CO and H I observations of an enigmatic interstellar cloudY. Libert1, E. Gérard2, T. Le Bertre1, L. Matthews3, C. Thum4, and J. M. Winters4
1 LERMA, UMR 8112, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
2 GEPI, UMR 8111, Observatoire de Paris, 5 Place J. Janssen, 92195 Meudon Cedex, France
3 MIT Haystack Observatory, Off Route 40, Westford, MA 01886, USA
4 IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France
Received 6 March 2009 / Accepted 6 April 2009
Context. An isolated H I cloud with peculiar properties has recently been discovered by Dedes et al. (2008, A&A, 491, L45) with the 300-m Arecibo telescope, and subsequently imaged with the VLA. It has an angular size of ~6', and the H I emission has a narrow line profile of width ~3 km s-1.
Aims. We explore the possibility that this cloud could be associated with a circumstellar envelope ejected by an evolved star.
Methods. Observations were made in the rotational lines of CO with the IRAM-30m telescope, on three positions in the cloud, and a total-power mapping in the H I line was obtained with the Nançay Radio Telescope.
Results. CO was not detected and seems too underabundant in this cloud to be a classical late-type star circumstellar envelope. On the other hand, the H I emission is compatible with the detached-shell model that we developed for representing the external environments of AGB stars.
Conclusions. We propose that this cloud could be a fossil circumstellar shell left over from a system that is now in a post-planetary-nebula phase. Nevertheless, we cannot rule out that it is a Galactic cloud or a member of the Local Group, although the narrow line profile would be atypical in both cases.
Key words: stars: AGB and post-AGB -- circumstellar matter -- ISM: clouds -- planetary nebulae -- radio lines: ISM
© ESO 2009