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The first stars are thought to be one of the dominant sources of hydrogenreionization in the early Universe, with their high luminosities and surfacetemperatures expected to drive high ionizing photon production rates. In thiswork, we take our Geneva stellar evolution models of zero-metallicity stars andpredict their production rates of photons capable to ionize H, He I and He II,based on a blackbody approximation. We present analytical fits in the range1.7-500 solar masses. We then explore the impact of stellar initial mass,rotation, and convective overshooting for individual stars. We have found thationizing photon production rates increase with increasing initial mass. For therotational velocities considered we see changes of up to 25% to ionizingphotons produced. This varies with initial mass and ionizing photon species andreflects changes to surface properties due to rotation. We have also found thathigher convective overshooting increases ionizing photon production byapproximately 20% for the change in overshooting considered here. For stellarpopulations, we explore how the production of ionizing photons varies as afunction of the initial mass function (IMF) slope, and minimum and maximuminitial masses. For a fixed population mass we have found changes of the orderof 20-30% through varying the nature of the IMF. This work presents ionizingphoton production predictions for the most up to date Geneva stellar evolutionmodels of Population III stars, and provides insight into how key evolutionaryparameters impact the contribution of the first stars to reionization.

Ionizing photon production of Population III stars: effects of rotation, convection, and initial mass function