Galactic globular cluster stars: From theory to observation

We use evolutionary calculations presented in a recent paper (Cassisi et al . 1997a: hereinafter Paper I) to predict B, V, I magnitudes for stars in ga lactic globulars. The effect of the adopted mixing length on stellar magnit udes and colors is discussed, showing that the uncertainty on such a theore tical parameter prevents the use of MS stars as bona fide theoretical stand ard candles. However, comparison with Hipparcos data for field subdwarfs di scloses a substantial agreement between theory and observation. Present pre dictions concerning the magnitude of TO and of HB stars are compared with s imilar results appeared in the recent literature. We find that our predicti ons about the dependence on metallicity of ZAHB magnitudes appear in good a greement with observational constraints as recently discussed by Gratton et al. (1997c). We present and discuss a theoretical calibration of the diffe rence in magnitude between HE and TO as evaluated with or without element s edimentation. The effect of a variation of the original helium content on t he magnitude of MS, TO and HB stars is explored and discussed. Finally we u se theoretical HB magnitudes to best fit the CM diagram of M 68 and M 5, ta ken as representative of metal poor and intermediate metallicity galactic g lobulars, deriving an age of 11 +/- 1.0 Gyr and 10 +/- 1.0 Gyr, respectivel y, for the adopted chemical compositions, plus an additional uncertainty of +/-1.4 Gyr if the uncertainty on the chemical composition is taken into ac count. This result is discussed on the basis of current evaluations concern ing cluster ages and distance moduli.