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A&A 367, 826-830 (2001)
DOI: 10.1051/0004-6361:20000496

Probing the post-AGB nature of HD 179821

A new radio study of its circumstellar envelope

E. Josselin^{1, 2} and A. Lèbre<sup>2

1</sup>  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

(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.

Key words: stars: AGB and post-AGB -- stars: circumstellar matter -- stars: individual: HD 179821

Offprint request: E. Josselin, josselin@graal.univ-montp2.fr

SIMBAD Objects in preparation

© ESO 2001

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