Probing the post-AGB nature of HD 179821 - A new radio study of its circumstellar envelope

In spite of recent detailed studies, the nature of IID 179821 is still puzz ling. It is classified either as an intermediate-mass post-AGB star or as a massive supergiant. We present here new high sensitivity radio observation s of HD 179821 in the circumstellar (CO)-C-12 and (CO)-C-13 lines. We also report the new detection of HCO+ in this object. The (CO)-C-12 line intensi ties: allow estimations of the mass-loss rate of HD 179821 and of the dynam ical age of its circumstellar envelope. The comparison het cc ccn (CO)-C-12 and (CO)-C-13 lines indicates an isotopic ratio for carbon of at most 5. S uch a low value is characteristic of (post-)AGB st ars wit ii low-mass prog enitors. Furthermore, the HCO+ molecular ion indicates that an active photo chemistry occurs in the envelope of HD 179821, probably induced by UV photo ns emitted as the central post-AGE star warms up and/or by shocks generated by interacting winds.