Fossil Imprints of the First-Generation Supernova Ejecta in Extremely Metal-deficient Stars

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Abstract
Using the results of nucleosynthesis calculations for theoretical core-collapse supernova models with various progenitor masses, it is shown that the abundance patterns of C, Mg, Si, Ca, and H that are seen in extremely metal-deficient stars with [Fe/H]-2.5 follow those seen in the individual first-generation supernova remnants (SNRs). This suggests that most of the stars with [Fe/H]-2.5 were made from individual supernova (SN) events. To obtain the ratio of heavy elements to hydrogen, a formula is derived to estimate the mass of hydrogen swept up by an SNR when it occurs in the interstellar matter with the primordial abundances. We use [Mg/H] to indicate the metallicities instead of [Fe/H]. The metallicities [Mg/H] predicted from these SNRs range from ~-4 to ~-1.5, and the mass of Mg in an SN is well correlated with its progenitor mass. Thus, the observed [Mg/H] in an extremely metal-deficient star has a correspondence to the progenitor mass. A larger [Mg/H] corresponds to a larger progenitor mass. Therefore, the so-called "age-metallicity relation" does not hold for stars with [Fe/H]-2.5. In contrast, the [Mg/Fe] ratios in the theoretical SNRs have a different trend from those in extremely metal-deficient stars. It is also shown that from the observed trend of [Mg/Fe], one can predict the Fe yield of each SN given the correspondence of [Mg/H] to the progenitor mass. The Fe yields thus obtained are consistent with those derived from SN light-curve analyses. This indicates that there is still a problem in modeling a core-collapse supernova at the beginning of its explosion or mass cut. The abundance determination of O in extremely metal-deficient stars, which has not been done from observational analyses, is strongly desired in order to test the hypothesis that the elements in an extremely metal-deficient star come from a single SN event and to obtain reliable yields for SNe.
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