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Globular clusters contain multiple stellar populations with peculiar chemicalcompositions. Pollution of the intracluster gas by an early population offast-evolving stars is the most common scenario for explaining theobservations. Stars with masses in excess of 1000 Msun have recently beensuggested as potential polluters. We investigate the spectral properties ofproto-GCs that would host a supermassive star. Our main goal is to quantify howsuch a star would affect the integrated light of the cluster, and to study thedetectability of such objects. We computed non-LTE atmosphere models for SMSwith various combinations of stellar parameters (luminosity, effectivetemperature, and mass) and metallicities appropriate for GCs, and we predicttheir emergent spectra. Using these spectra, we calculated the total emissionof young proto-GCs with SMS as predicted in a previously reported scenario, andwe computed synthetic photometry in UV, optical, and near-IR bands, inparticular for the JWST. At an effective temperature of 10000 K, the spectrumof SMSs shows a Balmer break in emission. This feature is due to strong non-LTEeffects (implied by the high luminosity) and is not observed in normal stars.The hydrogen lines also show a peculiar behavior, with Balmer lines in emissionwhile higher series lines are in absorption. At 7000 K, the Balmer break showsa strong absorption. At high effective temperatures, the Lyman break is foundin emission. Cool and luminous SMSs are found to dominate the integratedspectrum of the cluster, except for the UV range. The predicted magnitudes ofthese 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 coolSMSs imply that they might in principle be detected in color-color diagramsthat probe the spectral energy distribution below and above the Balmer break.

Spectral properties and detectability of supermassive stars in protoglobular clusters at high redshift