Probing the post-AGB nature of HD 179821^*

A new radio study of its circumstellar envelope

¹ Observatorio Astronómico Nacional (OAN), IGN, Apartado 1143, 28800 Alcalá de Henares, Spain
² GRAAL-CC72, UMR 5024-ISTEEM, CNRS/Univ. Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex, France

Corresponding author: E. Josselin, josselin@graal.univ-montp2.fr

Received: 25 April 2000
Accepted: 13 November 2000

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 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.