SMA Spectral Line Survey of the Proto-Planetary Nebula CRL 618

Carbon-rich Asymptotic Giant Branch (AGB) stars are major sources of gas and dust in the interstellar medium. During the brief (∼1000 yr) period in the evolution from AGB to the Planetary Nebula (PN) stage, the molecular composition evolves from mainly diatomic and small polyatomic species to more complex molecules. Using the Submillimeter Array (SMA), we have carried out a spectral line survey of CRL 618, covering a frequency range of 281.9 to 359.4 GHz. More than 1000 lines were detected in the ∼60 GHz range, most of them assigned to HC₃N and c-C₃H₂, and their isotopologues. About 200 lines are unassigned. Lines of CO, HCO⁺, and CS show the fast outflow wings, while the majority of line emission arises from a compact region of ∼1" diameter. We have analyzed the lines of HC₃N, c-C₃H₂, CH₃CN, and their isotopologues with rotation temperature diagrams.