A Molecular Line Survey of the Highly Evolved Carbon Star CIT 6

Abstract

We present a spectral-line survey of the C-rich envelope CIT 6 in the 2 mm and 1.3 mm bands carried out with the Arizona Radio Observatory 12 m telescope and the Heinrich Hertz Submillimeter Telescope. The observations cover the frequency ranges of 131--160 GHz, 219--244 GHz, and 252--268 GHz with a typical sensitivity limit of T_R < 10 mK. A total of 74 individual emission features are detected, of which 69 are identified to arise from 21 molecular species and isotopologues, with five faint lines remaining unidentified. Two new molecules (C₄H and CH₃CN) and seven new isotopologues (C¹⁷O, ²⁹SiC₂, ²⁹SiO, ³⁰SiO, ¹³CS, C³³S, and C³⁴S) are detected in this object for the first time. The column densities, excitation temperatures, and fractional abundances of the detected molecules are determined using rotation-diagram analysis. Comparison of the spectra of CIT 6 to that of IRC+10216 suggests that the spectral properties of CIT 6 are generally consistent with those of IRC+10216. For most of the molecular species, the intensity ratios of the lines detected in the two objects are in good agreement with each other. Nevertheless, there is evidence suggesting enhanced emission from CN and HC₃N and depleted emission from HCN, SiS, and C₄H in CIT 6. Based on their far-IR spectra, we find that CIT 6 probably has a lower dust-to-molecular ratio than IRC+10216. To investigate the chemical evolution of evolved stars, we compare the molecular abundances in the asymptotic giant branch envelopes CIT 6 and IRC+10216 and those in the bright proto-planetary nebula CRL 618. The implication on the circumstellar chemistry is discussed.