Using numerical simulations to identify observational signatures of self-gravitating protostellar discs
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Date
30/11/2017Author
Hall, Cassandra
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Abstract
In this thesis, I study numerical and semi-analytical models of self-gravitating
protostellar discs, with the aim of furthering our understanding of the role of
disc-self gravity in planet formation. At the time of writing, the ALMA era of
observational astronomy is upon us. Therefore, I place my research into this
context with synthetic images of both numerical and semi-analytical models.
I begin with an examination into the apparent lack of convergence, with increasing
resolution, of the fragmentation boundary in Smoothed Particle Hydrodynamics
(SPH) simulations of a protostellar disc. I run a suite of SPH with different
numerical implementations, and find that even very similar implementations can
fundamentally change the final answer.
I analyse a suite of SPH simulations that fragment to form gravitationally
bound objects, with the motivation of informing future population synthesis
model development. I find that fragment-fragment and fragment-disc interaction
dominates the orbital evolution of the system even at very early times, and any
attempt to produce a population of objects from the gravitational instability
process must include these interactions.
Before a disc fragments, it will go through a self-gravitating phase. If the disc cools
globally on a timescale such that it is balanced by heating due to gravitational
stresses, the disc will be in a state of quasi-equilibrium. So long as the disc
mass is sufficiently low, and spirals are sufficiently tightly wound, then angular
momentum transport can be described by the local approximation, for which there
is an analytical description.
Using this analytical description, I develop an existing 1D model into 3D, and
examine a wide range of parameter space for which disc self-gravity produces
significant non-axisymmetry. Using radiative transfer calculations coupled with
synthetic observations, I determine that there is a very narrow range of parameter
space in which a disc will have sufficiently large gravitational stresses so as to
produce detectable spirals, but the stresses not be so large as to cause the disc to
fragment. By developing a simple analytical prescription for dust, I show that this
region of parameter space can be broadened considerably. However, it requires
grains that are large enough to become trapped by pressure maxima in the disc,
so I conclude that if self-gravitating spiral arms are detected in the continuum, it
is likely that at least some grain growth has taken place.