The molecular envelopes of planetary nebulae

Abstract

We report the results of a survey of millimeter CO emission in 91 planetary nebulae using the IRAM 30 m and SEST 15 m telescopes. The observations provide new detections or improved data for 23 nebulae in the CO(2-1) and/or CO(1-0) line, and sensitive limits for those not seen in CO. Analysis of the results together with previous observations confirms the existence of an important class of planetary nebulae with massive (10-2-a few M⊙) envelopes of molecular gas. These nebulae typically have abundance ratios of N/O ≳ 0.3 and bipolar morphologies indicative of a young disk population. The column density through the envelopes and their mass relative to the mass of ionized gas show dramatic decreases with increasing nebular size, documenting the expansion of the envelopes and the growth of the optical nebulae at the expense of the molecular gas. The molecular envelopes remain a major mass component in these objects until the nebulae reach a radius of R ≈ 0.1 pc. The nebulae not detected in CO have little or no molecular gas (≲ 10-2-10-3 M⊙), and their envelopes must be rapidly photo-dissociated before or during the compact phase. The large differences in the molecular gas content of the nebulae highlight the different evolutionary paths for planetary nebula formation which result from the range in mass of the progenitors and the structure of their circumstellar envelopes.