RAPIDLY ACCRETING SUPERGIANT PROTOSTARS: EMBRYOS OF SUPERMASSIVE BLACK HOLES?

Direct collapse of supermassive stars (SMSs) is a possible pathway for generating supermassive black holes in the early universe. It is expected that an SMS could form via very rapidmass accretion with M-* similar to 0.1-1 M-circle dot yr(-1) during the gravitational collapse of an atomic-cooling primordial gas cloud. In this paper, we study how stars would evolve under such extreme rapid mass accretion, focusing on the early evolution until the stellar mass reaches 10(3) M-circle dot. To this end, we numerically calculate the detailed interior structure of accreting stars with primordial element abundances. Our results show that for accretion rates higher than 10(-2) M-circle dot yr(-1), stellar evolution is qualitatively different from that expected at lower rates. While accreting at these high rates, the star always has a radius exceeding 100 R-circle dot, which increases monotonically with the stellar mass. The mass-radius relation for stellar masses exceeding similar to 100 M-circle dot follows the same track with R-* proportional to M-*(1/2) in all cases with accretion rates greater than or similar to 10(-2) M-circle dot yr(-1); at a stellar mass of 10(3) M-circle dot, the radius is similar or equal to 7000 R-circle dot (similar or equal to 30 AU). With higher accretion rates, the onset of hydrogen burning is shifted toward higher stellar masses. In particular, for accretion rates exceeding M-* greater than or similar to 0.1 M-circle dot yr(-1), there is no significant hydrogen burning even after 10(3) M-circle dot have accreted onto the protostar. Such supergiant protostars have effective temperatures as low as T-eff similar or equal to 5000 K throughout their evolution and because they hardly emit ionizing photons, they do not create an H II region or significantly heat their immediate surroundings. Thus, radiative feedback is unable to hinder the growth of rapidly accreting stars to masses in excess of 10(3) M-circle dot as long as material is accreted at rates M-* greater than or similar to 10(-2) M-circle dot yr(-1).