Population studies

Abstract

We present CCD and photoelectric BR photometry of the globular clusters NGC 288 and NGC 362 over the range 14 < B < 22. The data were obtained to test in as differential a manner as possible the hypothesis of Searle and Zinn that age is the second parameter affecting horizontal-branch morphology, and that there is an age spread in the globular cluster system of the Galaxy. Superposition of color-magnitude data for the two clusters, together with comparison of the morphology in the vicinity of the main-sequence turnoff, demonstrate basic differences most readily explained in terms of NGC 362 being some 3 ± 1 Gyr younger than NGC 288, as has been advocated by Bolte. The data are most consistent with the view that the halo globular cluster system, and by inference the Galactic halo, formed in a chaotic manner as advocated by Searle and Zinn, rather than in a short-lived monolithic collapse as suggested by Eggen, Lynden-Bell, and Sandage. Show more