The Stellar Abundances for Galactic Archaeology (SAGA) Database – III. Analysis of enrichment histories for elements and two modes of star formation during the early evolution of the Milky Way

We study the enrichment histories for nine elements, namely C, the four -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 at the three metallicities [Fe/H] -1.8, -2.2 and -3.3. Across the first break, the mean abundance ratios decrease with metallicity by similar amounts for all elements with sufficient data. Across the latter two, downward trends with metallicity are also detected but for only some elements, namely for C, Co, Zn and possibly Sc for the second break, and for Co and Zn for the third. The breaks define four stellar populations with different abundance patterns that are dominant in each metallicity range divided by the breaks, namely Populations IIa, IIb, IIc and IId in order of increasing metallicity. We also explore their spatial distributions with spectroscopic distances to demonstrate that Populations IIa and IIb are spread over the Galactic halo, while Populations IIc and IId are observed near the Galactic plane. In particular, Population IIc stars emerge around [Fe/H] -2.6 and coexist with Population IIb stars, segregated by the spatial distributions. Our results reveal two distinct modes of star formation during the early stages of Galaxy formation, which are associated with variations of the initial mass function (IMF) and the spatial distribution of remnant low-mass stars. For the two lower-metallicity populations, the enhancements of Zn and Co indicate a high-mass and top-heavy IMF, in addition to large fraction of the carbon-enhanced stars. For the two higher-metallicity populations, 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 rate comparable to that in the present-day Galactic disc. We discuss the relevance to the kinematically resolved structures of the Galactic halo and the possible sites of these populations within the framework of a hierarchical structure formation scenario.