Main Sequence Fitting Distances and absolute ages of Galactic globular clusters

People involved at OAB: Bragaglia, Clementini, Fusi Pecci

Extensively applied in the eighties, although affected by rather large error bars ($\pm 0.25$ mag in distance and $\pm$4 Gyr in age), the GC Main Sequence Fitting (MSF) technique derives distances from the comparison of the GC Main Sequence to a suitable ``template'' formed by metal-poor subdwarfs in the solar neighborhood, whose distances are accurately measured via trigonometric parallaxes. This method has been substantially renewed by the release of the Hipparcos trigonometric parallax catalogue in June 1997.

The Hipparcos based MSF technique has produced a ``stretching'' of the GC distances which definitely favors the long astronomical distance scale, and, in turn, an inferred age scale younger by 2-3 Gyrs (Carretta et al. 2000, and references therein).

An ESO large programme, carried out in 2000-2002, has permitted to reduce the residual uncertainty in the MSF distances to about $\pm 0.07$ mag (i.e., dominated by the parallax error) and the corresponding errors in the GC ages to $\pm$1 Gyr, by addressing these effects. We have published results (Gratton et al. 2003) for NGC 104, NGC 6397, and NGC 6752. From a strictly homogeneous analysis of cluster main sequence turn-off (MSTO) stars and field subdwarfs with well determined parallax, we have been able to use a homogeneous metallicity and reddenings scale. This has reduced the final error on GC distances to $\pm 0.07$, and on ages to $\pm 1.1$ Gyr. From these 3 Galactic GCS, the age of the oldest globulars in our Galaxy is $13.7 \pm 0.8 \pm 0.6$ Gyr (random and systematic errors; Gratton et al. 2003), fully compatible with the very recent determination of the age of the universe by WMAP. New observing time with FLAMES has been assigned in P73 to extend this study to many other GCs.

This work is in collaboration with Carretta, Gratton (INAF-Padova Obs.), Grundahl (Aarhus Univ., DK).