CO and H I observations of an enigmatic interstellar cloud

¹ LERMA, UMR 8112, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France e-mail: Thibaut.LeBertre@obspm.fr
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

Abstract

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.