On the Formation of Massive Primordial Stars

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Abstract
We investigate the formation by accretion of massive primordial protostars in the range of 10-300 M☉. The high accretion rate used in the models ( = 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. After an initial expansion of the radius (for M* 12 M☉), the protostar undergoes an extended phase of contraction (up to M* 60 M☉). 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 80 M☉ is reached. As the interior luminosity approaches the Eddington luminosity, the protostellar radius rapidly expands, reaching a maximum at around 100 M☉. 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 300 M☉. 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.

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