On the detection of supermassive primordial stars

The collapse of supermassive primordial stars in hot, atomically cooled halos may have given birth to the first quasars at z ~ 15–20. Recent numerical simulations of these rapidly accreting stars reveal that they are cool, red hypergiants shrouded by dense envelopes of pristine atomically cooled gas at 6000–8000 K, with luminosities L  1010 L ⊙. Could such luminous but cool objects be detected as the first stage of quasar formation in future near-infrared (NIR) surveys? We have now calculated the spectra of supermassive primordial stars in their birth envelopes with the Cloudy code. We find that some of these stars will be visible to the James Webb Space Telescope at z  20 and that with modest gravitational lensing, Euclid and the Wide-Field Infrared Space Telescope could detect them out to z ~ 10–12. Rather than obscuring the star, its accretion envelope enhances its visibility in the NIR today by reprocessing its short-wavelength flux into photons that are just redward of the Lyman limit in the rest frame of the star.