J/A+A/706/A127ivo://CDS.VizieR/J/A+A/706/A127CDSNguyen-Luong Q.Nakamura F.Taniguchi K.Dinh-V-TrungLiu S.-Y.,Shimoikura T.Chiong C.-C.Dobashi K.Hirano N.Yonekura Y.,Nishimura A.Ogawa H.Chien C.Ho C.-T.Hwang Y.-J.Yeh Y.-T.,Lai S.-P.Fujii Y.Yamasaki Y.Kawabe R.Nolan A.Dung Pham V.2026-03-02T06:25:50Z2026-02-09T07:56:20ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frgiant-starsradio-spectroscopyAt the end of the long H-burning phase (main sequence), low-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 into the interstellar medium. Therefore, these stars significantly contribute to the Galactic astrochemical evolution. 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 circumstellar envelopes on the sky. We conducted 30-50GHz observations toward both sources using the high-sensitivity wide-band extended Q-band receiver (eQ) of the Nobeyama 45 m telescope. We analysed the data of CIT~6 and used the data of IRC+10216 for comparison. We applied the rotational-diagram method to derive 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. Molecular column densities in CIT~6 are systematically lower than those in IRC+10216, typically by about 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, consistent with emission arising 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, indicating non-solar isotopic compositions. 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 modulate the molecular composition.2026A&A...706A.127Nhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/706/A127CatalogResearchIsServedByTAP VizieR generic servicehttps://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/706/A127https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/706/A127http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/706/A127https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/706/A127https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/706/A127http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/706/A127GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapRadiodefaultJ/A+A/706/A127/listList of spectrarecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintNameStar namemeta.id;meta.maincharInst[LSB/USB] Instrumentmeta.id;instrcharBandFrequency intervalmeta.code;instr.bandpasscharspShow the spectrummeta.ref.urlcharFileNameName of the spectrum table in subdirectory spmeta.id;meta.tablechar