The Initial mass function of the first stars inferred from extremely metal-poor stars

TL;DR

This study analyzes elemental abundances in extremely metal-poor stars to infer that the first stars were mostly under 40 solar masses, with many being hypernovae, impacting early universe chemical enrichment.

Contribution

It provides new constraints on the initial mass function of the first stars by comparing observed abundances with supernova yield models across various masses and energies.

Findings

Most EMP stars are best fitted with supernova models of initial mass less than 40 M_0.

Over half of the stars are best fitted with 25 M_0 hypernova models.

Primordial supernovae ejected 10^{-2} to 10^{-1} M_\u001d0 of 56Ni, leaving remnants of 1.5-5 M_0.

Abstract

We compare elemental abundance patterns of $\sim 200$ extremely metal-poor (EMP; [Fe/H]$<-3$) stars with supernova yields of metal-free stars in order to obtain insights into the characteristic masses of the first (Population III or Pop III) stars in the Universe. Supernova yields are prepared with nucleosynthesis calculations of metal-free stars with various initial masses ($M=$13, 15, 25, 40 and 100 $M_{\odot}$) and explosion energies ($E_{51}=E/10^{51}$[erg]$=0.5-60$) to include low-energy, normal-energy, and high-energy explosions. We adopt the mixing-fallback model to take into account possible asymmetry in the supernova explosions and the yields that best-fit the observed abundance patterns of the EMP stars are searched by varying the model parameters. We find that the abundance patterns of the EMP stars are predominantly best-fitted with the supernova yields with initial masses $M<40 M_{\odot}$, and that more than than half of the stars are best fitted with the $M=25 M_\odot$ hypernova ($E_{51}=10$) models. The results also indicate that the majority of the primordial supernovae have ejected $10^{-2}-10^{-1} M_\odot$ of $^{56}$Ni leaving behind a compact remnant, either a neutron star or a black hole, with mass in a range of $\sim 1.5-5 M_{\odot}$. The results suggest that the masses of the first stars responsible for the first metal-enrichment are predominantly $< 40 M_{\odot}$. This implies that the higher mass first stars were either less abundant or directly collapsing into a blackhole without ejecting heavy elements or that a supernova explosion of a higher-mass first star inhibits the formation of the next generation of low-mass stars at [Fe/H]$<-3$.