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 (Ṁ = 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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