Surface gravities of very metal-poor stars from HIPPARCOS parallaxes

Abstract

We discuss some consequences for the determination of the surface gravity parameter of very metal-poor halo dwarfs in the new light shed upon these objects from the Hipparcos astrometry. Whereas it has become clear that standard spectroscopic LTE iron ionization equilibria are prone to systematic errors, in the sense, that stages of evolution are predicted that are actually not yet reached, the method that circumvents this shortcoming by means of pressure-dependent strong line wings, faces the problem that such tracers are considerably weakened, or even absent, in the visible spectra of very metal-poor stars. We exemplify this circumstance by means of two nearby halo stars, the well-known HD 140283 and the turnoff star G 84-29, both significantly below one hundredth of the solar metallicity. Based on observations collected at the German Spanish Astronomical Center, Calar Alto, Spain