DIFFERENTIAL PROPER-MOTION STUDY OF THE CIRCUMSTELLAR DUST SHELL OF THE ENIGMATIC OBJECT, HD 179821

HD 179821 is an enigmatic evolved star that possesses characteristics of both a post-asymptotic giant branch (post-AGB) star and a yellow hypergiant, and there has been no evidence that unambiguously defines its nature. These two hypotheses are products of an indeterminate distance, presumed to be 1 kpc or 6 kpc. We have obtained the two-epoch Hubble Space Telescope Wild Field Planetary Camera 2 data of its circumstellar shell, which shows multiple concentric arcs extending out to about 8''. We have performed differential proper-motion measurements on distinct structures within the circumstellar shell of this mysterious star in hopes of determining the distance to the object, and thereby distinguishing the nature of this enigmatic stellar source. Upon investigation, rather than azimuthal radially symmetric expansion, we discovered a bulk motion of the circumstellar shell of (2.41 ± 0.43, 2.97 ± 0.32) mas yr-1. This corresponded to a translational interstellar medium (ISM) flow of (1.28 ± 0.95, 7.27 ± 0.75) mas yr-1 local to the star. This finding implies that the distance to HD 179821 should be rather small in order for its circumstellar shell to preserve its highly intact spherical structure in the presence of the distorting ISM flow, therefore favoring the proposition that HD 179821 is a post-AGB object.