First star formation - Protostellar evolution from prestellar cores to main-sequence stars

We review formation process of the first stars. After forming by fragmentation of a first object, a primordial prestellar core (∼ 10³M_⊙) collapses in a self-similar fashion until the formation of a protostar at the number density of about 10²²cm^-3. The end product of the protostellar collapse is a tiny protostar (∼ 10^-3M_⊙) surrounded by a large amount of gravitationally unstable matter (∼ 10³M_⊙). Subsequently, the protostar grows in mass by orders of magnitude owing to accretion of the ambient matter. The accretion rate Ṁ is very high (10^-(2-3)M_⊙/yr) for the first stars, because of high prestellar temperature (> 300 K) owing to H₂ cooling in the primordial gas. During the accretion phase, there are a few distinctive evolutionary stages; the adiabatic accretion phase (M* < 3-10 M_⊙), the Kelvin-Helmhotz contraction phase (M* < 30-100 M_⊙). The subsequent stage is either (a) settling to the zero-age main sequence with continuing accretion (M* < 4 × 10^-3M_⊙/yr). or (b) halting of the accretion by the radiation pressure (Ṁ > 4 × 10^-3M_⊙/yr), depending on the value 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 Ṁ. Since the protostar is shrouded by an optically thick envelope throughout the main accretion phase, the hot stellar surface is not visible from outside until the protostellar mass grows to about 500M_⊙. During this period, the effective temperature remains about 6000 K for a wide range of the luminosity.