The Stellar Abundances for Galactic Archaeology (SAGA) data base - II. Implications for mixing and nucleosynthesis in extremely metal-poor stars and chemical enrichment of the Galaxy

We discuss the characteristics of known extremely metal-poor (EMP) stars in the Galaxy using the Stellar Abundances for Galactic Archaeology (SAGA) data base. We find the transition of the initial mass function to be at [Fe/H] similar to -2 from the viewpoint of the distribution of carbon abundance and the frequency of carbon-enhanced stars. Analyses of carbon-enhanced stars in our sample suggest that nucleosynthesis in asymptotic giant branch (AGB) stars can contribute to carbon enrichment in a different way depending on whether the metallicity is above or below [Fe/H] similar to -2.5, which is consistent with the current models of stellar evolution at low metallicity. For observed EMP stars, we confirm that some, though not all, observed stars might have undergone at least two types of extra mixing to change their surface abundances. One is the depletion of lithium abundance during the early phase of the red giant branch; the other is a decrease of the C/N ratio by one order of magnitude during the red giant branch phase. Observed small scatters of abundances for alpha-elements and iron-group elements suggest that the chemical enrichment of our Galaxy takes place in a well-mixed interstellar medium. The abundance trends of alpha-elements are highly correlated with each other including alpha-enhanced and depleted stars, while the abundances of iron-group elements are subject to different slopes relative to the iron abundance. This implies that the supernova yields of alpha-elements are almost independent of mass and metallicity, while those of iron-group elements have a metallicity dependence or mass dependence on the variable initial mass function. The occurrence of the hot-bottom burning for M greater than or similar to 5 M-circle dot is consistent with an initial mass function of the Galaxy peaked at similar to 10-12 M-circle dot, compatible with the statistics of carbon-enhanced stars with and without s-process element enhancement and nitrogen-enhanced stars. For s-process elements, we find not only a positive correlation between carbon and s-process element abundances, but also an increasing slope of the abundance ratio between them with increasing mass number of s-process elements. The dominant site of the s-process is still an open question because none of the known mechanisms for the s-process is able to account for this observed correlation. In spite of the evidence of AGB evolution in observed abundances of EMP stars, any evidence of binary mass transfer is elusive by pursuing the effect of dilution in the convective envelope. We find the dependence of sulphur and vanadium abundances on the effective temperatures, in addition to the previously reported trends for silicon, scandium, titanium, chromium and cobalt.