Variations in the Abundance Pattern of Extremely Metal‐Poor Stars and Nucleosynthesis in Population III Supernovae

Abstract

We calculate nucleosynthesis in Population III supernovae (SNe) and compare the yields with various abundance patterns of extremely metal-poor (EMP) stars. We assume that the observed EMP stars are second-generation stars, which have the metal-abundance patterns of Population III SNe. Previous theoretical yields of Population III SNe cannot explain the trends in the abundance ratios among Fe-peak elements (Mn, Co, Ni, Zn)/Fe or the large C/Fe ratio observed in certain EMP stars with [Fe/H] ≲ -2.5. In this paper we show that if we introduce higher explosion energies and mixing fallback in the core-collapse SN models of M ~ 20-130 M ☉ , the above abundance features of both typical and C-rich EMP stars can be much better explained. We suggest that the abundance patterns of the [Fe/H] ~ -2.5 stars correspond to supernova yields with normal explosion energies, while those of the carbon unenhanced ([C/Fe] < 1) stars with [Fe/H] ≃ -4 to -3 correspond to high-energy supernova yields. The abundance patterns of the C-rich ([C/Fe] ≳ 2) and low [Fe/H](≃ -5.5 ~ -3.5) stars can be explained with the yields of faint SNe that eject little 56 Ni as observed in SN 1997D. In the supernova-induced star formation model, we can qualitatively explain why the EMP stars formed by the faint or energetic supernovae have lower [Fe/H] than the EMP stars formed by normal supernovae. We also examine how the abundance ratios among Fe-peak elements depend on the electron mole fraction Y e and conclude that a large explosion energy is still needed to realize the large Co/Fe and Zn/Fe ratios observed in typical EMP stars with [Fe/H] ≲ -3.5.