ANALYSIS OF EVOLUTIONARY PERIOD-SHIFT PREDICTIONS FOR RR LYRAE STARS IN GALACTIC GLOBULAR-CLUSTERS

On the basis of detailed synthetic horizontal-branch (SHB) models, we analyze how different choices of HB evolutionary tracks can affect the predicted period shifts for the RR Lyrae stars in Galactic globular c lusters. Previous suggestions that choosing Sweigart's [ApJS, 65, 95 ( 1987)] models leads to a decrease in the predicted period shifts betwe en Oosterhoff type I and II clusters with respect to those obtained th rough use of the Lee & Demarque [ApJS, 73, 709 (1990)] tracks are conf irmed and quantified. For OoII clusters with not-too-blue HB morpholog ies (i.e., M15-like), we find that the increase in predicted period-sh ift values which results from the inclusion of the final 5%-10% of HB evolution in the evolutionary tracks is clearly not sufficient to over come the decrease which results from the adoption of the former tracks instead of the latter. The choice of Sweigart's models leads to a sma ller slope for the evolutionary DELTA log P(T(eff)-[Fe/H] relation by congruent-to 0.010 if the mean luminosity level of the RR Lyrae variab les in the SHB models are employed as a reference for reading the peri od values, and by congruent-to 0.017 if the lower envelopes of the cor responding distributions are used instead-thus further complicating th e explanation of the Sandage period-shift effect.