2004MNRAS.350.1127A

We present Mg abundances derived from high-resolution spectra using several Mgi and two high-excitation Mgii lines for 19 metal-poor stars with [Fe/H] values between -1.1 and +0.2. The main goal is to search for systematic differences in the derived abundances between the two ionization state lines. Our analysis shows that the one-dimensional local thermodynamic equilibrium (LTE) and non-LTE (N-LTE) study finds a very good agreement between these features. The [Mg/Fe] versus [Fe/H] relationship derived, despite the small sample of stars, is also in agreement with the classical figure of increasing [Mg/Fe] with decreasing metallicity. We find a significant scatter however, in the [Mg/Fe] ratio at [Fe/H]∼-0.6 which is currently explained as a consequence of the overlap at this metallicity of thick- and thin-disc stars, which were probably formed from material with different nucleosynthesis histories. We speculate on the possible consequences of the agreement found between Mgi and Mgii lines on the very well-known O problem in metal-poor stars. We also study the [O/Mg] ratio in the sample stars using O abundances from the literature and find that the current observations and nucleosynthetic predictions from Type II supernovae disagree. We briefly discuss some alternatives to solve this discrepancy.