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We investigate the upper stellar mass limit set by radiative feedback by theforming star with various accretion rates and metallicities. To this end, wenumerically solve the structures of both a protostar and its surroundingaccretion envelope assuming a spherical symmetric and steady flow. The opticaldepth of the dust cocoon, a dusty part of the accretion envelope, differs amongthe direct light from the stellar photosphere and the diffuse light re-emittedas dust thermal emission. As a result, varying the metallicity qualitativelychanges the way that the radiative feedback suppresses the accretion flow. Witha fixed accretion rate of $10^{-3}M_{\odot} {\rm yr^{-1}}$, the both direct anddiffuse lights jointly operate to prevent the mass accretion at $Z \gtrsim10^{-1}Z_{\odot}$. At $Z \lesssim 10^{-1}Z_{\odot}$, the diffuse light is nolonger effective, and the direct light solely limits the mass accretion. At $Z\lesssim 10^{-3}Z_{\odot}$, the HII region formation plays an important role interminating the accretion. The resultant upper mass limit increases withdecreasing metallicity, from a few $\times~10~M_\odot$ to $\sim 10^3~M_\odot$over $Z = 1Z_{\odot} - 10^{-4}~Z_\odot$. We also illustrate how the radiationspectrum of massive star-forming cores changes with decreasing metallicity.First, the peak wavelength of the spectrum, which is located around $30 \mu{\rm m}$ at $1Z_{\odot}$, shifts to $< 3 \mu {\rm m}$ at $Z \lesssim 0.1Z_\odot$. Second, a characteristic feature at $10 \mu \rm{m}$ due to theamorphous silicate band appears as a dip at $1Z_{\odot}$, but changes to a bumpat $Z \lesssim 0.1 Z_{\odot}$. Using these spectral signatures, we can searchmassive accreting protostars in nearby low-metallicity environments withup-coming observations.

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