Analysis of enrichment histories for elements and two modes of star formation during the early evolution of Milky Way

Abstract

We study the enrichment histories for nine elements, C, the four α-elements Mg, Si, Ca and Ti, Sc, and the three iron-peak elements Co, Ni and Zn, by using a large amount of stellar data, collected in the Stellar Abundances for Galactic Archaeology (SAGA) database. We find statistically significant changes, or breaks, of the mean abundance ratios to iron, [X/Fe]m, among these elements at the three metallicities of [Fe/H] ≃−1.8,−2.2 or −3.3. Across the first break, [X/Fe]m decreases with metallicity by similar amounts of ≃0.08 dex, common to all the elements with sufficient data in the relevant metallicity range. Across the latter two, downward trends with metallicity are also detected but only for some elements, i.e., for C, Co, Zn and possibly Sc across the second break, and for Co and Zn across the third one. These breaks define four stellar populations with different abundance patterns, Populations IIa, IIb, IIc and IId in order of increaesing metallicity, which dominate over each metallicity range divided by the breaks. We explore the correlations with the vertical distance of stars from the disk to reveal that the break of [Zn/Fe] at [Fe/H] ≃ −2.2 is detected for distant stars of |Z|≳1 kpc but not for nearby stars and that two populations of Pop IIb and IIc, characterized by [Zn/Fe]m = 0.22 and 0.09 respectively, coexist in the same metallicity range of −2.6≲[Fe/H]≲ −2.2. This indicates that the break cannot be ascribed to the metallicity dependence of supernova yields, but is explicable only in terms of the change of initial mass function (IMF) from high-mass/top-heavy to low-mass one, coupled with hypernova yielding high-[Co, Zn/Fe] (Umeda & Nomoto 2005), which is also claimed from a large fraction of carbon enhanced metal-poor stars (Komiya et al. 2007). For the two higher-metallicity populations, Pops. IIc and IId, on the other hand, the difference in the abundance patterns is attributable to the delayed contribution of Type Ia supernovae, indicative of a low-mass IMF and a specific star formation ra te comparable to that in the present-day Galactic disc. We discuss the relevance to the kinematically resolved structures of Galactic halo and the possible sites of these populations within the framework of the hierarchical structure formation scenario.