Authors: Cernicharo, J.; Pardo, J. R.; Cabezas, C.; Agundez, M.; Tercero, B.; Marcelino, N.; Fuentetaja, R.; Guelin, M.; de Vicente, P.
Journal: ASTRONOMY & ASTROPHYSICS
Publication date: 2023/02/15
DOI: 10.1051/0004-6361/202245816
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(2)Pi and a dipole moment of similar to 1 D. Because of this low value of mu and to its much larger rotational partition function, its lines are expected to be well below the sensitivity of our data for both sources.
