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 (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.
Based on observations carried out with the Yebes 40m telescope (projects 19A003, 20A014, 20D023, and 21A011). The 40m radiotelescope at Yebes Observatory is operated by the Spanish Geographic Institute (IGN, Ministerio de Transportes, Movilidad y Agenda Urbana).- Publication:
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Astronomy and Astrophysics
- Pub Date:
- February 2023
- DOI:
- 10.1051/0004-6361/202245816
- arXiv:
- arXiv:2301.12819
- Bibcode:
- 2023A&A...670L..19C
- Keywords:
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- molecular data;
- line: identification;
- ISM: molecules;
- ISM: individual objects: TMC-1;
- ISM: individual objects: IRC+10216;
- astrochemistry;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Accepted as a Letter to the Editor in Astronomy &