SIMBAD references

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 α-enhanced evolutionary models accounting for the gravitational settling of heavy elements at fixed chemical composition (,, and). The isochrones–assuming the same true distance modulus , the same reddening [], 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, 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 α- and CNO-enhanced (by a factor of 3) models, both at fixed total metallicity and at fixed iron abundance. We found that the isochrones based on the former set give the same cluster age as the canonical α-enhanced isochrones. The isochrones based on the latter set also give a similar cluster age . These findings support previous results concerning the weak sensitivity of cluster isochrones to CNO-enhanced chemical mixtures.