OPTICAL SPECTROSCOPY OF THE RED HORIZONTAL-BRANCH STARS OF NGC-6791

We have obtained 3600-4600 Angstrom spectra at similar to 5 Angstrom r esolution for 31 stars in the field of NGC 6791, one of the oldest and most ''globular-cluster-like'' of the Galactic open clusters. Based o n spectroscopic analysis of 19 of these stars considered to be red hor izontal branch (RHB) members of NGC 6791, we conclude that there is no evidence for significant variations in the CH band or the Ca and Fe l ine strengths among these stars, although there are strong indications of intrinsic differences among the CN band strengths. There is also n o evidence for correlations between the CN band, CH band, or Fe line s trengths of these stars, although there may be a weak correlation betw een CN band and Ca line strengths. The nature of the CN-band inhomogen eity is in marked contrast to the bimodal CN distributions discovered among the RHB stars of the two globular clusters 47 Tuc and M71 by Nor ris and Freeman [ApJ, 254, 143 (1982)] and Smith & Penny [AJ, 97, 1397 (1989)]. These results imply that either the environment within which globular clusters formed resulted in initial C and N abundance inhomo geneities that were lacking in NGC 6791, or that the evolutional proce sses which alter stellar surface abundances prior to or during the cor e helium-burning phase of evolution, differ in some respect between th e disk globular clusters and NGC 6791. (C) 1995 American Astronomical Society.