2001MNRAS.328..301G

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>=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 array at a resolution of 0.4arcsec and 0.35km.s^–1. The OH emission lies in a thick shell with inner and outer radii of 1.3 and 2.9×10^15 m (D=6 kpc) and expansion velocity of 30km.s^–1. Although we find some evidence 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 supports a distance estimate of 6kpc. However, the shell is incomplete and appears to have been disrupted by more recent collimated outflow activity within the last 1500yr. 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 evolution. We suggest that the SiO thermal emission arises from the same detached envelope as the OH maser emission as in IRC +10420. If so then this would strengthen the connection between these two stars and probably rule out a post-AGB status for HD 179821.