CO and HI observations of an enigmatic cloud

TL;DR

This paper investigates an unusual HI cloud with peculiar properties, exploring its possible origins as a fossil circumstellar shell from an evolved star, and analyzing its composition and structure through various radio observations.

Contribution

It provides the first detailed analysis of this enigmatic cloud, proposing a novel hypothesis that it is a fossil circumstellar shell from a post-planetary-nebula phase.

Findings

CO was not detected, indicating low abundance incompatible with a classical circumstellar envelope.

HI emission matches the detached-shell model for AGB star environments.

The cloud could be a fossil shell, a Galactic cloud, or a Local Group member, but the narrow line profile is unusual.

Abstract

An isolated HI cloud with peculiar properties has recently been discovered by Dedes, Dedes, & Kalberla (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 HI emission has a narrow line profile of width ~ 3 km/s. We explore the possibility that this cloud could be associated with a circumstellar envelope ejected by an evolved star. 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 HI line was obtained with the Nancay Radio Telescope. 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 HI emission is compatible with the detached-shell model that we developed for representing the external environments of AGB stars. 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.