Thermal and Dynamical Evolution of Primordial Gas Clouds: On the Formation of First Luminous Objects

We investigate the thermal and dynamical evolution of primordial gas cloudsin the universe after decoupling. Comparing the time-scale of dynamicalevolution with that of fragmentation, we can estimate the typical fragmentationscale. We propose the following scenario of the formation process of firstluminous objects consisting of large number stars. First, by pancake collapseof the overdensity regions in the expanding universe or collision betweenclouds in potential wells, quasi-plane shocks form. If the shock-heatedtemperature is higher than about $10^4$ K, the postshock gas cools down toseveral hundred K by \Hbun line cooling, and the shock-compressed layerfragments into filamentary clouds. The filamentary cloud collapses dynamicallyonce more and fragments into cloud cores. Finally, a primordial star forms in acloud core. We show that the minimum mass of the first star is essentiallydetermined by the Chandrasekhar mass. Also, we investigate the dynamicalcollapse of cloud cores by numerical simulation and show that the evolutionpaths of the central regions of the cores depend only very weakly on the totalcore mass. After mass accretion, a massive star may be formed in a core, sincethe estimated mass accretion rate is very large. In such a case, it may bepossible for many massive stars form almost simultaneously. Then the clouds canbe luminous objects. On the other hand, if the shock-heated temperature islower, effective star formation is delayed significantly.Comment: PTP Tex file 23 pages, 10 figure