OH maser mapping of the evolved star HD 179821: evidence for interacting outflows

The evolved star HD 179821 continues to be the subject of much debate as to whether it is a nearby (D = 1 kpc) post-asymptotic giant branch (post-AGB) star or a distant (D = 6 kpc) high initial mass (M-i greater than or equal to 30 M.) post-red supergiant. We have mapped the OH maser emission around HD 179821 in the 1612- and 1667-MHz lines with the MERLIN interferometer a rray at a resolution of 0.4 arcsec and 0.35 km s(-1). The OH emission lies in a thick shell with inner and outer radii of 1.3 and 2.9 X 10(15) m (D = 6 kpc) and expansion velocity of 30 km s(-1). Although we find some evidenc e for acceleration and for deviations from spherical symmetry, the bulk of the maser emission is consistent with a constant-velocity spherical shell. The extent of the shell agrees with H2O and OH dissociation models and supp orts a distance estimate of 6 kpc. However, the shell is incomplete and app ears to have been disrupted by more recent collimated outflow activity with in the last 1500 yr. We suggest that this activity is also responsible for the active envelope chemistry (in particular the presence of HCO+) and for the apparent offset of the star from the centre of the shell. The luminous yellow hypergiant star IRC + 10420 also shows signs of recent outflows, and HD 179821 may be at a similar, perhaps slightly earlier, phase of evolutio n. We suggest that the SiO thermal emission arises from the same detached e nvelope as the OH maser emission as in IRC + 10420. If so then this would s trengthen the connection between these two stars and probably rule out a po st-AGB status for HD 179821.