Thermal Evolution of Primordial Gas Clouds and Mass Range of Zero-Metal Stars

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We investigate the evolution and fragmentation of a gas cloud with the primordial chemical composition. A hydrogen gas cloud collapses isothermally at 500-100C K when a low fraction of molecular hydrogen works as a coolant, and breaks into small subcondensations with mass less than 10 M_⦿ due to thermal instability associated with molecular dissociation. On the other hand a hydrogen gas cloud which contains no molecular hydrogen collapses isothermally at 6000-8000 K due to two-photon cooling of atomic hydrogen in a thermally stable condition, and enters the region where thermal energy exceeds radiation energy when thermal equilibrium between matter and radiation is achieved in the cloud. Consideration of energetics in the subsequent stage of the cloud evolution gives an estimate of the mass range 0.180 M_⦿ for stable nuclear-burning protostars of the first generation. The thermal behavior of a gas cloud in the regime of z (the ratio of heavy-element abundance to solar one) less than 10^<-4> is essentially similar to that in case of no heavy elements, and the heavy-element cooling brings about thermal instability in a wide range of density and temperature in the regime of z greater than 10^<-3>. Linear perturbation analysis gives a growth time of the instability much shorter than the free-fall time, and suggests an efficient excitation of density fluctuations driven by thermal instability.FragmentationPrimordial cloudsThermal evolutionzero-metal stars
1984-06-25

Thermal Evolution of Primordial Gas Clouds and Mass Range of Zero-Metal Stars

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