Age as the Second Parameter in NGC 288/NGC 362? I. Turnoff Ages: A Purely Differential Comparison
- 1 November 2001
- journal article
- research article
- Published by American Astronomical Society in The Astronomical Journal
- Vol. 122 (5) , 2569-2586
- https://doi.org/10.1086/323711
Abstract
We present deep V, I photometry of the globular clusters NGC 288, 362, and 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 (ΔV) or color [δ(V-I)@2.5] of the main-sequence turnoff point.Keywords
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