SIMBAD references

We present the results of new stellar evolution computations in the heavy element and helium abundance ranges appropriate to describe the evolutionary status of galactic globular cluster stars, for main sequence, turn-off, subgiant and horizontal branch phases. We also present and discuss a few red giant tracks. The main rationales of these computations are: (o) 1. Availability of new opacities both for stellar interiors (Rogers & Iglesias 1992, 1994; OPAL) and for low temperature external layers (Kurucz 1991; Alexander & Ferguson 1994), whose influence on stellar structures has not yet been widely tested. (o) 2. Test on Population II stars of the recent Canuto & Mazzitelli (1991, 1992; CM) model for overadiabatic convection. (o) 3. Availability of Kurucz' colour - T_eff transformations (1993), a homogeneous set based on Kurucz' atmospheric models, covering the whole range of Populations I and II chemical abundances, and a large fraction of the HR diagram.
Result. are given for five sets of chemical abundances (Y=0.23, Z=0.0001; Y=0.23, Z=0.0003; Y=0.235, Z=0.001; Y=0.235, Z=0.002; Y=0.24, Z=0.004) and two treatments of external convection: according to the standard mixing length theory (MLT), and according to Canuto and Mazzitelli. Calibration of MLT is made following the general rule of adopting the ratio of the mixing length to the pressure scale height (α=l/H_p) which fits the solar model (and is consistent with subdwarfs location). Also the CM models have been tuned in a marginal way, as will be discussed in the following. New theoretical HB loci are more luminous at low metallicity, in agreement with recent observations of RR Lyrae and their magnitude-metallicity relations. Adopting the derived distance moduli, the turn-off location is consistent with the observed loci, without requiring arbitrary adjustments of the main sequence colour, which was often necessary in previous comparisons appeared in the astronomical literature. The resulting ages for globular clusters (GCs) are younger than previously found: MLT isochrones plus horizontal branch fitting give ages of ∼14Gyr for the most metal poor GCs. With the CM treatment, the isochrones turn out redder and, on the average, the observed turn-off (TO) colours are matched by CM isochrones still younger by ∼2Gyr, also thanks to the peculiar topology of the CM tracks around the TO which, due to the extreme thinness of the convective envelope, leads to an "apparent decrease" in the TO luminosity. On the whole, then, these new models provide younger GCs, and smaller spread in age between metal poor and metal rich GCs (ages 12/14Gyr).