The local stability of a gravitationally collapsing hydrogen gas cloud is studied by the linear perturbation analysis, including the non-stationary effects of microscopic processes of ionization and molecular reactions, as well as those of self-gravitation and radiative cooling. We derive the fifth-order characteristic equation and examine the condition of occurrence of instability in some limiting cases of wavenumber to see the nature of the instabilities. Numerical estimates of growth rates are performed along the evolutionary sequence of a pre-galactic gas cloud, which is supposed to collapse due to cooling by a low fraction of molecular hydrogen. It is shown that the instabilities for various modes arise during the overall contraction of the gas cloud, and that in a linear regime of perturbation the mode for self-gravitation is not strongly coupled with the other modes for thermal processes. Especially the thermal instability of molecular mode is brought about by dissociation process of molecular hydrogen and the e-folding time of the instability becomes much shorter than the free-fall time. This confirms the results of a previous paper (Sabano and Yoshii 1977) that a pre-galactic gas cloud breaks into fragments with mass less than 10Msun.
Publications of the Astronomical Society of Japan
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