using a simplified method such that the cooling time in the outer parts of the disc is shorter than in the inner regions, as expected in real discs. We find that both giant (>MSat) and low mass (<MNep) planets initially migrate inwards very rapidly, but are able to slow down in the inner gravitationally stable regions of the disc without needing to open up a gap. This is in contrast to previous studies where the cooling was modelled in a more simplified manner where regardless of mass, the planets were unable to slow down their inward migration. This shows the important effect the thermodynamics has on planet migration. In a broader context, these results show that planets that form in the early stages of the discs’ evolution, when they are still quite massive and self-gravitating, can survive
Large grains can grow in circumstellar discs
We perform coagulation & fragmentation simulations to understand grain growth
in T Tauri & brown dwarf discs. We present a physically-motivated approach
using a probability distribution function for the collision velocities and
separating the deterministic & stochastic velocities. We find growth to larger
sizes compared to other models. Furthermore, if brown dwarf discs are
scaled-down versions of T Tauri discs (in terms of stellar & disc mass, and
disc radius), growth at the same location with respect to the outer edge occurs
to similar sizes in both discs.Comment: Submitted to the conference proceedings of the IAU Symposium 299 -
Exploring the formation and evolution of planetary systems. 2 pages; 2
figure
On the gap-opening criterion of migrating planets in protoplanetary disks
We perform two-dimensional hydrodynamical simulations to quantitatively
explore the torque balance criterion for gap-opening (as formulated by Crida et
al. 2006) in a variety of disks when considering a migrating planet. We find
that even when the criterion is satisfied, there are instances when planets
still do not open gaps. We stress that gap-opening is not only dependent on
whether a planet has the ability to open a gap, but whether it can do so
quickly enough. This can be expressed as an additional condition on the
gap-opening timescale versus the crossing time, i.e. the time it takes the
planet to cross the region which it is carving out. While this point has been
briefly made in the previous literature, our results quantify it for a range of
protoplanetary disk properties and planetary masses, demonstrating how crucial
it is for gap-opening. This additional condition has important implications for
the survival of planets formed by core accretion in low mass disks as well as
giant planets or brown dwarfs formed by gravitational instability in massive
disks. It is particularly important for planets with intermediate masses
susceptible to Type III-like migration. For some observed transition disks or
disks with gaps, we expect that estimates on the potential planet masses based
on the torque balance gap-opening criterion alone may not be sufficient. With
consideration of this additional timescale criterion theoretical studies may
find a reduced planet survivability or that planets may migrate further inwards
before opening a gap.Comment: Accepted by ApJ, 22 pages, 13 figures, 6 table
On the fragmentation of self-gravitating discs
I have carried out three-dimensional numerical simulations of self-gravitating discs to determine under what circumstances they fragment to form bound clumps that may grow into giant planets. Through radiation hydrodynamical simulations using a Smoothed Particle Hydrodynamics code, I find that the disc opacity plays a vital role in determining whether a disc fragments. Specifically, opacities that are smaller than interstellar Rosseland mean values promote fragmentation (even at small radii, R < 25AU) since low opacities allow a disc to cool quickly. This may occur if a disc has a low metallicity or if grain growth has occurred. Given that the standard core accretion model is less likely to form planets in a low metallicity environment, I predict that gravitational instability is the dominant planet formation mechanism in a low metallicity environment. In addition, I find that the presence of stellar irradiation generally acts to inhibit fragmentation (since the discs can only cool to the temperature defined by stellar irradiation). However, fragmentation may occur if the irradiation is sufficiently weak that it allows the disc to attain a low Toomre stability parameter.
With specific reference to the HR 8799 planetary system, I find that it is only possible for fragments to form in the radial range where the HR 8799 planets are located (approximately 24-68 AU) if the disc is massive. In such a high mass regime, mass transport occurs in the disc causing the surface mass density to alter. Therefore, fragmentation is not only affected by the disc temperature and cooling, but also by any restructuring due to the gravitational torques. The high mass discs also pose a problem for the formation of this system because the protoplanets accrete from the disc and end up with masses greater than those inferred from observation and thus, the growth of planets would need to be inhibited. In addition, I find that further subsequent fragmentation at small radii also takes place.
By way of analytical arguments in combination with hydrodynamical simulations using a parameterised cooling method, I explore the fragmentation criteria which in the past, has placed emphasis on the cooling timescale in units of the orbital timescale, beta. I find that at a given radius the surface mass density (i.e. disc mass and profile) and star mass also play a crucial role in determining whether a disc fragments or not as well as where in the disc fragments form. I find that for shallow surface mass density profiles (p<2, where the surface mass density is proportional to R^{-p}), fragments form in the outer regions of the disc. However for steep surface mass density profiles (p is greater than or similar to 2), fragments form in the inner regions of a disc. In addition, I find that the critical value of the cooling timescale in units of the orbital timescale, beta_crit, found in previous simulations is only applicable to certain disc surface mass density profiles and for particular disc radii and is not a general rule for all discs. I obtain an empirical fragmentation criteria between the cooling timescale in units of the orbital timescale, beta, the surface mass density, the star mass and the radius. Finally, I carry out crucial resolution testing by performing the highest resolution disc simulations to date. My results cast some serious doubts on previous conclusions concerning fragmentation of self-gravitating discs
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