THE EVOLVED HOT STARS OF THE OLD, METAL-RICH GALACTIC CLUSTER NGC-6791

Optical spectrophotometry of seven faint, extremely blue stars found b y Kaluzny & Udalski [AcA, 42, 29 (1999)] in the open cluster NGC 6791 is presented and analyzed using model atmosphere fits to the Balmer an d He I line profiles, Four are subdwarf B (sdB) stars with 24 000 K le ss than or similar to T(eff) less than or similar to 32 000 K and +4.3 less than or similar to M(V) less than or similar to +4.5, at least t hree of which are likely to be extended horizontal branch (EHB) member s of this old, metal-rich cluster. One is a subdwarf O star, M(V) = +4 .0, and could be a post-horizontal branch or AGB-manque star in the cl uster. One cataclysmic variable is found, also a possible member. The brightest and coolest candidate is most likely a nonmember B star of l ower gravity. As a by-product of the analysis, the mean reddening of t he cluster is estimated to be E(B - V) = 0.14. These results imply tha t the core helium-burning, horizontal branch (HB) of NGC 6791 has an e xtremely bimodal distribution of hydrogen envelope mass. Most stars wi th total masses approximately 0.7. M. form a normal red ''clump'' in t he color-magnitude diagram, while greater than or similar to 15% lie i n a tight group on the EHB with very little (<0.01 M.) hydrogen envelo pe overlying the approximately 0.5. M. core. It is difficult to see ho w such a large fraction of EHB stars with almost identically small env elope masses could have been produced by single star mass loss rates w ith any reasonable dispersion. It seems equally difficult to understan d how a binary mass transfer mechanism could be so fine-tuned. The pot entially analogous faint blue EHB tails in globular cluster horizontal branches have often been thought to be due to binary star interaction s of the kind suggested by Mengel et al. [ApJ, 204, 488 (1976)]. These are expected to produce both a more centrally concentrated distributi on of EHB stars and a significant range of envelope masses, at least i n populous clusters with high central concentrations. While EHB stars in most globulars are observed to have both of these properties, neith er is seen for the sdB/sdO stars in NGC 6791. The latter's spatial dis tribution is not significantly different from that of the red clump or the brightest red giant stars. The very small range of M(V) for the E HB stars in this cluster, at precisely the brightness expected for sta rs having the helium core flash mass {and very different from the fain t sdB sequence observed in M71 by Drukier et al. [ApJ, 342, L27 (1989) ]}, also argues against the double degenerate binary explanation descr ibed by Iben & Tutukov [ApJ, 282, 615 (1984)]. Though we cannot satisf actorily explain the unusual HB morphology in NGC 6791, the EHB stars in this younger, metal-rich disk cluster must have formed in a much lo wer density environment under a very different set of circumstances th an their globular cluster counterparts. We further suggest that the si milar sdB/EHB stars of the field are produced mainly by the most metal -rich component of the disk population. The observed sdB space density of the field disk stars could be accounted for by assuming that only the fraction of the galactic disk population having [Fe/H] greater tha n or similar to +0.15 forms EHB stars, in the same proportion as obser ved in NGC 6791. In support of this hypothesis, there seem to be few, if any, EHB stars in old open clusters of solar metallicity or less. T he implied far-ultraviolet (<2000 angstrom) radiation appropriate for the hot, evolved population of NGC 6791 will yield an integrated clust er L(UV)/L(bol) comparable to those observed in the metal-rich ellipti cal galaxies and spiral bulges. Since the stellar sources in the latte r may come from a population similar to NGC 6791 in metallicity, age, and stellar density, we suggest that a like fraction of evolved, hot s tars exists in the unresolved extragalactic populations. Current data are consistent with the working hypothesis that the trend of increasin g far-UV radiation with increasing metallicity found for elliptical ga laxies may also apply to the galactic disk.