47 Tucanae: The Spectroscopic versus Color-Magnitude Diagram Age Discrepancy

Vazdekis, A.; Salaris, Maurizio; Arimoto, Nobuo; Rose, James A.
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The Astrophysical Journal, Volume 549, Issue 1, pp. 274-280.

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We investigate current problems in obtaining reliable ages for old stellar systems based on stellar population synthesis modeling 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 turnoff 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 provide a good fit to the CMD of 47 Tuc but also lead to a spectroscopic age in better agreement with the CMD age.