HB Morphology and Age Indicators for Metal-Poor Stellar Systems with Age in the Range of 1 to 20 Gyr

Abstract

Isochrone computations and horizontal branch (HB) models for Y(MS)=0.23 and two values of Z (0.0001, 0.0004)are used to derive constraints on the age indicators and HB morphology of metal-poor clusters with age t (in Gyrs) in the range of 1< t < 20.It is found that for fixed metallicity the luminosity L(3.83) of the zero age horizontalbranch (ZAHB) at the RR Lyrae gap depends on age and that the difference in visual magnitude between main sequence turn-off (TO) and ZAHB may be calibrated in terms of age and metallicity. The results of synthetic HB computations show that at fixed metallicity the mean luminosity of RR Lyraes depends on HB morphology, as well as that the mean luminosity of purely red horizontal branches depends on the age and amount of mass loss during the red giant phase. Such features 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). Moreover, it is shown that the observed HB morphology provides us with an easy estimate of the upper limit of the cluster age. Some useful constraints to the cluster metallicity could be also derived.