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

Using results of nucleosynthesis calculations for theoretical core-collapse supernova models with various progenitor's masses, it is shown that abundance patterns of C, Mg, Si, Ca, and H 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 a 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 a SN is well correlated with its progenitor's mass. Thus the observed [Mg/H] in an extremely metal deficient star has a correspondence to the progenitor's mass. A larger [Mg/H] corresponds to a larger progenitor's mass. Therefore, 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 the observed trend of [Mg/Fe] can predict the Fe yield of each SN given the correspondence of [Mg/H] to the progenitor's 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 modelling a core-collapse supernova at its beginning of explosion or mass cut.Comment: 6 pages, 4 figures, 1 table; Accepted for publication in the Astrophysical Journal Letter