The Mass Distribution of Population III Stars

Extremely metal-poor stars are uniquely informative on the nature of massive Population III stars. Modulo a few elements that vary with stellar evolution, the present-day photospheric abundances observed in extremely metal-poor stars are representative of their natal gas cloud composition. For this reason, the chemistry of extremely metal-poor stars closely reflects the nucleosynthetic yields of supernovae from massive Population III stars. Here we collate detailed abundances of 53 extremely metal-poor stars from the literature and infer the masses of their Population III progenitors. We fit a simple initial mass function to a subset of 29 of theinferred Population III star masses, and find that the mass distribution is well-represented by a power law IMF with exponent $\alpha = 2.35^{+0.29}_{-0.24}$. The inferred maximum progenitor mass for supernovae from massive Population III stars is $M_{\rm{max}} = 87^{+13}_{-33}$ M$_\odot$, and we find no evidence in our sample for a contribution from stars with masses above $\sim$120 M$_\odot$. The minimum mass is strongly consistent with the theoretical lower mass limit for Population III supernovae. We conclude that the IMF for massive Population III stars is consistent with the initial mass function of present-day massive stars and there may well have formed stars much below the supernova mass limit that could have survived to the present day.