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We analyze the relations between the relative magnesium abundances,metallicities, and Galactic orbital elements for halo stars. We show that therelative magnesium abundances in protodisk halo stars are virtually independentof metallicity and lie within a fairly narrow range while presumably accretedstars demonstrate a large spread in relative magnesium abundances up tonegative [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 whenpassing from stars with low azimuthal velocities to those with high ones at\Delta[Fe/H]=0.5dex. The mean apogalactic radii and inclinations of the orbitsalso increase while their eccentricities decrease. As a result negative radialand vertical gradients in relative magnesium abundances are observed in theaccreted halo in the absence of correlations between the [Mg/Fe] ratios andother orbital elements, while these correlations are found at a highsignificance level for genetically related Galactic stars. We surmise that asthe masses of dwarf galaxies decrease, the maximum SNII masses and hence, theyield of \alpha-elements in them also decrease. In this case, the relationbetween the [Mg/Fe] ratios and the inclinations and sizes of the orbits ofaccreted stars is in complete agreement with numerical simulations of dynamicalprocesses during the interaction of galaxies. Thus the behavior of themagnesium abundance in accreted stars suggests that the satellite galaxies aredisrupted and lose their stars en masse only after dynamical friction reducessignificantly the sizes of their orbits and drags them into the Galactic plane.Less massive satellite galaxies are disrupted even before their orbits changeappreciably under tidal forces.Comment: accepted 2006, Astronomy Letters, Vol. 32 No. 8, P.545, 18 pages, 6  figure

Formation of galactic subsystems in light of the magnesium abundance in field stars: The halo