GLOBULAR-CLUSTER AGES AND THE FORMATION OF THE GALACTIC HALO

Main-sequence turnoff magnitudes from the recent set of Yale isochrone s published by Chaboyer et al. in 1995 have been combined with a varie ty of relations for the absolute magnitude of RR Lyrae stars CM,(RR)I to calibrate age as a function of the difference in magnitude between the main-sequence turnoff and the horizontal branch (Delta VHR/TO). A best estimate for the calibration of M(v)(RR) is derived from a survey of the current literature: M(v)(RR) = 0.2[Fe/H] + 0.98. This estimate , together with other calibrations (with slopes ranging from 0.15 to 0 .30), has been used to derive Delta VHR/TO ages for 43 Galactic globul ar clusters. Independent of the choice of M,(RR), there is no strong e vidence for an age-Galactocentric distance relationship among the 43 g lobular clusters. However, an age-metallicity relation exists, with th e metal-poor clusters being the oldest. A study of the age distributio n reveals that an age range of 5 Gyr exists among the bulk of the glob ular dusters. In addition, about 10% of the sample are substantially y ounger, and including them in the analysis increases the age range to 9 Gyr. Once again, these statements are independent of the M(v)(RR) re lation. Evidence for age being the second parameter governing horizont al-branch morphology is found by comparing the average Delta V-HB(TO) age of the second parameter clusters to the normal clusters. The secon d parameter clusters are found to be on average 2-3 Gyr younger than t he other clusters, which is consistent with age being the second param eter. These results suggest that globular clusters were formed over an extended period of time, with progressively more metal-rich globular clusters ([Fe/H] greater than or similar to -1.7) being formed at late r times.