Non-LTE Abundances and Consequences for the Evolution of the alpha elements in the Galaxy

Abstract

Abundances of alpha-elements such as Ca and Mg in disk and halo stars are usually derived from equivalent widths 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 CaI and MgI 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.