Ages of Globular Clusters from Hipparcos Parallaxes of Local Subdwarfs

We report here first but still strongly conclusive results on absolute ages of galactic globular clusters (GGCs). This study is based on high precision trigonometric parallaxes from the HIPPARCOS satellite coupled with accurate metal abundances ([Fe/H], [O/Fe], and [\alpha/Fe]) from high resolution spectroscopy for a sample of about 100 subdwarfs. The new data allowed us to reliably define the absolute location of the main sequence (MS) as a function of metallicity. These results are then used to derive distances and ages for a carefully selected sample of nine globular clusters having metallicities determined from high dispersion spectra of individual giants according to a procedure totally consistent with that used for the field subdwarfs. Very precise and homogeneous reddening values have also been independently determined for these clusters. We find that Hipparcos parallaxes are smaller than the corresponding ground-based measurements leading, in turn, to longer distance moduli ($\sim 0.2$ mag) and younger ages ($\sim 2.8$ Gyr). The relation between the HB absolute magnitude and metallicity for the nine programme clusters turns out to be: $ M_V(HB) = (0.29 \pm 0.09)([Fe/H]+1.5) + (0.43\pm 0.04)$ The corresponding LMC distance modulus is $(m-M)_0=18.63\pm 0.06$. The age of the bona fide old globular clusters (Oosterhoff II, BHB, 47 Tuc) based on the absolute magnitude of the turn-off, a theoretically robust indicator, is: Age = {12.1^{+1.2}_{-3.6}} Gyr.