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Session 107 - Open Clusters.
Display session, Thursday, January 18
North Banquet Hall, Convention Center
The open cluster NGC 6791 is as old or older than any other open cluster and has an iron abundance about twice solar. Yet several hot blue horizontal branch (BHB) stars are probable members (Kaluzny amp; Udalski 1992; Liebert, Saffer, amp; Green 1994). We have performed an abundance analysis of a high-resolution spectrum of the coolest BHB star, II-17, of type A5\,III. The program SYNTHE (Kurucz amp; Avrett 1981) was used with the atomic line list of Peterson, Dalle Ore, amp; Kurucz (1993). An excellent match between synthetic and observed spectra was found using a Kurucz (1991) model with effective temperature \Teff\ = 7250\K, \logg = 3.0, an overall metallicity twice solar, [Fe/H] = +0.3, and somewhat higher abundances of the lighter elements Na, Si, and Ca.
The star II-17 has all the properties of a true BHB member of NGC\,6791: its proper motion suggests membership (Cudworth 1994); its radial velocity agrees with the cluster mean to \pm 2 km/s, and is constant to 1 km/s from night to night; its rotational velocity is 15 km/s, slower than that of normal main-sequence A stars of this temperature and faster than expected for a post-asymptotic-giant-branch star; and it is metal-rich, with an abundance distribution that shows no A-type spectral peculiarities but rather a halo-type composition. Understanding its existence may provide the key to the puzzling upturn in ultraviolet flux below 1500Å\ seen in many high metallicity systems. The abundances of this star establish the cluster metallicity and abundance ratios of critical elements, constraining the timescale of disk formation and the rapidity of the buildup of heavy elements in the disk. They will provide a detailed calibration of the abundances of metal-rich giants, putting the metal-rich scale on a quantitative foundation. Finally, the star's properties illustrate the need for surveys and syntheses to include metallicity-sensitive effects; for example, the UBV colors of II-17 are so red (due to excess line blanketing) that it falls in the normal RR Lyrae domain, and so it would have been missed in many searches for BHB field stars.
Program listing for Thursday