A 3 mm molecular line survey toward the C-star envelope CIT 6

Abstract

We present an unbiased molecular line survey toward the carbon-rich circumstellar envelope CIT 6 carried out between 90 and 116 GHz with the Arizona Radio Observatory 12 m telescope. A total of 42 lines assigned to 10 molecular species and four isotopologues are detected. Despite the absence of any newly identified circumstellar molecules, several transitions are freshly reported for this object. This work is a complement to our previous line survey toward CIT 6 in the frequency ranges of 36–49 GHz and 131–268 GHz. Based on the measurements and in combination with previously published data, we perform a rotation-diagram analysis to determine the column densities, excitation temperatures, and fractional abundances of the molecules. The excitation temperature varies along the radius. The abundance pattern of CIT 6 is broadly consistent with that of the prototype C-star IRC +10216 at the specified detection sensitivity, indicating that the molecular richness in IRC +10216 cannot be attributed to an interpretation of unusual chemistry. Nevertheless, subtle distinctions between the two C-stars are found. The higher abundance of carbon-chain molecules and lower 12C$/$13C ratio in CIT 6 compared to IRC +10216 imply that the former is more massive or in a more evolved phase than the latter.