Atomic diffusion in metal poor stars - The influence on the Main Sequence fitting distance scale, subdwarfs ages and the value of Delta Y/Delta Z

The effect of atomic diffusion on the Main Sequence (MS) of metal-poor low mass stars is investigated. Since diffusion alters the stellar surface chem ical abundances with respect to their initial values, one must ensure - by calibrating the initial chemical composition of the theoretical models - th at the surface abundances of the models match the observed ones of the stel lar population under scrutiny. When properly calibrated, our models with di ffusion reproduce well within the errors the Hertzsprung-Russell diagram of Hipparcos subdwarfs with empirically determined T-eff values and high reso lution spectroscopical [Fe/H] determinations. Since the observed surface abundances of subdwarfs are different from the i nitial ones due to the effect of diffusion, while the globular clusters ste llar abundances are measured in Red Giants, which have practically recovere d their initial abundances after the dredge-up, the isochrones to be employ ed for studying globular clusters and Halo subdwarfs with the same observat ional value of [Fe/H] are different and do not coincide. This is at odds wi th the basic assumption of the MS-fitting technique for distance determinat ions. However, the use of the rather large sample of Hipparcos lower MS sub dwarfs with accurate parallaxes keeps at minimum the effect of these differ ences, for two reasons. First, it is possible to use subdwarfs with observe d [Fe/H] values close to the cluster one; this minimizes the colour correct ions (which are derived from the isochrones) needed to reduce all the subdw arfs to a mono-metallicity sequence having the same [Fe/H] than the cluster . Second, one can employ objects sufficiently faint so that the differences between the subdwarfs and cluster MS with the same observed value of [Fe/H ] are small (they increase for increasing luminosity). We find therefore th at the distances based on standard isochrones are basically unaltered when diffusion is taken properly into account. On the other hand, the absolute ages, the age dispersion, the age-metallici ty relation for Halo subdwarfs, as well as the value of the helium enrichme nt ratio Delta Y/Delta Z obtained from the width of the empirical Halo subd warfs MS, are all significantly modified when the properly calibrated isoch rones with diffusion are used.