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Astron. Astrophys. 355, 299-307 (2000)

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6. Summary

We have analyzed the influence of heavy elements and helium diffusion on the MS of metal poor low mass stars in connection with the determination of GC distances via MS-fitting technique, field subdwarfs ages, and the helium enrichment ratio [FORMULA] derived from the width of the subdwarfs MS. These three quantities are all of paramount importance for cosmological and Galactic evolution issues.

The necessity of this analysis was prompted by the recognition that isochrones for MS subdwarfs and GC with direct spectroscopical determinations of the metallicity are not the same if diffusion is taken properly into account; moreover, differences in the MS location and TO position between subdwarfs standard and diffusive isochrones are metallicity and age dependent.

We have considered the full effect of atomic diffusion, without any allowance for possible hydrodynamical mixing phenomena which could reduce the efficiency of diffusion in Population II stars, as some observations appear to suggest (see the discussion in Sect. 1).

Our main results are:

i) [FORMULA]-enhanced calibrated diffusive isochrones reproduce well within the observational errors the position in the [FORMULA] plane of metal poor field subdwarfs with accurate parallaxes and empirical values of [FORMULA] (from the IRFM method), for reasonable assumptions about their age. However, with the observational sample considered here and taking into account the existing observational uncertainties, it is impossible from this comparison to demonstrate that diffusion is fully efficient in Population II stars.

ii) MS-fitting distances obtained using current samples of Hipparcos subdwarfs and standard isochrones are negligibly affected by atomic diffusion.

iii) The estimated subdwarfs ages and age dispersion are strongly modified when diffusion is properly considered for the subdwarfs isochrones (absolute ages are significantly reduced). Since the actual metallicity of subdwarfs in the TO region can be very different from the initial one, one must take into account this effect when deriving age-metallicity relations.

iv) The value of [FORMULA] in the Galactic Halo metallicity range turns out to be systematically underestimated (by [FORMULA]([FORMULA])[FORMULA]1-2) if standard isochrones are employed.

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© European Southern Observatory (ESO) 2000

Online publication: March 17, 2000