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 galactic globulars. The effect of the adopted mixing length on stellar magnitudes and colors is discussed, showing that the uncertainty on such a theoretical parameter prevents the use of MS stars as bona fide theoretical standard candles. However, comparison with Hipparcos data for field subdwarfs discloses a substantial agreement between theory and observation. Present predictions concerning the magnitude of TO and of HB stars are compared with similar results appeared in the recent literature. We find that our predictions about the dependence on metallicity of ZAHB magnitudes appear in good agreement with observational constraints as recently discussed by Gratton et al. (1997c). We present and discuss a theoretical calibration of the difference in magnitude between HE and TO as evaluated with or without element sedimentation. The effect of a variation of the original helium content on the magnitude of MS, TO and HB stars is explored and discussed. Finally we use theoretical HB magnitudes to best fit the CM diagram of M 68 and M 5, taken as representative of metal poor and intermediate metallicity galactic globulars, deriving an age of 11 +/- 1.0 Gyr and 10 +/- 1.0 Gyr, respectively, for the adopted chemical compositions, plus an additional uncertainty of +/-1.4 Gyr if the uncertainty on the chemical composition is taken into account. This result is discussed on the basis of current evaluations concerning cluster ages and distance moduli.