A Study of Characteristics and Origins of Stars of Ultra High Mg Abundances

Chemical abundances of stars of ultra high Mg abundances are rather different from those of the vast majority "normal" stars. Their typical abundance pattern cannot be completely explained by current models of stellar origin and evolution. Therefore, detailed studies of chemical abundances of a sufficient amount of such stars can improve our understanding of some special processes of stellar nuclear syntheses and related issues of galaxy evolution. In this paper we first describe stars of ultra high Mg abundances that have been identified in previous research and are suitable for abundance analyses. These stars were identified through High Resolution Spectra (HRS); their chemical-abundance patterns and kinetics have been well studied. We then present our statistical analyses of characteristics of stellar parameters and kinetics of certain candidates for stars of ultra high Mg abundances. The candidates were selected by us from the SDSS. We compare characteristics of the candidates to those of the stars of ultra high Mg abundances identified through HRS. We have selected from the candidates those having ultra high C abundances using their plots of CH(G) index versus (g-r)₀. We find that all of our candidates show ultra high C abundances except a small fraction. We notice that among stars of ultra high Mg abundances those showing ultra high abundances of elements generated through neutron capture almost all belong to binary-star systems, suggesting that their chemical-evolution processes are affected by their AGB (Asymptotic Giant Branch) companions. In contrast, our analysis reveals that other stars of ultra high Mg abundances mostly have high motion speeds, which indicates their origins from materials ejected by low-energy supernovae. Our candidates of motion speeds more than 300km/s should thus constitute an excellent sample within which stars of the first generation (Fe/H<-5.0) in the Galaxy are to be identified.