The Metallicity Dependence of RR Lyrae Absolute Magnitudes from Synthetic Horizontal-Branch Models

Abstract

A grid of synthetic horizontal-branch (SHB) models based on HB evolutionary tracks with improved physics has been constructed to reconsider the theoretical calibration of the dependence of M_v(RR) on metallicity in globular clusters, and the slope of the mean -[Fe/H] relation. The SHB models confirm Lee's earlier finding (Lee 1991) that the slope of the -[Fe/H] relation is itself a function of the metallicity range considered, and that in addition, for a given [Fe/H], RR Lyrae luminosities depend on HB morphology. This is due to the fact that HB stars pass through the RR Lyrae instability strip at different evolutionary stages, depending on their original position on the HB. At [Fe/H]=-1.9, and for HB type 0, the models yield M_v(RR)=0.47 \pm 0.10. The mean slope for the zero-age HB models is 0.204. Since there is no simple universal relation between M_v(RR) and metallicity that is applicable to all globular clusters, the HB morphology of each individual cluster must be taken into account, in addition to [Fe/H], in deriving the appropriate M_v(RR). Taking HB morphology into account, we find that the slope of the mean -[Fe/H] relation varies between 0.36 for the clusters with galactocentric distances R_gc less than 6 kpc and 0.22 for clusters with 6<R_gc<20 kpc. Implications for interpreting observations of field RR Lyrae variables and for absolute globular cluster ages and galactic chronology are briefly discussed.