On the Absolute Age of the Globular Cluster M92

• Published in: Publications of the Astronomical Society of the Pacific 2010-09, Vol.122 (895), p.991-999
• Main Authors: DI CECCO, A, BECUCCI, R, CALAMIDA, A, CAPUTO, F, CORSI, C. E, FERRARO, I, IANNICOLA, G, PULONE, L, ROMANIELLO, M, WALKER, A. R, BONO, G, MONELLI, M, STETSON, P. B, DEGL'INNOCENTI, S, PRADA MORONI, P. G, NONINO, M, WEISS, A, BUONANNO, R
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology
• ISSN: 0004-6280; 1538-3873
• DOI: 10.1086/656017

We present precise and deep optical photometry of the globular M92. Data were collected in three different photometric systems: Sloan Digital Sky Survey (, , , and ; MegaCam at CFHT), Johnson-Kron-Cousins (, , and ; 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 alpha -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 = 0.248). The isochrones-assuming the same true distance modulus ([micro] = 14.74 mag), the same reddening [ (-) = 0.025 A+/- 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 (, - ; , and - ; , - ; , and - ; 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 A+/- 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 A+/- 1.5 Gyr) as the canonical alpha -enhanced isochrones. The isochrones based on the latter set also give a similar cluster age (10 A+/- 1.5 Gyr). These findings support previous results concerning the weak sensitivity of cluster isochrones to CNO-enhanced chemical mixtures.