1994A&A...291..895A

Accurate stellar parameters have been obtained from the analyses of more than thousand spectra of 115 metal-poor dwarfs and subgiants with visual magnitudes brighter than V≃12. The stellar sample was selected mainly by high proper motion, with additional restrictions from B-V colours and U-B excesses. The effective temperatures cover a range from 5000 to 6500K while metal abundances are found between [M/H]=-0.1 and -3.0dex. For the first time a representative number of unevolved cool metal-poor stars has been analysed individually using purely spectroscopic methods. Based on homogeneous ODF blanketed model atmospheres in LTE and working differentially with respect to the Sun we derive a consistent set of stellar parameters, effective temperature, surface gravity, metal abundance and microturbulence velocity. Individual profile synthesis is applied to a number of spectral lines for each star, which has led as a rule to accuracies in T_eff of better than 100K, in logg of better than 0.15, and in [Fe/H] of better than 0.1dex. Because of the consistent treatment with only one type of model atmosphere, this sample provides an opportunity to examine the individual parameters statistically and investigate in detail their relation to the formation and evolution of the Galaxy. One aspect of this analysis is a general shift to higher iron abundances for the most metal-poor stars. Along with the recently preferred meteoritic solar iron abundance and effective temperatures from consistent Balmer line profile fits that tend to be 100-200K hotter than found from photometric calibrations, discrepancies of up to 0.5dex in [Fe/H] can be explained in comparison with other abundance analyses. The most important results refer to the evolutionary status of the bona fide subdwarf sample. Irrespective of the different effective temperatures found here, there exists a severe problem when comparing post main sequence evolutionary models of cool stars with our observed parameters. Even more interesting is the fact, demonstrated by the results of a consistent analysis of the iron ionization equilibrium, that roughly half of the subdwarfs are subgiants, some of them having nearly reached the bottom of the giant branch.