Formation of Primordial Supermassive Stars by Rapid Mass Accretion

Supermassive stars (SMSs) forming via very rapid mass accretion ( ) could be precursors of supermassive black holes observed beyond a redshift of about six. Extending our previous work, here we study the evolution of primordial stars growing under such rapid mass accretion until the stellar mass reaches 10 4 − 5 M ☉ . Our stellar evolution calculations show that a star becomes supermassive while passing through the "supergiant protostar" stage, whereby the star has a very bloated envelope and a contracting inner core. The stellar radius increases monotonically with the stellar mass until ≃ 100 AU for M * ≳ 10 4 M ☉ , after which the star begins to slowly contract. Because of the large radius, the effective temperature is always less than 10 4 K during rapid accretion. The accreting material is thus almost completely transparent to the stellar radiation. Only for M * ≳ 10 5 M ☉ can stellar UV feedback operate and disturb the mass accretion flow. We also examine the pulsation stability of accreting SMSs, showing that the pulsation-driven mass loss does not prevent stellar mass growth. Observational signatures of bloated SMSs should be detectable with future observational facilities such as the James Webb Space Telescope . Our results predict that an inner core of the accreting SMS should suffer from the general relativistic instability soon after the stellar mass exceeds 10 5 M ☉ . An extremely massive black hole should form after the collapse of the inner core.