We study the enrichment histories for nine elements, C, four alpha-elements of Mg, Si, Ca, and Ti, Sc, and three iron-peak elements of Co, Ni, and Zn, by using a large number of stellar data, collected by the Stellar Abundances for Galactic Archaeolo
gy (SAGA) database. We find statistically significant changes, or breaks, of the mean abundance ratios to iron at three metallicities of [Fe/H]-1.8, -2.2, and -3.3. Across the first one, the mean abundance ratios decrease with the metallicity by similar extents for all the elements with the sufficient data. Across the latter two, downward trends with the metallicity are also detected but for limited elements, C, Co, Zn, and possibly Sc, and for two of Co and Zn, respectively. The breaks define four stellar populations with the different abundance patters which are dominant in each metallicity range divided by the breaks, Pop IIa, IIb, IIc, and IId in order of increasing metallicity. We also explore their spatial distributions with the spectroscopic distances to demonstrate that Pops IIa and IIb spread over the Galactic halo while Pops IIc and IId are observed near the Galactic plane. In particular, Pop IIc stars emerge around [Fe/H] -2.6 and coexist with Pop 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 the variations of 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 together with the statistics on the carbon-enhanced stars. We discuss the relevance to the kinematically resolved structures of the Galactic halo and the possible sites of these populations within the framework of hierarchical structure formation scenario.
We investigate the tidal interactions of a red giant with a main sequence in the dense stellar core of globular clusters by Smoothed Particle Hydrodynamics method. Two models of $0.8 msun$ red giant with the surface radii 20 and $85 R_sun$ are used w
ith 0.6 or $0.8M_sun$ main sequence star treated as a point mass. We demonstrate that even for the wide encounters that two stars fly apart, the angular momentum of orbital motion can be deposited into the red giant envelope to such an extent as to trigger rotational mixing and to explain the fast rotation observed for the horizontal branch stars, and also that sufficient mass can be accreted on the main sequence stars to disguise their surface convective zone with the matter from the red giant envelope. On the basis of the present results, we discuss the parameter dependence of these transfer characteristics with non-linear effects taken into account, and derive fitting formulae to give the amounts of energy and angular momentum deposited into the red giant and of mass accreted onto the perturber as functions of stellar parameters and the impact parameter of encounter. These formulae are applicable to the encounters not only of the red giants but also of the main sequence stars, and useful in the study of the evolution of stellar systems with the star-star interactions taken into account.
