A 3 mm Molecular Line Survey toward The C-star Envelope CIT 6

TL;DR

This study conducts a molecular line survey of the CIT 6 C-star envelope between 90-116 GHz, identifying molecular species and analyzing their properties to compare with IRC+10216, revealing insights into stellar evolution and chemistry.

Contribution

First unbiased molecular line survey of CIT 6 in this frequency range, providing new molecular transition data and comparative analysis with IRC+10216.

Findings

42 lines from 10 molecules detected

Molecular abundance patterns similar to IRC+10216

CIT 6 shows higher carbon-chain molecules and lower 12C/13C ratio

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 4 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 CIT6 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.