Upper stellar mass limit by radiative feedback at low-metallicities: metallicity and accretion rate dependence

We investigate the upper stellar mass limit set by radiative feedback for a forming star with various accretion rates and metallicities. Thus, we numerically solve the structures of both a protostar and its surrounding accretion envelope assuming a spherical symmetric and steady flow. The optical depth of the dust cocoon, a dusty part of the accretion envelope, differs for direct light from the stellar photosphere and diffuse light re-emitted as dust thermal emission. As a result, varying the metallicity qualitatively changes the way that the radiative feedback suppresses the accretion flow. With a fixed accretion rate of 10(-3)M(circle dot) yr(-1), both direct and diffuse light jointly operate to prevent mass accretion at Z greater than or similar to 10(-1) Z(circle dot). At Z less than or similar to 10(-1) Z(circle dot), the diffuse light is no longer effective and the direct light solely limits the mass accretion. At Z less than or similar to 10(-3) Z(circle dot), formation of the HII region plays an important role in terminating the accretion. The resultant upper mass limit increases with decreasing metallicity, from a few x 10 M-circle dot to similar to 10(3) M-circle dot over Z = 1Z(circle dot)-10(-4) Z(circle dot). We also illustrate how the radiation spectrum of massive star-forming cores changes with decreasing metallicity. First, the peak wavelength of the spectrum, which is located around 30 mu m at 1Z(circle dot), shifts to < 3 mu m at Z less than or similar to 0.1 Z(circle dot). Secondly, a characteristic feature at 10 mu m due to the amorphous silicate band appears as a dip at 1 Z(circle dot), but changes to a bump at Z less than or similar to 0.1 Z(circle dot). Using these spectral signatures, we can search massive accreting protostars in nearby low-metallicity environments with upcoming observations.