A spectral-line survey of CIT 6 between 30 and 50 GHz with the eQ receiver at NRO 45 m

Context . At the end of the long H-burning phase (main sequence), low-mass to intermediate-mass stars evolve into asymptotic giant branch (AGB) stars that can have carbon-rich envelopes depending on the initial C/O ratio. In their circumstellar envelopes (CSEs), dust particles and molecules are formed and shed back to the interstellar space. Therefore, these stars significantly contribute to the galactic astro-chemical evolution. Aims . To shed light on the chemical properties of carbon-rich CSEs, especially carbon- and silicon-bearing molecules, we performed parallel spectral-line surveys of IRC+10216 and CIT 6, the brightest and second-brightest carbon-rich star envelopes on the sky. Methods . We conducted 30−50 GHz observations towards both sources using a high-sensitivity-wide-band extended Q-band receiver (eQ) of the Nobeyama 45−m telescope. We then analysed data of CIT 6 and used data of IRC+10216 for comparison purposes. We applied the rotational-diagram method to derive their rotational temperatures and column densities for HC 5 N and HC 7 N. For other molecules, we assumed an excitation temperature to derive their column densities. Results . Molecular column densities in CIT 6 are systematically lower than those of IRC+10216, typically by one order of magnitude. Silicon- and sulfur-bearing species such as SiS and CS show the strongest depletion, whereas carbon-chain molecules (HC n N, C 6 H) remain relatively prominent, indicating that carbon-chain formation is still efficient in CIT 6. Rotational temperatures are higher in CIT 6, which is consistent with the fact that emission arises from warmer and more compact regions of its envelope. Both sources show low 12 C/ 13 C ratios and mildly sub-solar 28 Si/ 29 Si values, which are non-solar isotopic ratios. Conclusions . Both envelopes display canonical, carbon-rich AGB chemistry and comparable isotopic compositions. CIT 6, however, shows slightly higher excitation temperatures, stronger carbon-chain growth, and deeper depletion of Si- and S-bearing species. These signatures point to a more evolved circumstellar environment, where dust condensation and shock processing further modulated the molecular composition.