We present magnesium abundances derived from high resolution spectra using several Mg I and two high excitation Mg II 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 ionisation state lines .
Our analysis shows that the one-dimensional LTE and N-LTE study finds a very good agreement between these features .
The [ Mg/Fe ] vs. [ 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 ] \sim - 0.6 which is currently explained as a consequence of the overlap at this metallicity of thick and thin disk stars , which were probably formed from material with different nucleosynthesis histories .
We speculate on the possible consequences of the agreement found between Mg I and Mg II lines on the very well known oxygen problem in metal-poor stars .
We also study the [ O/Mg ] ratio in the sample stars using oxygen 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 .