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

Abundances of alpha-elements such as Ca and Mg in disk and halo stars areusually derived from equivalent widths lines measured on high resolutionspectra, and assuming Local Thermodynamic Equilibrium (LTE) . In this paper, wepresent non-LTE differential abundances derived by computing the statisticalequilibrium of CaI and MgI atoms, using high resolution equivalent widthsavailable in the literature for 252 dwarf to subgiant stars. These non-LTEabundances 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 galactichalo and disk phases, revealing new and surprising structures. These resultshave important consequences for chemical evolution models of the Galaxy. Inaddition, non-LTE abundance ratios for stars belonging to the M92 clusterapparently have the same behavior. More high resolution observations, mainly ofglobular clusters, are urgently needed to confirm our results.Comment: 15 pages, 3 tables, 7 figure