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Session 107 - Open Clusters.
Display session, Thursday, January 18
North Banquet Hall, Convention Center

[107.05] The Hot Stars of Old Open Clusters: I. M67 and NGC 188

W. B. Landsman (HSTX/GSFC), B. Dorman (NRC/GSFC), R. W. O'Connell (UVA), R. Bohlin (STScI), M. S. Roberts (NRAO), S. G. Neff, A. M. Smith, A. Sweigart, T. P. Stecher (NASA/GSFC)

During the second flight of the Astro observatory in March 1995, we used the Ultraviolet Imaging Telescope (UIT) to obtain ultraviolet (\sim 1600 Åimages of five old open clusters. Most stars in an old open cluster are too cool to be detected in the ultraviolet, but among the possible UV-bright sources are blue stragglers, hot white dwarfs, and sdB or sdO stars. In NGC 188, we detect only the hot subdwarf and possible member II-91, and the blue straggler E43 (from Caputo et al.\ 1990, AJ, 99, 261). In M67, we detect ten blue stragglers, two white dwarf candidates (including the ROSAT source G152) and, surprisingly, the red giant S1040 from the catalogue of Sanders (1977).

The star S1040 (V = 11.52, B--V = 0.82) is a single-lined spectroscopic binary with a period of 42.8 days, and an astrometric and radial-velocity member of M67 (Mathieu et al.\ 1990, AJ, 100, 1859). It occupies a ``red straggler'' position in the color-magnitude diagram (CMD), 0.2 mag blueward of the giant branch. Our detection of S1040 in the ultraviolet implies that the secondary is likely a hot (\sim 25,000 K) white dwarf, and (2) the unusual position of S1040 in the CMD is likely due to previous mass transfer from the white dwarf progenitor. We discuss the implications our observations for the study of mass transfer binaries, and the origin of the blue stragglers in M67.

Program listing for Thursday