Models for old, metal-poor stars with enhanced alpha-element abundances. I. Evolutionary tracks and ZAHB loci; Observational constraints

Stellar evolutionary tracks have been computed for 17 [Fe/H] values from -2 .31 to -0.30 assuming, in each case, [alpha/Fe] = 0.0, 0.3, and 0.6. The he lium abundance was assumed to vary from Y = 0.2352 at [Fe/H] = -2.31 to Y = 0.2550 at [Fe/H] = -0.30 and held constant for the different choices of [a lpha/Fe] at a fixed iron content. Masses in the range 0.5 less than or equa l to M. less than or equal to 1.0, in 0.1 M. steps, were generally consider ed, though sequences for higher mass values were computed, as necessary, to ensure that isochrones as "young" as 8 Gyr could be generated for each gri d. All of the stellar models are based on an equation of state that treats nonideal effects, the latest nuclear reaction and neutrino cooling rates, a nd opacities that were computed specifically for the adopted chemical mixtu res. The tracks were extended to the tip of the giant branch or to an age o f 30 Gyr, whichever came first, and zero-age horizontal-branch (ZAHB) loci were constructed using the helium core masses and chemical profiles from ap propriate red giant precursors. Selected models have been compared with tho se computed by A. V. Sweigart, for the same masses and chemical composition s, to demonstrate that the results obtained from two entirely independent s tellar evolution codes agree well with one another when very similar input physics is assumed. In the case of extremely metal-deficient stars, an enha ncement in the abundance of the alpha-elements causes a single, fairly sign ificant bump in the opacity at a temperature just above 10(6) K, which is c aused by absorption processes involving the K shell of oxygen. This peak be comes steadily more pronounced as the overall metallicity increases and a s econd bump, arising from the L edges of Ne, Mg, and Si, eventually appears near log T = 5.6. As far as the tracks and isochrones are concerned, we fin d that, as already reported by others, it is possible to mimic the computat ions for [alpha/Fe] > 0 remarkably well by those for scaled-solar mixes sim ply by requiring the total mass-fraction abundance of the heavy elements, Z , to be the same. However, this result holds only for metallicities signifi cantly less than solar. Above [Fe/H] greater than or similar to -0.8, track s and isochrones for enhanced alpha-element mixtures begin to have systemat ically hotter/bluer turnoffs and red giant branches than those for scaled-s olar mixtures of the heavy elements. Also addressed is the extent to which our models satisfy the constraints posed by the local subdwarfs, the distan ces of which are based on Hipparcos parallax measurements. Our analysis sug gests that the predicted metallicity dependence of the location of the lowe r main sequence on the C-M diagram is in good agreement with the observed d ependence In fact, we do not find any compelling evidence from the local Po pulation II calibrators that the colors of our models require significant a djustments. In further support of our calculations, we find that, both in z ero point and slope, the computed giant branches on the (M-bol, log T-eff)- plane agree well with those inferred for globular clusters from observation s in the infrared. Moreover, our ZAHB models have luminosities that are jus t outside the 1 sigma error bars of the mean M-V's inferred for RR Lyrae st ars from Baade-Wesselink, statistical parallax, and trigonometric parallax studies. Lower helium contents or higher alpha-element abundances or an inc rease in the conductive opacities are among the possible ways of reducing t he differences that remain. To facilitate comparisons with observations, th e tracks/ZAHBs are provided with predicted BV(RI)(C) photometry.