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HD179821 is an enigmatic evolved star that possesses characteristics of botha post-asymptotic giant branch star and a yellow hyper-giant, and there hasbeen no evidence that unambiguously defines its nature. These two hypothesesare products of an indeterminate distance, presumed to be 1 kpc or 6 kpc. Wehave obtained the two-epoch Hubble Space Telescope WFPC2 data of itscircumstellar shell, which shows multiple concentric arcs extending out toabout 8 arcsec. We have performed differential proper-motion measurements ondistinct structures within the circumstellar shell of this mysterious star inhopes of determining the distance to the object, and thereby distinguishing thenature of this enigmatic stellar source. Upon investigation, rather thanazimuthal radially symmetric expansion, we discovered a bulk motion of thecircumstellar shell of (2.41+-0.43, 2.97+-0.32) mas/yr. This corresponded to atranslational ISM flow of (1.28+-0.95, 7.27+-0.75) mas/yr local to the star.This finding implies that the distance to HD 179821 should be rather small inorder for its circumstellar shell to preserve its highly intact sphericalstructure in the presence of the distorting ISM flow, therefore favoring theproposition that HD 179821 is a post-AGB object.

Differential Proper-Motion Study of the Circumstellar Dust Shell of the Enigmatic Object, HD 179821