New Insights into the Early Stage of the Galactic Chemical Evolution

The supernova yields of several heavy elements, including α-, iron-group, and r -process elements, are obtained as a function of the mass of their progenitor main-sequence stars M ms from the abundance patterns of extremely metal-poor stars with a procedure recently proposed by Shigeyama & Tsujimoto. The ejected masses of α- and iron-group elements increase with M ms , whereas more Eu is ejected from supernovae with lower M ms . For these several heavy elements, it is shown that the average abundance ratios that are weighted by the Salpeter initial mass function coincide with the ratios observed in stars with -2 < [Fe/H] < -1 within 0.1 dex. It follows that the correlations of stellar abundance ratios with the metallicity are twofold. One is the abundance ratios for [Fe/H] < -2.5 imprinted by the nucleosynthesis in individual supernovae on a timescale of ~10 7 yr, and the other, for [Fe/H] > -2, results from the mixing of the products from a whole site of the nucleosynthesis, taking place on a timescale longer than 10 9 yr.