On the Absolute Age of the Globular Cluster M92

We present precise and deep optical photometry of the globular M92. Data were collected in three different photometric systems: Sloan Digital Sky Survey (g', r', i', and z'; MegaCam at CFHT), Johnson-Kron-Cousins (B, V, and I; various ground-based telescopes), and Advanced Camera for Surveys (ACS) Vegamag (F475W, F555W, and F814W; Hubble Space Telescope). Special attention was given to the photometric calibration, and the precision of the ground-based data is generally better than 0.01 mag. We computed a new set of a-enhanced evolutionary models accounting for the gravitational settling of heavy elements at fixed chemical composition ([alpha/Fe] = +0.3, [Fe/H] = -2.32 dex, and Y = 0.248). The isochrones-assuming the same true distance modulus (mu = 14.74 mag), the same reddening [E(B - V) = 0.025 +/- 0.010 mag], and the same reddening law-account for the stellar distribution along the main sequence and the red giant branch in different color-magnitude diagrams (i', g' - i'; i', and g' - r'; i', g' - z'; I, and B - I; and F814W and F475W-F814W). The same outcome applies to the comparison between the predicted zero-age horizontal-branch (ZAHB) and the HB stars. We also found a cluster age of 11 +/- 1.5 Gyr, in good agreement with previous estimates. The error budget accounts for uncertainties in the input physics and the photometry. To test the possible occurrence of CNO-enhanced stars, we also computed two sets of alpha- and CNO-enhanced (by a factor of 3) models, both at fixed total metallicity ([M/H] = -2.10 dex) and at fixed iron abundance. We found that the isochrones based on the former set give the same cluster age (11 +/- 1.5 Gyr) as the canonical a- enhanced isochrones. The isochrones based on the latter set also give a similar cluster age (10 +/- 1.5 Gyr). These findings support previous results concerning the weak sensitivity of cluster isochrones to CNO-enhanced chemical mixtures.