Distances, ages and epoch of formation of Globular Clusters

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  • 5 February 1999
Abstract
We review the results on distances and absolute ages of galactic globular clusters (GCs) obtained after the release of the Hipparcos catalogue. Several methods aimed at the definition of the Population II local distance scale are discussed. While some methods yield distances to the center of the LMC (the bar), other give distances to the system of globular clusters. There is some rough evidence that the two distances may not coincide. Emphasis is given to the discussion of distances and ages of GCs derived using Hipparcos parallaxes of local subdwarfs. Distances and ages for the 9 clusters discussed in Gratton et al 1997 (ApJ 491, 749, Paper I) are re-derived using an enlarged sample of local subdwarfs, which includes about 90% of the metal-poor dwarfs with accurate parallaxes (Delta p/p < 0.12) in the whole Hipparcos catalogue. On average, our revised distance moduli are decreased by 0.04 mag with respect to Paper I. The corresponding age of the globular clusters is t=11.8+/-2.6 Gyr. The corresponding (true) LMC distance modulus (18.55+/-0.12 mag for the clusters, and 18.66+/ 0.12 mag for the bar)is compared with other existing determinations. We conclude that at present the best estimate for the distance to the bar of the LMC is: 18.54+/-0.04 suggesting that distances from the Subdwarf fitting method are 1 sigma too long. Our best estimate for the age of the globular clusters is thus revised to: Age = 13.2+/-2.9 Gyr (95% confidence range). The best relation between ZAHB absolute magnitude and metallicity is: Mv(ZAHB) = (0.18+/-0.09)([Fe/H]+1.5) + (0.65+/-0.11).

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