Age as the Second Parameter in NGC 288 / NGC 362? I. Turnoff Ages: a Purely Differential Comparison

Abstract

We present deep V,I photometry of the globular clusters NGC 288, NGC 362 and NGC 1851 obtained during a single observational run under strictly homogeneous conditions. We use the bimodal horizontal branch (HB) of NGC 1851 as a ``bridge'' to obtain the optimum relative match between the HBs of NGC 288 and NGC 362. In this way we can effectively remove the uncertainties associated with distance, reddening and inhomogeneities in the absolute calibration, thus obtaining a very robust, purely differential estimate of the age difference between these two clusters. According to the bridge test, NGC 288 is found to be older than NGC 362 by 2 +- 1 Gyr. This result is fully confirmed also by all classical differential age diagnostics, either based on the luminosity or color of the main sequence turnoff point.

Accepted by The Astronomical Journal - 30 pages, 6 tables, 14 figures, LaTeX, the emulateapj5.sty macro is used. Fig. 2 and Fig. 3 are provided in reduced resolution. Full resolution versions available upon request to the first author ([email protected]). Minor changes due to modifications in the companion paper (Pap II)

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