On the Formation of Massive Primordial Stars

Abstract

We investigate the formation by accretion of massive primordial protostars in the range 10 to 300 Mo. The high accretion rate used in the models (M = 4.4 × 10⁻³ M _⊙ yr ¹) causes the structure and evolution to differ significantly from those of both present-day protostars and primordial zero-age main sequence stars. The stellar surface is not visible throughout most of the main accretion phase, since a photosphere is formed in the infalling envelope. Significant nuclear burning does not take place until a protostellar mass of about 80 M _⊙. As the interior luminosity approaches the Eddington luminosity, the protostellar radius rapidly expands owing to the radiation pressure. Eventually, a final swelling occurs when the stellar mass reaches about 300 Mo. This expansion is likely to signal the end of the main accretion phase, thus setting an upper limit to the protostellar mass formed in these conditions.