Globular Cluster Ages and the Formation of the Galactic Halo

Abstract

Main sequence turnoff magnitudes from the recent set of Yale isochrones (Chaboyer ea 1995) have been combined with a variety of relations for the absolute magnitude of RR Lyr stars (mvrr) to calibrate age as a function of the difference in magnitude between the main sequence turn-off and the horizontal branch (dv). A best estimate for the calibration of mvrr is derived from a survey of the current literature: $mvrr = 0.20 feh + 0.98$. This estimate, together with other calibrations (with slopes ranging from 0.15 to 0.30) has been used to derive dv ages for 43 Galactic globular clusters. Independent of the choice of mvrr, there is no strong evidence for an age-Galacto-centric distance relationship among the 43 globular clusters. However, an age-metallicity relation exists, with the metal-poor clusters being the oldest. A study of the age distribution reveals that an age range of 5 Gyr exists among the bulk of the globular clusters. In addition, about 10% of the sample are substantially younger, and including them in the analysis increases the age range to 9 Gyr. Once again, these statements are independent of the mvrr ~relation. Evidence for age being the second parameter governing horizontal branch morphology is found by comparing the average dv ~age of the second parameter clusters to the normal clusters. The second parameter clusters are found to be on average 2 - 3 Gyr younger than the other clusters, which is consistent with age being the second parameter. These results suggest that globular clusters were formed over an extended period of time, with progressively more metal-rich globular clusters ($feh ga -1.7$) being formed at later times.Comment: to appear in ApJ, 16 pages LaTeX, uses aaspptwo.sty, complete postscript file (including figures) available via anonymous from: ftp://ftp.cita.utoronto.ca/cita/chaboyer/papers/halo.p