Distances, Ages and Epoch of Formation of Globular Clusters

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, and their results compared. We notice that while some methods yield distances to the center of the LMC (identified here with the bar), other give distances to the system of globular clusters. There is some rough evidence that the two distances may not coincide, likely due to small number statistics for the LMC cluster RR Lyrae variables. Furthermore, uncertainties in the analysis enlarge error bars of some distance indicators to values larger then previously assumed. These two factors together may reconcile the so-called Short and Long Distance Scales. Emphasis is given in the paper to the discussion of distances and ages of GCs derived using Hipparcos parallaxes of local subdwarfs. We find that the selection criteria adopted to choose the local subdwarfs, as well as the size of the corrections applied to existing systematic biases, are the main culprit for the differences found among the 3 independent studies that first used Hipparcos parallaxes and the Subdwarf Fitting technique (Reid 1997, 1998; Pont et al. 1998; and Gratton et al. 1997, Paper I). We also caution that the absolute age of M92 (usually considered one of the oldest clusters) still remains uncertain due to zero-point offsets in color existing between the two, so far, most accurate photometries for this cluster. Distances and ages for the 9 clusters discussed in Paper I are re-derived using an enlarged sample of local subdwarfs, which includes about 90% of the metal-poor dwarfs with accurate parallaxes (∆π/π ≤ 0.12) in the whole Hipparcos catalogue. On average, our revised distance moduli are decreased