2002MNRAS.333..825C

We have developed an homogeneous evolutionary scenario for H- and He-burning low-mass stars by computing updated stellar models for a wide metallicity and age range [0.0002≤Z ≤0.004 and 9≤t (Gyr)≤15, respectively] suitable to study globular clusters. This theoretical scenario allows us to provide self-consistent predictions about the dependence of selected observational features of the colour-magnitude diagram, such as the brightness of the turn-off (TO), the zero-age horizontal branch (ZAHB) and the red giant branch bump (BUMP), on the cluster metallicity and age.

Taking into account these predictions, we introduce a new observable based on the visual magnitude difference between the TO and the ZAHB [ΔM_V(TO-ZAHB)], and the TO and the BUMP [ΔM_V(TO-BUMP)], given by A=ΔM_V(TO-BUMP)-0.566ΔM_V(TO-ZAHB). We show that the parameter A does not depend at all on the cluster age, but that it does strongly depend on the cluster global metallicity. The calibration of the parameter A as a function of Z is then provided, as based on our evolutionary models. We tested the reliability of this result by also considering stellar models computed by other authors, employing different input physics. Eventually, we present clear evidence that the variation of ΔM_V(TO-BUMP) with ΔM_V(TO-ZAHB) does supply a powerful probe of the global metal abundance, at least when homogeneous theoretical frameworks are adopted. Specifically, we show that the extensive set of models by Vanden Berg et al. suggests a slightly different calibration of A versus Z calibration, which however provides global metallicities higher by only 0.08±0.06dex with respect to the results from our computations.

We provide an estimate of the global metallicity of 36 globular clusters in the Milky Way, based on our A -Z calibration, and a large observational data base of Galactic globular clusters. By considering the empirical [Fe/H] scales by both Zinn & West and Carretta & Gratton, we are able to provide an estimate of the α -element enhancement for all clusters in our sample. We show that the trend of [α/Fe] with respect to the iron content significantly depends on the adopted empirical [Fe/H] scale, with the Zinn & West one suggesting α -element enhancements in fine agreement with current spectroscopic measurements.