Chemical compositions of four high-latitude A-F supergiants

HR 6144, HD 161796, 89 Her, and HR 7671 are four representative members of the class of A- and F-type supergiants lying at high galactic latitudes. We have determined their atmospheric parameters and chemical compositions on the basis of model-atmosphere analyses of high-S/N digital spectra. The abundance results for HR 6144, HD 161796, and 89 Her are strongly atypical of Population I supergiants. (1) The stars are slightly metal-poor ([Fe/H] ≈ -0.4), indicating an origin in the Galaxy's thick disk, (2) Both carbon and nitrogen are overabundant in all three stars, and oxygen is overabundant in the first two, showing that products of both hydrogen and helium burning have been mixed to the stellar surfaces. Moreover, several of the program stars show emission features in the wings of Ha and strong infrared dust excesses, two properties also not typical of high-mass, Population I supergiants. Their locations in the H-R diagram, and the difficulty of assigning them to Population I, suggest that high-latitude supergiants are low-mass stars in a post-AGB evolutionary stage (and that the dust shells are protoplanetary nebulae). However, our analyses of HR 6144, HD 161796, and 89 Her do not reveal the enhancements of heavy s-process elements seen in highly evolved AGB stars, which are understood theoretically as a consequence of dredge-up of material exposed to hot protons. Alternatives to a post-AGB interpretation, such as binary star coalescence, should be explored. HR 7671 differs from our other three program stars in that it is very metal-poor ([Fe/H] = -1.1), is slightly C and O deficient, shows s-process abundances enhanced by a factor of 4 above solar, and has a surprising amount of lithium [log e(Li) = 2.4]. This object may indeed be a post-AGB star, which is descended from a Li-rich S-type star.