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

The first stars are thought to be one of the dominant sources of hydrogen reionization in the early Universe, with their high luminosities and surface temperatures expected to drive high ionizing photon production rates. In this work we take our Geneva stellar evolution model grid of zero-metallicity stars and predict their production rates of photons capable to ionize H, He I and He II. We explore the impact of stellar initial mass, rotation, convective overshooting, and initial mass function of the host population. We have found that ionizing photon production rates increase with increasing initial mass, therefore populations with more top heavy initial mass functions produce more ionizing photons. For the rotational velocities in our model grid we see changes of up to 25% to ionizing photons produced. This varies with ionizing photon species and reflects changes to surface properties due to rotation. We have also found that higher convective overshooting increases ionizing photon production at all initial masses for all ionizing photon species, by approximately 20% for the change in overshooting considered here. For stellar populations, the variations in the slope of the initial mass function and the maximum initial mass have a significant effect on the production of ionizing photons. This work presents ionizing photon production predictions for the most up to date Geneva stellar evolution models of Population III stars ahead of future observations from facilities such as JWST, and provides insight into how key evolutionary parameters impact the contribution of the first stars to reionization.