AGE-DIFFERENCES BETWEEN OLD STELLAR POPULATIONS FROM THE HB MORPHOLOGY METALLICITY DIAGRAM

Isochrones in the horizontal-branch (HB) morphology-metallicity diagra m have been constructed on the basis of (1) detailed synthetic HB mode ls, as recently presented by Catelan [A&AS, 98, 547 (1993a)], and here in extended to completely red and blue HB morphologies; and (2) the re d giant branch (RGB) models of Sweigart & Gross [ApJS, 36, 405 (1978)] . The effect of variations in the several relevant input parameters th at characterize the present (standard) isochrones-namely, main-sequenc e helium content (Y(MS)), abundances of the alpha-capture elements ([a lpha/Fe]), and mean overall mass loss on the RGB (DELTAM)-has been ext ensively studied. We find that the age difference which may be estimat ed between any two (old) stellar populations from their distributions in the HB morphology-[Fe/H] plane, holding Y(MS), [alpha/Fe], and DELT AM fixed, depends strongly on the absolute values which are assumed fo r the latter quantities. In other words, unless the absolute values fo r such parameters can be accurately determined for the objects subject to examination, the HB morphology-[Fe/H] diagram does not constrain a ge differences in a reliable way. As examples, we have analyzed the fo llowing cases. (1) The classical ''second-parameter'' pair of globular clusters (GCs) NGC 288-NGC 362; (2) the (reportedly ''young'') GC Rup recht 106; (3) the old LMC GCs; and (4) the bulge RR Lyrae population. Our isochrones for fixed Y(MS), [alpha/Fe], and DELTAM suggest that: (1) NGC 288 and NGC 362 may differ in age by any amount in the range a pproximately 3-14 Gyr; (2) Ruprecht 106 may be younger than the GCs in side the solar circle by approximately 3-12 Gyr; (3) the latter cluste rs may be older than the old LMC ones by approximately 2-8 Gyr; and, f inally, (4) the RR Lyrae stars in the Galactic bulge may be older than the clusters in the inner halo by approximately 0.8-3.5 Gyr. The iden tification of the true second parameter that controls HB morphology (b esides [Fe/H]) is thus seen to depend on the accurate determination of cluster-to-cluster differences in (at least) the three parameters lis ted above, whereas the magnitude of the problem itself depends on the absolute values of those parameters which may have been excluded as se cond-parameter candidates. We also find that similar age differences b etween any two populations, as obtained for different (Y(MS), [alpha/F e], DELTAM) combinations, imply similar absolute ages for these popula tions. On the basis of this result, we show that the classical second- parameter pair NGC 288-NGC 362 can only be explained in terms of an ag e difference as small as congruent-to 3 Gyr {as found by VandenBerg et al. [AJ, 100, 445 (1990)] and others through independent age-dating m ethods}, if both clusters are younger than approximately 10 Gyr. This follows irrespective of the (Y(MS), [alpha/Fe], DELTAM) combination ch osen which is capable of reproducing the reported age difference of 3 Gyr between these two clusters. The far-reaching implications of this result must clearly be further explored.