The evolved hot stars of the old, metal-rich galactic cluster NGC 6791

Optical spectrophotometry of seven faint, extremely blue stars found by Kałużny & Udalski [AcA, 42, 29 (1992)] in the open cluster NGC 6791 is presented and analyzed using model atmosphere fits to the Balmer and He I line profiles. Four are subdwarf B (sdB) stars with 24000 K≲T_eff≲32 000 K and +4.3≲M_V≲+4.5, at least three of which are likely to be extended horizontal branch (EHB) members 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-manqué star in the cluster. One cataclysmic variable is found, also a possible member. The brightest and coolest candidate is most likely a nonmember B star of lower gravity. As a by-product of the analysis, the mean reddening of the cluster is estimated to be E(B-V)=0.14. These results imply that the core helium-burning, horizontal branch (HB) of NGC 6791 has an extremely bimodal distribution of hydrogen envelope mass. Most stars with total masses ∼0.7Script M sign_⊙ form a normal red "clump" in the color-magnitude diagram, while ≳15% lie in a tight group on the EHB with very little (<0.01Script M sign_⊙) hydrogen envelope overlying the ∼0.5Script M sign_⊙ core. It is difficult to see how such a large fraction of EHB stars with almost identically small envelope masses could have been produced by single star mass loss rates with any reasonable dispersion. It seems equally difficult to understand how a binary mass transfer mechanism could be so fine-tuned. The potentially analogous faint blue EHB tails in globular cluster horizontal branches have often been thought to be due to binary star interactions of the kind suggested by Mengel et al. [ApJ, 204, 488 (1976)]. These are expected to produce both a more centrally concentrated distribution of EHB stars and a significant range of envelope masses, at least in populous clusters with high central concentrations. While EHB stars in most globulars are observed to have both of these properties, neither is seen for the sdB/sdO stars in NGC 6791. The latter's spatial distribution 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 EHB stars in this cluster, at precisely the brightness expected for stars having the helium core flash mass {and very different from the faint sdB sequence observed in M71 by Drukier et al. [ApJ, 342, L27 (1989)]}, also argues against the double degenerate binary explanation described by Iben & Tutukov [ApJ, 282, 615 (1984)]. Though we cannot satisfactorily explain the unusual HB morphology in NGC 6791, the EHB stars in this younger, metal-rich disk cluster must have formed in a much lower density environment under a very different set of circumstances than their globular cluster counterparts. We further suggest that the similar 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]≳+0.15 forms EHB stars, in the same proportion as observed 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. The implied far-ultraviolet (<2000 Å) radiation appropriate for the hot, evolved population of NGC 6791 will yield an integrated cluster L_UV/L_bol comparable to those observed in the metal-rich elliptical galaxies and spiral bulges. Since the stellar sources in the latter may come from a population similar to NGC 6791 in metallicity, age, and stellar density, we suggest that a like fraction of evolved, hot stars exists in the unresolved extragalactic populations. Current data are consistent with the working hypothesis that the trend of increasing far-UV radiation with increasing metallicity found for elliptical galaxies may also apply to the galactic disk.