AN ATLAS OF THEORETICAL CONSTRAINTS FOR HORIZONTAL-BRANCH STARS

Synthetic horizontal branches (SHB) are constructed from a grid of up- to-date canonical ([alpha/Fe]=0) horizontal branch (HB) evolutionary t racks with an original helium content Y(MS)=0.23 and metallicity Z in the range 0.0001-0.02. The relevant constraints on the properties of H B and RR Lyrae stars are presented and discussed. Present results conf irm that the mean luminosity [log L(RR)] and light-to-mass ratio [A(RR )] of RR Lyraes depend on the metallicity as well as HB morphology. Fo r red to moderately blue HB types, i.e. for RR Lyrae-rich synthetic mo dels, we find [log L(RR)]=1.420-0.086 log Z and [A(RR)]=1.747-0.026 lo g Z, for log Z < -2.5, while for log Z > -2.5 we get [log L(RR)]=1.133 -0.198 log Z and [A(RR)]=1.397-0.161 log Z. The comparison with observ ations of field and Galactic globular cluster RR Lyrae variables shows that all the observed light-to-mass ratios can be fitted with a const ant original helium Y(MS)=0.23. Present HB models and recent isochrone computations at Y(MS)=0.23 are used to calibrate in terms of age (t i n Gyr) and metal content the difference in bolometric magnitude DELTAM (bol)(TO-HB) between the main-sequence turnoff and the zero-age horizo ntal branch at the RR Lyrae gap. On this ground, the Buonanno et al. o bservations of a sample of Galactic globular clusters are analyzed yie lding the mean relationships [t]=13.92-1.34[Fe/H] at [alpha/Fe]=0, and [t]=13.69-1.26 [Fe/H] at [alpha/Fe]=0.48. Concerning the age spread w hich is present among globular clusters of the same metallicity, prese nt estimates could confirm a relation between age and HB morphology, w ith the blue HB clusters older by approximately 2 Gyr than red HB clus ters, provided that no systematic error occurs in the definition of th e observed zero-age horizontal branch level.