Last modified: Sat Feb 19 17:52:51 JST 2000
Doctoral Dissertation
A Study of Gravitational Collapse of Filaments
in Molecular Clouds
with Radiation Hydrodynamics Simulations
Koji OGOCHI
A dissertation submitted to the Doctoral Program
in Physics, the University of Tsukuba
in partial fulfillment of the requirements for the
degree of Doctor of Philosophy (Science)
submitted on January 2000
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Abstract
We perform radiation hydrodynamics (RHD) simulations
to study the self-gravitational collapse of the filaments
in molecular clouds.
For simplicity, we assume that the filaments are in
1-D axisymmetric geometry
and that neither magnetic field nor angular momentum is present.
We focus our attention on the radiative transfer effects.
The dynamics of the collapsing filaments strongly depends
on thermodynamic properties.
Especially, the isothermality has special meanings
in the cylindrical geometry.
Therefore, the careful treatments of thermodynamics
are indispensable to study the collapse of the filaments.
For the purpose,
we take into account the effects of the radiative transfer
since it mainly determines the energy flows.
Unfortunately, simple approximated methods
in which the optical depth is assumed
are inadequate for our study.
Instead, we adopt the variable Eddington factor (VEF) method
%which is adequate for any optical depth
though its numerical costs are expensive.
This is the first RHD calculation using the VEF method
for the collapse of the filaments.
From both analytic and computational calculations,
we derive a criterion that the filaments can be isothermal
during the collapse.
The criterion is expressed in terms of the central density
when the isothermality breaks down.
We investigate the criterion under various conditions.
As a result, we find that
the criterion does not agree with an intuitive understanding that
the filaments become adiabatic when they become opaque to radiation.
Indeed, numerical results show that the collapsing filaments can be
still isothermal even when they become opaque,
if the cooling rate dominates the heating rate.
Alternatively,
the filament temperature can depart from constant even in
the optically thin regime if the heating rate exceeds the cooling rate.
Using the criterion,
we also estimate the mass of dense cores which will be formed
by the fragmentation of the filaments.
It is expected that
the estimated mass could explain the mass spectrum of
very low mass stars such as brown dwarfs in star forming sites.
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ogochi@rccp.tsukuba.ac.jp