S 1040 in M 67 : A Post - Mass - Transfer Binary with a Helium - Core White

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 42.8d 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 Å 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 Lyα absorption profile that confirms the white dwarf identification of the S1040 secondary. A model atmosphere fit to the GHRS spectrum yields Teff = 16,160 K, log g = 6.7, and a mass of ∼ 0.22 M , for an assumed cluster distance of 820 pc and 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 (∼ 2d) period binary, and evolved through a blue straggler phase to reach its current state. Hughes STX Corporation, NASA Goddard Space Flight Center, Laboratory for Astronomy and Solar Physics, Code 681, Greenbelt, MD 20771 Département de Physique, Université de Montréal, Succ. Centre-Ville, C.P. 6128, Montréal, Québec, Canada, H3C 3J7 Lockheed Martin Canada, 6111 av. Royalmount, Montréal, Québec, Canada, H4P 1K6 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 NASA Goddard Space Flight Center, Laboratory for Astronomy and Solar Physics, Code 681, Greenbelt, MD 20771