Metal-rich globular clusters in the galactic disk: new age determinations and the relation to halo clusters

New age determinations of the galactic disk globular clusters 47 Tuc, M71 and NGC6352 have been performed with our up-to-date alpha-enhanced stellar models. We find that all three clusters are about 9.2 Gyr old and therefore coeval with the oldest disk white dwarfs. Several arguments are presented which indicate that the initial helium content of the stars populating these clusters is close to the solar one. We also revisit a total of 28 halo clusters, for which we use an updated [Fe/H] scale. This new metallicity scale leads on average to an age reduction of around 0.8 Gyr relative to our previous results. We compare the predicted cluster distances, which result from our dating method, with the most recent distances based on HIPPARCOS parallaxes of local subdwarfs. We further demonstrate that for the most metal-rich clusters scaled-solar isochrones no longer can be used to replace alpha-enhanced ones at the same total metallicity. The implications of the presented age determinations are discussed in the context of the formation history of the Galaxy.