Introduction

Rapid progress has been made over the last few years in the study of the neutral gas in planetary nebulae (PNe), and it is now well established that at least some PNe are surrounded by massive envelopes of neutral gas (see, e.g., the review by Huggins 1993). The neutral envelopes provide a new perspective on the formation and evolution of the ionized nebulae, and allow the study of a range of circumstellar processes with different characteristics than those found in other circumstellar environments. In this paper we summarize the results of a recent survey of millimeter CO emission in PNe to study the molecular component of the neutral gas, and we comment on some of the issues raised by the observations.

The Molecular Gas in PNe

Millimeter CO emission has proved to be an especially useful probe of the neutral gas in PNe, since it can be used to determine the structure and kinematics of the envelopes, and to estimate the mass of the molecular component. In order to systematically study the differences between PNe, particularly evolutionary effects, we have undertaken extensive survey work in the 230 GHz CO (2–1) line using the IRAM 30 m and SEST 15 m telescopes. These provide access to both northern and southern PNe, with angular resolutions of 12″–24″. Our observations considerably extend the earlier survey work by Huggins ≈ Healy (1989), and are up to a factor of ≈ 6 times more sensitive, depending on the angular size of the PNe.