Horizontal-Branch Morphology and the Photometric Evolution of Old Stellar Populations

Theoretical integrated broad-band colors ranging from far-UV to near-IR havebeen computed for old stellar systems from our evolutionary populationsynthesis code. These models take into account, for the first time, thedetailed systematic variation of horizontal-branch (HB) morphology with age andmetallicity. Our models show that some temperature-sensitive color indices aresignificantly affected by the presence of `blue HB stars'. Our models arecalibrated in the B-V, V-I, C-T1, and M-T1 vs. [Fe/H] planes, usinglow-reddened Galactic globular clusters (GCs) [E(B-V) < 0.2] and the relativeage difference between the older inner halo Galactic GCs and younger outer halocounterparts is well reproduced. Several empirical linear color-metallicitytransformation relations are assessed with our models and it is noted that theymay not be safely used to estimate metallicity if there are sizable agedifferences amongst GCs within and between galaxies. It is anticipated that thedetailed population models presented here coupled with further precisespectrophotometric observations of globular cluster systems in externalgalaxies from the large ground-based telescopes and space UV facilities willenable us to accurately estimate their ages and metallicities. (abridged)Comment: 40 pages, 14 figures, Tentatively scheduled for the November 2002 issue of The Astronomical Journa