SPECTROSCOPIC ANALYSES OF METAL-POOR STARS .3. MAGNESIUM ABUNDANCES

We present magnesium abundances and Mg/Fe ratios for 56 metal-poor dwa rfs and subgiants based on differential LTE model atmosphere analyses of spectra that have a typical S/N of 50 - 100 and a resolution of abo ut 20000 - 30000. The distribution of Mg/Fe abundance ratios with Fe a bundances is characterized by a steep increase of [Mg/Fe] near [Fe/H] = -0.6. At that iron abundance we recognize an abrupt change from a so lar value for stars with [Fe/H] greater than or equal to -0.6 to an up per limit of +0.4 dex for the metal-poor stars. Our data suggest that this step in the Mg/Fe abundance ratio at [Fe/H] = -0.6 is the result of the onset of Galactic disk formation, whereas [Mg/Fe] = +0.4 appare ntly is an upper limit arising from the constraints of nucleosynthesis in massive SNII events during the first stellar generations, the yiel ds of which we observe in the most metal-poor halo stars.