2001A&A...367..826J

In spite of recent detailed studies, the nature of HD 179821 is still puzzling. It is classified either as an intermediate-mass post-AGB star or as a massive supergiant. We present here new high sensitivity radio observations of HD 179821 in the circumstellar ¹²CO and ¹³CO lines. We also report the new detection of HCO⁺ in this object. The ¹²CO line intensities allow estimations of the mass-loss rate of HD 179821 and of the dynamical age of its circumstellar envelope. The comparison between ¹²CO and ¹³CO lines indicates an isotopic ratio for carbon of at most 5. Such a low value is characteristic of (post-)AGB stars with low-mass progenitors. Furthermore, the HCO⁺ molecular ion indicates that an active photochemistry occurs in the envelope of HD 179821, probably induced by UV photons emitted as the central post-AGB star warms up and/or by shocks generated by interacting winds.