Na, Mg and Al abundances as a population discriminant for nearby metal-poor stars

Aims. Parameters for 55 nearby metal-poor stars are determined using high-resolution spectroscopy. Together with similar data taken from a recent analysis, they are used to show trends of their Galactic evolution with stellar [Fe/H] or [Mg/H] abundances. The separation of abundance ratios between disk and halo stars is used as a basic criterion for population membership. Methods. After careful selection of a clean subsample free of suspected or known binaries and peculiar stars, abundances of Mg, Na and Al are based on NLTE kinetic equilibrium calculations applied to spectrum synthesis methods. Results. The relation between [Na/Mg] and [Fe/H] is a continuous enrichment through all three Galactic populations spanning a range of values between a metal-poor plateau at [Na/Mg] = -0.7 and solar values. [Al/Mg] displays a step-like difference between stars of the Galactic halo with [Al/Mg] similar to -0.45 and the two disk populations with [Al/Mg] similar to +0.10. [Al/Mg] ratios, together with the [Mg/Fe] ratios, asymmetric drift velocities V, and stellar evolutionary ages, make possible the individual discrimination between stars of the thick disk and the halo. At present, this evidence is limited by the small number of stars, and by the theoretical and empirical uncertainties of stellar age determinations, but it achieves a high significance. Conclusions. While the stellar sample is not complete with respect to space volume, the resulting abundances indicate the necessity to revise current models of chemical evolution to allow for an adequate production of Al in early stellar generations.