S1040 in M67: A post-mass transfer binary with a helium core white dwarf

We have obtained spectra of the yellow giant S1040 in the open cluster M67 using the Goddard High-Resolution Spectrograph (GHRS) and the Faint Object Spectrograph on the Hubble Space Telescope. S1040 is a single-lined spectroscopic binary with a 42.8 day period that occupies a ''red straggler'' position in the M67 color-magnitude diagram (CMD), 0.2 mag blueward of the giant branch. A detection of S1040 at 1620 Angstrom with the Ultraviolet Imaging Telescope provided evidence that the secondary is a hot white dwarf and thus that the anomalous location of S1040 in the CMD is likely due to a prior episode of mass transfer. Our GHRS spectrum shows a broad Ly alpha absorption profile that confirms the white dwarf identification of the S1040 secondary. A model atmosphere fit to the GHRS spectrum yields T-eff = 16,160 K, log g = 6.7, and a mass of about 0.22 M., for an assumed cluster distance of 820 pc and a reddening of E(B - V) = 0.02. The unusually low mass derived for the white dwarf implies that it must have a helium core and that a mass transfer episode must have begun while the progenitor was on the lower giant branch. We construct a plausible mass transfer history for S1040 in which it originated as a short-period (similar to 2 days) binary and evolved through a blue straggler phase to reach its current state.