Ages for globular clusters with predominantly red horizontal branches

We have used the B-V color difference (d_B-v) between the horizontal branch (HB) and the red giant branch to explore the ages of globular clusters with red HB morphologies. Of these, 14 are Galactic, one belongs to the Large Magellanic Cloud (ESO 121-SC 03), and another to the Small Magellanic Cloud (Lindsay 1). Our observed values of d_B-v for these clusters are compared with theoretical values derived by constructing synthetic red HB models with a range of metallicities and ages. Based on ages determined via this comparison and main-sequence turnoff photometry for a subset of our clusters, we conclude that the usefulness of d_B-V as an age indicator seriously diminishes for clusters with [Fe/H] > -0.7. Furthermore, the Galactic globular clusters with [Fe/H] ≤ -0.7 and red HBs have a weighted mean age of 〈age〉 = 11.6 ± 0.3 Gyr, where the uncertainty is the standard error of the weighted mean. In contrast, from the work of Chaboyer, Sarajedini, and Demarque, clusters with HB morphologies bluer than those considered herein have 〈age〉 = 14.0 ± 0.3 Gyr, leading to the conclusion that the red HB clusters are younger than the remaining clusters by 2.4 ± 0.4 Gyr. This, and other evidence presented here, reinforces the conclusion of previous investigators that age is the second parameter, which, in addition to metal abundance, governs the morphology of the HB.