Spectral properties and detectability of supermassive stars in
  protoglobular clusters at high redshift
  Globular clusters contain multiple stellar populations with peculiar chemical
compositions. Pollution of the intracluster gas by an early population of
fast-evolving stars is the most common scenario for explaining the
observations. Stars with masses in excess of 1000 Msun have recently been
suggested as potential polluters. We investigate the spectral properties of
proto-GCs that would host a supermassive star. Our main goal is to quantify how
such a star would affect the integrated light of the cluster, and to study the
detectability of such objects. We computed non-LTE atmosphere models for SMS
with various combinations of stellar parameters (luminosity, effective
temperature, and mass) and metallicities appropriate for GCs, and we predict
their emergent spectra. Using these spectra, we calculated the total emission
of young proto-GCs with SMS as predicted in a previously reported scenario, and
we computed synthetic photometry in UV, optical, and near-IR bands, in
particular for the JWST. At an effective temperature of 10000 K, the spectrum
of SMSs shows a Balmer break in emission. This feature is due to strong non-LTE
effects (implied by the high luminosity) and is not observed in normal stars.
The hydrogen lines also show a peculiar behavior, with Balmer lines in emission
while higher series lines are in absorption. At 7000 K, the Balmer break shows
a strong absorption. At high effective temperatures, the Lyman break is found
in emission. Cool and luminous SMSs are found to dominate the integrated
spectrum of the cluster, except for the UV range. The predicted magnitudes of
these proto-GCs are m_AB~28-30 between 0.7 and 8 um and for redshifts z~4-10,
which is detectable with the JWST. The peculiar observational features of cool
SMSs imply that they might in principle be detected in color-color diagrams
that probe the spectral energy distribution below and above the Balmer break.
