New insights into the early stage of the Galactic chemical evolution

The supernova yields of several heavy elements, including alpha-, iron-grou p, 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 extremel y metal-poor stars with a procedure recently proposed by Shigeyama & Tsujim oto. The ejected masses of alpha- and iron-group elements increase with M-m s,M- 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 th e correlations of stellar abundance ratios with the metallicity are twofold . One is the abundance ratios for [Fe/H] < -2.5 imprinted by the nucleosynt hesis in individual supernovae on a timescale of similar to 10(7) yr, and t he other, for [Fe/H] > -2, results from the mixing of the products from a w hole site of the nucleosynthesis, taking place on a timescale longer than 1 0(9) yr.