Low-Metallicity Protostars and the Maximum Stellar Mass Resulting from
  Radiative Feedback
  The final mass of a newborn star is set at the epoch when the mass accretion
onto the star is terminated. We study the evolution of accreting protostars and
the limits of accretion in low metallicity environments. Accretion rates onto
protostars are estimated via the temperature evolution of prestellar cores with
different metallicities. The derived rates increase with decreasing
metallicity, from 10^-6 M_sun/yr at Z = Z_sun to 10^-3 M_sun/yr at Z = 0. With
the derived accretion rates, the protostellar evolution is numerically
calculated. We find that, at lower metallicity, the protostar has a larger
radius and reaches the zero-age main-sequence (ZAMS) at higher stellar mass.
Using this protostellar evolution, we evaluate the upper stellar mass limit
where the mass accretion is hindered by radiative feedback. We consider the
effects of radiation pressure exerted on the accreting envelope, and expansion
of the HII region. The mass accretion is finally terminated by radiation
pressure on dust grains in the envelope for Z > 10^-3 Z_sun and by the
expanding HII region for lower metallicty. The mass limit from these effects
increases with decreasing metallicity from 10 M_sun at Z = Z_sun to about 300
M_sun at Z = 10^-6 Z_sun. The termination of accretion occurs after the central
star arrives at the ZAMS at all metallicities, which allows us to neglect
protostellar evolution effects in discussing the upper mass limit by stellar
feedback. The fragmentation induced by line cooling in low-metallicity clouds
yields prestellar cores with masses large enough that the final stellar mass is
set by the feedback effects.
