Optical spectrophotometry of seven faint, extremely blue stars found by Kaluzny & Udalski (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 less than or approximately Teff less than or approximately 32 000 K and +4.3 less than or approximately Mv less than or approximately +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, Mv = +4.0, and could be a post-horizontal branch or AGB-manque 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 approximately 0.7 solar mass form a normal red 'clump' in the color-magnitude diagram, while greater than or approximately 15% lie in a tight group on the EHB with very little (less than 0.01 solar mass) hydrogen envelope overlying the approximately 0.5 solar mass 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. 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. 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 (less than 2000 A) radiation appropriate for the hot, evolved population of NGC 6791 will yield an integrated cluster LUV/ Lbol 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.
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Liebert et al. (1994) studied this question.