Chemical Compositions of Four High-Latitude A--F Supergiants

The atmospheric parameters and chemical composition of HR 6144, HD 161796, 89 Her, and HR 7671, four representative members of the class of A- and F-type supergiants lying at high Galactic latitudes, are determined. The abundance results for HR 6144, HD 161796, and 89 Her are strongly atypical of Population I supergiants in that they are slightly metal-poor and have an overabundance of both C and N. 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. However, they do not reveal the heavy s-process element enhancements seen in highly evolved B stars. HR 7671 differs from the other three stars in that it is very metal-poor, slightly deficient in C and O, has s-process abundances enhanced by a factor of four above solar, and has a surprising amount of Li. It may be a post-AGB star descended from a Li-rich S-type star.