Some First Stars Were Red: Detecting Signatures of Massive Population III Formation through Long-term Stochastic Color Variations

Identifying stars formed in pristine environments (Population III) within the first billion years is vital to uncovering the earliest growth and chemical evolution of galaxies. Population III galaxies, however, are typically expected to be too faint and too few in number to be detectable by forthcoming instruments without extremely long integration times and/or extreme lensing. In an environment, however, where star formation is suppressed until a halo crosses the atomic-cooling limit (e.g., by a modest Lyman-Werner flux, high baryonic streaming velocities, and/or dynamical heating effects), primordial halos can form substantially more numerous and more massive stars. Some of these stars will in turn be accreting more rapidly than they can thermally relax at any given time. Using high-resolution cosmological zoom-in simulations of massive star formation in high-z halos, we find that such rapidly accreting stars produce prominent spectral features that would be detectable by the James Webb Space Telescope. The rapid-accretion episodes within the halo lead to stochastic reprocessing of 0%-20% of the total stellar emission into the rest-frame optical over long timescales, a unique signature which may allow deep observations to identify such objects out to z ~ 10-13 using mid- and wide-band NIRCam colors alone.