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