Gemini Preprint #63

The White Dwarf Cooling Age of the Open Cluster NGC 2420

Ted von Hippel
Gemini Observatory Northern Operations Center, 670 N. A'ohoku Place, Hilo HI 96720

Gerard Gilmore
Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK


We have used deep HST WFPC2 observations of two fields in NGC 2420 to produce a cluster CMD down to V approximately equal to 27. After imposing morphological selection criteria we find eight candidate white dwarfs in NGC 2420. Our completeness estimates indicate that we have found the terminus of the WD cooling sequence. We argue that the cluster distance modulus is likely to be close to 12.10 with E(B - V) = 0.04. With these parameters we find a white dwarf cooling age for NGC 2420 of 2.0±0.20 (1sigma) Gyrs. The 0.20Gyr uncertainty includes errors in the photometry, sequence fitting, precursor time scales, and theoretical white dwarf cooling time scales. Comparing the cluster white dwarf cooling age to ages derived from stellar isochrone fitting we find a preference for ages derived from models incorporating convective overshoot.

To appear in the Astronomical Journal.

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Ruth A. Kneale / / June 5, 2000