SPECTROSCOPIC ANALYSES OF METAL-POOR STARS .2. THE EVOLUTIONARY STAGEOF SUBDWARFS

Models of post-main sequence stellar evolution of VandenBerg and Bell (1985) have been applied to determine spectroscopic masses and distanc es for metal-poor stars. Careful consideration of the most important e rror sources published in more recent papers such as VandenBerg (1992) for the first time allow us to draw firm statistical conclusions. It is shown that the evolutionary calculations qualitatively fit to the o bserved stellar parameters whereas quantitatively they predict too hig h ages for metal-poor stars. As an important result we confirm that ev olutionary sequences need to be calibrated with respect to their metal abundance in order to use their absolute predictions of temperature a nd luminosity. It turns out that this can be achieved by a simple shif t of the evolutionary tracks and isochrones in effective temperature w ith values Delta log T-eff less than or similar to 0.03 which accounts for possible changes of the mixing-length and the O/Fe ratio with met allicity. The stellar luminosities and surface gravities obtained from evolutionary models are much more reliable than their effective tempe ratures. Therefore we conclude that the accuracy of the corresponding spectroscopic stellar gravities is systematically affected by deviatio ns from LTE, in particular along the subgiant sequence where systemati c errors less than Delta log g approximate to 0.3 must be ascribed to the non-LTE ionization equilibrium of Fe II/Fe I. In our spectroscopic analyses the strong dependence between surface gravity and abundances determined from Fe I lines restricts the accuracy of Fe abundances in subgiants to 0.1 dex at best. The most remarkable result of our evolu tionary and kinematic investigations of halo stars refers to the large fraction of slightly evolved subgiants among the so-called subdwarfs. Since conventional photometric approaches often assume that the great majority of metal-poor stars are dwarfs this results in distances tha t are systematically too low for their samples. Consequently, signific ant differences are found when comparing evolutionary and kinematic pa rameters obtained from either photometric or spectroscopic data. We de monstrate this by comparing the space velocities of the stars. It appe ars that stars with particularly high space velocities derived from sp ectroscopic distances show very often much lower velocities based on t heir main sequence parallaxes. We find that results refering to main s equence parallaxes are doubtful and can be used only with greatest car e. An advantageous side-effect of the application of spectroscopic dat a to evolutionary calculations is the possibility to identify binary s ystems that are either standing out from the Toomre diagram with their unusually high space velocities, or from a log g - log T-eff diagram with apparently contradictory luminosities.