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 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 $10^{-3}M_{\odot} {\rm yr^{-1}}$, 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 $Z = 1Z_{\odot} - 10^{-4}~Z_\odot$. 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 $30 \mu
{\rm m}$ at $1Z_{\odot}$, shifts to $< 3 \mu {\rm m}$ at $Z \lesssim 0.1
Z_\odot$. Second, a characteristic feature at $10 \mu \rm{m}$ due to the
amorphous silicate band appears as a dip at $1Z_{\odot}$, 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.
