2023A&A...670L..19C

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.