1995A&A...298..833C

Isochrone computations and horizontal branch (HB) models with an original helium content Y(MS)=0.23 and two values of metallicity (Z=0.0001, 0.0004) are used to derive constraints on age indicators and HB morphology of metal-poor star clusters with age (t in Gyr) in the range of 1 to 20Gyr. It is found that the luminosity L(3.83) of the zero age horizontal branch (ZAHB) at the mean effective temperature (logT_e=3.83) of the RR Lyrae gap depends on metallicity as well as age, and that the resulting difference in visual magnitude to the main sequence turn-off (TO) may be calibrated in terms of age as log t=-0.186+0.40{DELTA}V(TO-ZAHB) and logt =-0.332+0.45{DELTA}V(TO-ZAHB) at Z=0.0001 and 0.0004, respectively. Synthetic horizontal branch models confirm that for fixed metallicity and age the mean luminosity of RR Lyrae depends on HB morphology. Moreover, the mean luminosity of purely red horizontal branches turns out to depend also on the amount of mass loss during the red giant phase, an occurrence which should be properly taken into account when the cluster age is derived from the observed difference in magnitude between main sequence TO and actual horizontal branch (rather than ZAHB). Finally, it is shown that for a given metallicity the observed HB morphology could provide an easy estimate of the upper limit of the cluster age. Since such a limit increases significantly with the adopted metallicity, while the absolute age which is derived from the difference {DELTA}V(TO-HB) is roughly constant, useful constraints to the cluster metallicity could be also derived.