Letter to the Editor

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

¹ Grupo de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
e-mail: jose.cernicharo@csic.es
² Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain
³ Observatorio Astronómico Nacional (OAN, IGN), Madrid, Spain
⁴ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d’Hères, France

Received: 29 December 2022
Accepted: 30 January 2023

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 (Q-band Ultrasensitive Inspection Journey to the Obscure TMC-1 Environment) 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 ²Π and a dipole moment of ∼1 D. Because of this low value of μ and to its much larger rotational partition function, its lines are expected to be well below the sensitivity of our data for both sources.