Tuc : the Spectroscopic versus Cmd Age Discrepancy

We investigate current problems in obtaining reliable ages for old stellar systems based on stellar population synthesis modelling of their integrated spectra. In particular, we address the large ages derived for the globular cluster 47 Tuc, which is at odds with its Color-Magnitude-Diagram (CMD) age. Using a new age indicator, Hγσ<130, which is particularly effective at breaking the degeneracy between age and metallicity, we confirm the discrepancy between the spectroscopic age and the CMD age of 47 Tuc, in that the spectroscopic age is much older. Nebular emission appears unlikely to be a source for weakening the observed Balmer lines. We then explore a number of key parameters affecting the temperature of Turn-Off stars, which are the main contributors to the Balmer lines for old metal-rich stellar populations. We find that α-enhanced isochrones with atomic diffusion included not only provides a good fit to the CMD of 47 Tuc, but also leads to a spectroscopic age in better agreement with the CMD age. Subject headings: galaxies: evolution — galaxies: stellar content — Galaxy: globular clusters: general — Galaxy: globular clusters: individual (47 Tuc) — stars: evolution