  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 <M_v(RR)>-[Fe/H] relation. The SHB models confirm
Lee's earlier finding (Lee 1991) that the slope of the <M_v(RR)>-[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
<M_v(RR)>-[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.
