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 10-3M_⊙ yr^-1, both direct and diffuse light jointly operate to prevent mass accretion at Z _⊙ 10^-1 Z_⊙. At Z ≲ 10^-1 Z_⊙, the diffuse light is no longer effective and the direct light solely limits the mass accretion. At Z ≲ 10^-3 Z_⊙, 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 × 10M_⊙ to ~103M_⊙ over Z = 1Z_⊙-10^-4 Z_⊙. 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 μm at 1Z_⊙, shifts to < 3 μm at Z ≲ 0.1 Z_⊙. Secondly, a characteristic feature at 10 μm due to the amorphous silicate band appears as a dip at 1 Z_⊙, but changes to a bump at Z ≲ 0.1 Z_⊙. Using these spectral signatures, we can search massive accreting protostars in nearby low-metallicity environments with upcoming observations.