On Calibrating Brightest Asymptotic Giant Branch Stars as Age Indicators: The Bulge Globular Cluster NGC 6553 and the Age of M32

Bright asymptotic giant branch (AGB) stars can be used as an age indicator for stellar populations that are too far away for their main-sequence turnoffs to be reachable. However, the method requires an empirical calibration, as the brightness of the AGB termination depends on both age and metallicity in ways that cannot be predicted from first principles. In this Letter, we present an attempt at providing such a calibration for old, near-solar metallicity populations. For this purpose, we use the bulge, metal-rich globular cluster NGC 6553, combining available Hubble Space Telescope (HST) optical photometry with near-IR data. HST observations have indicated the cluster to be nearly as old as halo globular clusters (Ortolani et al.), and here we use the same data to determine the reddening and distance of the cluster. When we combine these observations with the IR data, we obtain the absolute K-band and bolometric magnitudes of the brightest AGB stars in the cluster. It turns out that these brightest stars are nearly as bright as the brightest AGB stars in the dwarf elliptical galaxy M32, and we conclude that no appreciable age difference between the globular cluster and M32 is required to account for the brightest stars in this galaxy.

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