The effect of enhanced alpha-elements in helium-burning Population II stars

New opacity tables for Population II stars have been computed, including O, Ne, Mg, and Ca in quantities enhanced by a factor of five over the quantities given by the solar ratios. The tables are used to compute evolutionary sequences for low-mass stars along the He-burning phases from the zero-age horizontal branch (ZAHB) up to the onset of thermal pulses in an advanced phase of He shell burning. When the alpha-elements are enhanced, the sensitivity of the ZAHB luminosity to (Fe/H) is much greater than when a solar ratio composition is assumed. The resulting HB evolution is largely dominated by blueward loops caused by the increased efficiency of the H-burning shell. Both the HB and the AGB phases appear theoretically well defined and well separated, as observed in real clusters. Neither the AGB luminosity nor the lifetimes during the various stages of He burning are significantly affected by the increased quantity of alpha-elements. 33 refs