We investigate the upper stellar mass limit set by radiative feedback by the forming star with various accretion rates and metallicities .
To this end , 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 among the direct light from the stellar photosphere and the 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 _ { \odot } { yr ^ { -1 } } , the both direct and diffuse lights jointly operate to prevent the mass accretion at Z \gtrsim 10 ^ { -1 } Z _ { \odot } .
At Z \lesssim 10 ^ { -1 } Z _ { \odot } , the diffuse light is no longer 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 in terminating the accretion .
The resultant upper mass limit increases with decreasing metallicity , from a few \times~ { } 10 ~ { } M _ { \odot } to \sim 10 ^ { 3 } ~ { } M _ { \odot } over Z = 1 Z _ { \odot } -10 ^ { -4 } ~ { } Z _ { \odot } .
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 1 Z _ { \odot } , shifts to < 3 \mu { m } at Z \lesssim 0.1 Z _ { \odot } .
Second , a characteristic feature at 10 \mum due to the amorphous silicate band appears as a dip at 1 Z _ { \odot } , but changes to a bump at Z \lesssim 0.1 Z _ { \odot } .
Using these spectral signatures , we can search massive accreting protostars in nearby low-metallicity environments with up-coming observations .