On the Formation of Massive Primordial Stars

We investigate the formation by accretion of massive primordial protostars in the range 10 to 300 M⊙. The high accretion rate used in the models (Ṁ = 4.4 × 10 M⊙ yr ) causes the structure and evolution to differ significantly from those of both present-day protostars and primordial zero-age main sequence stars. After an initial expansion of the radius (for M∗ . 12M⊙), the protostar undergoes an extended phase of contraction (up to M∗ ≃ 60M⊙). The stellar surface is not visible throughout most of the main accretion phase, since a photosphere is formed in the infalling envelope. Also, significant nuclear burning does not take place until a protostellar mass of about 80M⊙. As the interior luminosity approaches the Eddington luminosity, the protostellar radius rapidly expands, reaching a maximum around 100M⊙. Changes in the ionization of the surface layers induce a secondary phase of contraction, followed by a final swelling due to radiation pressure when the stellar mass reaches about 300M⊙. 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. Subject headings: cosmology: theory — early universe — stars: formation — stars: pre-main-sequence