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K.A. Williams, M. Bolte (UCO/Lick Observatory), C.F. Claver (NOAO), M.A. Wood (Florida Inst. of Technology)
We present very deep photometry in three bands of the open star cluster M67, extending to V\approx 28, in an attempt to determine the white-dwarf cooling age of the cluster. White dwarf candidates were selected by comparing theoretical white dwarf cooling curves with all unresolved objects in the field. Fifteen white dwarf candidates were selected, including seven candidates fainter V=24.25, the faint magnitude limit expected based on a main sequence turn-off age of 4 Gyr and published white dwarf cooling models. These faintest white dwarf candidates are likely to be faint, unresolved background galaxies, given the high density of such objects. Spectroscopic follow-up of the brightest candidates is also presented, resulting in one confirmed white dwarf, one z=3.0 AGN, and two ambiguous spectra. This work illustrates the difficulty in identifying distant cool white dwarfs, even given exquisite observing conditions. Either spectroscopic or proper-motion confirmations will be required of faint white dwarf candidates before a white dwarf cooling age for M67 can be confirmed.
The author(s) of this abstract have provided an email address for comments about the abstract: williams@ucolick.org