FORMATION OF PRIMORDIAL SUPERMASSIVE STARS BY RAPID MASS ACCRETION

Supermassive stars (SMSs) forming via very rapid mass accretion (M* greater than or similar to 0.1M circle dot yr(-1)) 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) M-5 circle dot. 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 stellarmass until similar or equal to 100 AU for M* greater than or similar to 10(4) M circle dot, after which the star begins to slowly contract. Because of the large radius, the effective temperature is always less than 104 K during rapid accretion. The accreting material is thus almost completely transparent to the stellar radiation. Only for M* greater than or similar to 10(5) M circle dot 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 circle dot. An extremely massive black hole should form after the collapse of the inner