A Large Stellar Evolution Database for Population Synthesis Studies. II. Stellar Models and Isochrones for an α-enhanced Metal Distribution

We present a large, new set of stellar evolution models and isochrones for an α-enhanced metal distribution typical of Galactic halo and bulge stars; it represents a homogeneous extension of our stellar model library for a scaled-solar metal distribution already presented by Pietrinferni et al. The effect of the α-element enhancement has been properly taken into account in the nuclear network, opacity, equation of state, and for the first time in the bolometric corrections and color transformations. This allows us to avoid the inconsistent use—common to all α-enhanced model libraries currently available—of scaled-solar bolometric corrections and color transformations for α-enhanced models and isochrones. We show how bolometric corrections to magnitudes obtained for the U, B portion of stellar spectra for Teff ≤ 6500 K are significantly affected by the metal mixture, especially at the higher metallicities. Our models cover both an extended mass range (between 0.5 and 10 M☉, with a fine mass spacing) and a broad metallicity range, including 11 values of the metal mass fraction Z, corresponding to the range -2.6 ≤ [Fe/H] ≤ 0.05. The initial He mass fraction is Y = 0.245 for the most metal-poor models, and it increases with Z, according to ΔY/ΔZ = 1.4. Models with and without the inclusion of overshoot from the convective cores during the central H-burning phase are provided, as well as models with different mass loss efficiencies. We also provide complete sets of evolutionary models for low-mass, He-burning stellar structures cover the whole metallicity range. This database, used in combination with our scaled-solar model library, is a valuable tool for investigating both Galactic and extragalactic simple and composite stellar populations, using stellar population synthesis techniques.