THE ALPHA-ENHANCED ISOCHRONES AND THEIR IMPACT ON THE FITS TO THE GALACTIC GLOBULAR-CLUSTER SYSTEM

We analyze in detail the effect produced by the enhancement of the alpha-element O, Ne, Mg, Si,S, and Ca on the evolutionary properties of low-mass, low-metallicity stars. In particular we address the evolutionary phases which extend from the main sequence up to the H reignition on the asymptotic giant branch. We find that the alpha-elements other than 0 cannot be neglected. Our main results are as follows: 1. Evolutionary models (suitable for Population II stars) with alpha-enhanced chemical composition can be computed even though proper low-temperature opacities are not yet available, since we demonstrate that most of the evolutionary properties of Population II stellar models (i.e., the location, in the H-R diagram, of both the main sequence and the turn off; the central H-burning lifetime; the luminosity of the bump on the red giant branch; the efficiency of the first dredge-up; the He core mass and the luminosity at the He flash; and the properties all along the following He-burning phase) are not influenced by the metal content included in the opacity coefficient below 12,000 K. 2. If the condition [GRAPHICS] is satisfied once the alpha-elements have been enhanced (not necessarily by the same amount), then the alpha-enhanced isochrones are very well mimicked by the standard scaled solar isochrones of metallicity: Z = Z0(0.638f(alpha) + 0.362) , where f(alpha) is the chosen average enhancement factor of the alpha-elements and Z0 is the initial (nonenhanced) metallicity. 3. We discuss the fit to Groombridge 1830, the most ''reliable'' of the local subdwarfs, and we conclude that it is unnecessary, at present, to revise the value of the mixing-length parameter alpha [=l/H(p)] obtained by fitting the Sun. 4. The use of O-enhanced models is discouraged, since they lead to results which differ from those obtained by including the other alpha-elements. 5. We discuss the effect of the enhancement of the alpha-elements on the absolute and relative ages of the Galactic globular clusters by means of two independent methods, i.e., the DELTAV(HB - TO) and the DELTA(B - V) methods. Both lead to similar results: the absolute and the relative ages are only slightly affected by the enhancement of the alpha-elements. In particular, the ages obtained by adopting the DELTA(B - V) method are practically insensitive to the enhancement, since this quantity depends only weakly on the metallicity, while the ones obtained by using the DELTAV(HB-TO) method show a maximum decrease (by congruent-to 0.7 Gyr per 0.3 dex in the overabundance) at the very low metallicity end ([M/H] congruent-to -2.3), which progressively reduces as the metallicity increases.