Non-LTE Abundances and Consequences for the Evolution of the α-Elements in the Galaxy

Abundances of α-elements such as Ca and Mg in disk and halo stars are usually derived from equivalent width lines measured on high-resolution spectra and assuming local thermodynamic equilibrium (LTE). In this paper, we present non-LTE differential abundances derived by computing the statistical equilibrium of Ca I and Mg I atoms, using high-resolution equivalent widths available in the literature for 252 dwarf to subgiant stars. These non-LTE abundances, combined with recent determination of non-LTE abundances of iron, seem to remove the dispersion of the [Ca/Fe] and [Mg/Fe] ratios in the galactic halo and disk phases, revealing new and surprising structures. These results have important consequences for chemical evolution models of the Galaxy. In addition, non-LTE abundance ratios for stars belonging to the M92 cluster apparently have the same behavior. More high-resolution observations, mainly of globular clusters, are urgently needed to confirm our results.