Variations in the Abundance Pattern of Extremely Metal-poor Stars and Nucleosynthesis in Population III Supernovae

We calculate nucleosynthesis in Population (Pop) 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 the second generation stars, which have the metal-abundance patterns of Pop III SNe. Previous theoretical yields of Pop III SNe cannot explain the trends in the abundance ratios among iron-peak elements (Mn, Co, Ni, Zn)/Fe as well as the large C/Fe ratio observed in certain EMP stars with [Fe/H] <~ -2.5. In the present paper, we show that if we introduce higher explosion energies and mixing-fallback in the core-collapse SN models of M ~ 20 - 130 Msun, 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 un-enhanced ([C/Fe] < 1) stars with [Fe/H] =~ -4 ~ - 3 correspond to high-energy supernova yields. The abundance patterns of the C-rich ([C/Fe]>~ 2) and low [Fe/H] (=~ -5 \~ -3.5) stars can be explained with the yields of faint SNe that eject little 56Ni as observed in SN1997D. 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 iron-peak elements depend on the electron mole fraction Ye, 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.Comment: 33 pages, 17 figures, 7 tables, To appear in the Astrophysical Journal 2005, January 1

The first chemical enrichment in the universe and the formation of hyper metal-poor stars

The recent discovery of a hyper metal-poor (HMP) star, whose metallicity Fe/H is smaller than 1/100,000 of the solar ratio, together with one earlier HMP star, has raised a challenging question if these HMP stars are the actual first generation, low mass stars in the Universe. We argue that these HMP stars are the second generation stars being formed from gases which were chemically enriched by the first generation supernovae. The key to this solution is the very unusual abundance patterns of these HMP stars with important similarities and differences. We can reproduce these abundance features with the core-collapse ``faint'' supernova models which undergo extensive matter mixing and fallback during the explosion.Comment: To be published in Science. 12 pages, 3 figure