ISOCHRONES FOR H-BURNING GLOBULAR-CLUSTER STARS .2. THE METALLICITY RANGE -2.3 LESS-THAN [FE/H] LESS-THAN -0.5

We present a new set of theoretical isochrones ranging between 10 and 20 Gyr and between Z = 10(-4) and Z = 6 x 10(-3) in order to extend the set of isochrones recently published by Chieffi and Straniero. All the isochrones fully extend from the lower main sequence up to the He flash. In addition to the tables containing the full set of isochrones, we present a series of analytical fitting formulae giving the helium core mass at the He flash, the amount of He brought at the surface as a consequence of the first dredge-up, the (B - V) color of the main sequence at M(V) = 6, the visual magnitude, the (B - V) color and the mass at the turnoff, the visual magnitude of the clump along the red giant branch evolution, the effective temperature of the red giant branch at M(bol) = -3, the visual magnitude and the mass at the red giant tip-all as a function of age and metallicity. Moreover, in order to allow a prompt derivation of the luminosity functions, the isochrones have been tabulated by adopting a fixed step in the visual magnitude (delta-M(V) = 0.10 mag). A comparison with other sets of isochrones available in the literature shows that (1) our isochrones turn out to be very similar in shape but systematically redder by few hundredths of a magnitude (congruent-to-0.03 mag) with respect to those published by VandenBerg and coworkers; (2) the Revised Yale Isochrones differ profoundly both from our set and that of VandenBerg. A reanalysis of the seven field subdwarfs, often adopted as a reference sequence, suggests that these stars do not form a tight sequence of stars and hence that it is quite hazardous to use them as a reliable reference locus. However, a subset of five out of the seven field subdwarfs having similar metallicities and forming a tighter sequence is nicely fitted by our isochrones without the requirement of any color shift. We present also a fit to a selected group of Galactic globular clusters, namely, M92, M68, M30, NGC 6752, M10, M3, M5, NGC 288, NGC 362, 47 Tucanae, and Palomar 12. The method adopted to fit the fiducial sequences of these clusters consists of two steps: (1) by fitting the shape of the fiducial sequence of each cluster, we fixed the age within, say, 2 Gyr, independent of any constraint on the reddening, distance modulus, and metallicity; (2) when the range of suitable ages has been fixed, the range of suitable values of metallicity and distance modulus can be readily derived once a value for the reddening has been adopted. Our main findings are the following: 1. Eight out of the 11 clusters show an age spread which does not exceed 3 Gyr; the three remaining clusters, i.e., NGC 362, 47 Tuc, and Palomar 12 (note that these are the most metal-rich clusters in the sample), turn out to be significantly younger by congruent-to-5 Gyr compared with the others. 2. The metallicities we infer for these clusters fairly agree with the Zinn and West metallicity scale; however, if one adopted the empirical (B - V)-log T(eff) relation recently published by Arribas and Martinez instead of that of VandenBerg and Bell, the inferred metallicities would be in better agreement with the ones quoted by Gratton and Ortolani instead of those quoted by Zinn and West. 3. The distance moduli derived for these clusters turn out to be largely within the range of values currently quoted in the literature. 4. By adopting our derived distance moduli and metallicities, and the apparent visual magnitude of the horizontal branches as quoted by Buonanno, Corsi, and Fusi Pecci, we determined the absolute visual magnitude of the horizontal branches of the clusters in our sample as a function of the metallicity. A comparison between such data and the theoretical relation, obtained by means of a new set of horizontal-branch models, shows fairly good agreement: it is to be noted that the new theoretical relation giving the variation of the horizontal-branch luminosity as a function of the metallicity is much better approximated by a parabola than by a straight line. 5. If the fiducial sequences published by Bolte and concerning the two clusters NGC 288 and NGC 362 were correct, at least differentially, these two clusters would differ in age by several Gyr and in metallicity by 0.3-0.4 dex, NGC 288 turning out to be the most metal-poor of the two.