Discovery of the C7N- anion in TMC-1 and IRC+10216

Abstract

We report on the discovery of the C7N- anion towards the starless core TMC-1 and towards the carbon-rich evolved star IRC+10216. We used the data of the QUIJOTE line survey towards TMC-1 and found six lines in perfect harmonic frequency relation from J=27-26 up to J=32-31. The frequency of the lines can be reproduced with a rotational constant and a distortion constant of B=582.68490+/-0.00024 MHz and D=4.01+/-0.13 Hz, respectively. The standard deviation of the fit is 4 kHz. Towards IRC+10216, we identify 17 lines from J=27-26 up to J=43-42; their frequencies are also in harmonic relation, providing B=582.6827+/-0.00085 MHz and D=3.31+/-0.31 Hz. The nearly exact coincidence of the rotational and distortion constants in both sources points unambiguously to a common molecular carrier. Taking into account the chemical peculiarities of both sources, the carrier could be a radical or an anion. The radical can be discarded, as the observed lines belong to a singlet species. Hence, the most plausible carrier is an anion. High-level ab initio calculations indicate that C7N-, for which we compute a rotational constant of B=582.0 MHz and a dipole moment of 7.5 D, is the carrier of the lines in both sources. We predict the neutral C7N to have a ground electronic state 2Pi and a dipole moment around 1 D. Because of this low dipole moment value and to its much larger rotational partition function, its lines are expected to be well below the sensitivity of our data for both sources.

Accepted as a Letter to the Editor in Astronomy & Astrophysics. Submitted on 29/12/2022 and accepted on 30/1/2023

References in corpus (7)

• Detection of C5N- and vibrationally excited C6H in IRC +10216
• Discovery of benzyne, o-C6H4, in TMC-1 with the QUIJOTE line survey
• Yebes 40 m radio telescope and the broad band NANOCOSMOS receivers at 7 mm and 3 mm for line surveys
• Search for anions in molecular sources: C4H- detection in L1527
• The Effects of Molecular Anions on the Chemistry of Dark Clouds
• Discovery of fulvenallene in TMC-1 with the QUIJOTE line survey
• Interstellar nitrile anions: Detection of C3N- and C5N- in TMC-1

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• Discovery of H${}_2$CCCH${}^{+}$ in TMC-1
• The abundance and excitation of molecular anions in interstellar clouds
• The spatial distribution of an aromatic molecule, C6H5CN, in the cold dark cloud TMC-1
• Discovery of thiofulminic acid with the QUIJOTE line survey: A study of the isomers of HNCS and HNCO in TMC-1
• Discovery of two metallic cyanoacetylides in IRC+10216: HMgCCCN and NaCCCN
• Laboratory and astronomical discovery of the cyanovinyl radical H2CCCN
• Charting Circumstellar Chemistry of Carbon-rich AGB Stars: I. ALMA 3 mm spectral surveys
• Infrared Spectroscopy of Pentagon-Containing PAHs: Indenyl and Fluorenyl Anions and Indenyl Cation
• Detection of the linear SiC${}_3$ and SiC${}_5$ radicals in IRC\,+10216
• CN and CCH derivatives of ethylene and ethane: Confirmation of the detection of CH${}_3$CH${}_2$CCH in TMC-1
• Charting circumstellar chemistry of carbon-rich asymptotic giant branch stars. II. Abundances and spatial distributions of CS