SIMBAD references

We present magnesium abundances and Mg/Fe ratios for 56 metal-poor dwarfs and subgiants based on differential LTE model atmosphere analyses of spectra that have a typical S/N of 50-100 and a resolution of about 20000-30000. The distribution of Mg/Fe abundance ratios with Fe abundances 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 solar value for stars with [Fe/H]≥-0.6 to an upper limit of +0.4dex 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 apparently is an upper limit arising from the constraints of nucleosynthesis in massive SN II events during the first stellar generations, the yields of which we observe in the most metal-poor halo stars.