Probing the Post-AGB Nature of HD 179821

Abstract

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 analyze here new high sensitivity radio observations of its circumstellar envelope in the ¹²C0 and ¹³CO lines. We also report the new detection of HCO⁺ in this object. The¹²C0 line intensities allow estimations of the mass-loss rate of HD 179821. The comparison between ¹²CO and¹³C0 lines indicates an isotopic ratio 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.