Abundances of Na, Mg and Al in nearby metal-poor stars

To determine the population membership of nearby stars we explore abundance results obtained for the light neutron-rich elements (23)Na and (27)Al in a small sample of moderately metal-poor stars. Spectroscopic observations are limited to the solar neighbourhood so that gravities can be determined from HIPPARCOS parallaxes, and the results are confronted with those for a separate sample of more metal-poor typical halo stars. Following earlier investigations, the abundances of Na, Mg and Al have been derived from NLTE statistical equilibrium calculations used as input to line profile synthesis. Compared with LTE the abundances require systematic corrections, with typical values of + 0.05 for [Mg/Fe], -0.1 for [Na/Fe] and + 0.2 for [Al /Fe] in thick disk stars where [Fe/H] similar to - 0.6. In more metal-poor halo stars these values reach + 0.1, - 0.4, and + 0.5, respectively, differences that can no longer be ignored. After careful selection of a clean subsample free from suspected or known binaries and peculiar stars, we find that [Na/Mg] and [Al/Mg], in combination with [Mg/Fe], space velocities and stellar evolutionary ages, make possible an individual discrimination between thick disk and halo stars. At present, this evidence is limited by the small number of stars analyzed. We identify a gap at [Al /Mg] similar to - 0.15 and [Fe/H] similar to - 1.0 that isolates stars of the thick disk from those in the halo. A similar separation occurs at [Na /Mg] similar to - 0.4. We do not confirm the age gap between thin and thick disk found by Fuhrmann. Instead we find an age boundary between halo and thick disk stars, however, with an absolute value of 14 Gyr that must be considered as preliminary. While the stellar sample is by no means complete, the resulting abundances indicate the necessity to revise current models of chemical evolution and/or stellar nucleosynthesis to allow for an adequate production of neutron-rich species in early stellar generations.