First Star Formation : Protostellar Evolution from Prestllar Cores to Main-Sequence Stars

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We review formation process of the first stars. After forming by fragmentation of a firstobject, a primordial prestellar core (〜 10^3M_〓) collapses in a self-similar fashion until theformation of a protostar at the number density of about 10^<22>cm^<-3>. The end product ofthe protostellar collapse is a tiny protostar (〜 10^<-3>M_〓) surrounded by a large amount ofgravitationally unstable matter (〜 10^3M_〓). Subsequently, the protostar grows in mass byorders of magnitude owing to accretion of the ambient matter. The accretion rate M is very high (10^<-(2-3)>M_〓/yr) for the first stars, because of high prestellar temperature (> 300 K) owing to H_2 cooling in the primordial gas. During the accretion phase, there are a fewdistinctive evolutionary stages : the adiabatic accretion phase (ML_* < 3-10 M_〓), the Kelvin-Helmhotz contraction phase (ML_* < 30-10 M_〓). The subsequent stage is either (a) settlingto the zero-age main sequence with continuing accretion (M < 4 × 10^<-3>M_〓/yr), or (b) halting of the accretion by the radiation pressure (M > 4 × 10^<-3>M_〓/yr), depending on thevalue of the accretion rate. Under a realistic accretion rate proposed by Abel et al. [Science 295 (2002), 93], the actual evolution resembles the case of small M Since the protostaris shrouded by an optically thick envelope throughout the main accretion phase, the hotstellar surface is not visible from outside until the protostellar mass grows to about 500M_〓). During this period, the effective temperature remains about 6000K for a wide range of theluminosity.
理論物理学刊行会の論文
2003-02-28

First Star Formation : Protostellar Evolution from Prestllar Cores to Main-Sequence Stars

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