Closing in on Pop-III Stars: Constraints and Predictions Across the Spectrum

Abstract: The absence of direct high redshift observations poses a significant challenge in understanding the properties of first stars. Nonetheless, the cumulative effect of entire stellar populations can be studied with current data. In this work we use a combination of high redshift observables in order to infer the formation and emission properties of the first stellar populations: high redshift UVLFs, the optical depth of CMB photons to reionization, hydrogen absorption lines in quasar spectra, and measurements of the soft cosmic X-ray background. We study two minimal models of stellar population: i) a single, Pop-II, stellar population which dominates throughout Cosmic Dawn, ii) two distinct stellar populations, Pop-II and Pop-III, dominating at different times with the transition between them taken as a free parameter. We set strong constraints on the properties of Pop-II stars, and upper limits on the formation and multi-wavelength emission of Pop-III stars. After applying the constraints above, we present the viable envelopes of the 21-cm global signal with and without Pop-III stars. We identify a region in the parameter space of the two population model which predicts a global 21-cm signal distinctive from that of the single population one. A measurement of such a signal would be a strong indication for the presence of Pop-III stars at early times.