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