The White Dwarf Cooling Age of M67

Abstract

A deep imaging survey covering the entire 23' diameter of the old open cluster M67 to V=25 has been carried out using the mosaic imager (UHCam) on the Canada-France-Hawaii Telescope. The cluster color-magnitude diagram (CMD) can be traced from stars on its giant branch at MV=+1 down through main-sequence stars at least as faint as MV=13.5. Stars this low in luminosity have masses below 0.15 M☉. A modest white dwarf (WD) cooling sequence is also observed commencing slightly fainter than MV=10 and, after correction for background galaxy and stellar field contamination, terminating near MV=14.6. The observed WDs follow quite closely a theoretical cooling sequence for 0.7 M☉ pure carbon core WDs with hydrogen-rich atmospheres (DA WDs). The cooling time to an MV of 14.6 for such WDs is 4.3 Gyr, which we take as the WD cooling age of the cluster. A fit of a set of isochrones to the cluster CMD indicates a turnoff age of 4.0 Gyr. The excellent agreement between these results suggests that ages derived from white dwarf cooling should be considered as reliable as those from other dating techniques. The WDs currently contribute about 9% of the total cluster mass, but the number seen appears to be somewhat low when compared with the number of giants observed in the cluster.