Formation of Galactic Systems in Light of the Magnesium Abundance in Field Stars.III.the Halo

Abstract

We analyze the relations between the relative magnesium abundances, metallicities, and Galactic orbital elements for halo stars. We show that the relative magnesium abundances in protodisk halo stars are virtually independent of metallicity and lie within a fairly narrow range while presumably accreted stars demonstrate a large spread in relative magnesium abundances up to negative [Mg/Fe]. The mean metallicity of magnesium-poor ([Mg/Fe]<0.2 dex) accreted stars has been found to be displaced toward the negative values when passing from stars with low azimuthal velocities to those with high ones at Δ[Fe/H]=0.5dex. The mean apogalactic radii and inclinations of the orbits also increase while their eccentricities decrease. As a result negative radial and vertical gradients in relative magnesium abundances are observed in the accreted halo in the absence of correlations between the [Mg/Fe] ratios and other orbital elements, while these correlations are found at a high significance level for genetically related Galactic stars. We surmise that as the masses of dwarf galaxies decrease, the maximum SNII masses and hence, the yield of α-elements in them also decrease. In this case, the relation between the [Mg/Fe] ratios and the inclinations and sizes of the orbits of accreted stars is in complete agreement with numerical simulations of dynamical processes during the interaction of galaxies. Thus the behavior of the magnesium abundance in accreted stars suggests that the satellite galaxies are disrupted and lose their stars en masse only after dynamical friction reduces significantly the sizes of their orbits and drags them into the Galactic plane. Less massive satellite galaxies are disrupted even before their orbits change appreciably under tidal forces.