Issue |
A&A
Volume 688, August 2024
|
|
---|---|---|
Article Number | L29 | |
Number of page(s) | 5 | |
Section | Letters to the Editor | |
DOI | https://doi.org/10.1051/0004-6361/202451319 | |
Published online | 15 August 2024 |
Letter to the Editor
Space and laboratory discovery of iminopentadienylidene, HNC5⋆
1
Dept. de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
2
Department of Applied Chemistry, Science Building II, National Yang Ming Chiao Tung University, 1001 Ta-Hsueh Rd., Hsinchu 300098, Taiwan
3
Observatorio de Yebes (IGN), Cerro de la Palera s/n, 19141 Yebes, Guadalajara, Spain
4
Observatorio Astronómico Nacional (OAN, IGN), C/ Alfonso XII, 3, 28014 Madrid, Spain
Received:
1
July
2024
Accepted:
19
July
2024
We report the discovery of HNC5 in TMC-1. Six lines have been found in harmonic relation, with quantum numbers J = 12−11 up to J = 17−16. The lines can be reproduced with the standard frequency relation for linear molecules with B = 1361.75034 ± 0.00033 MHz and D = 32.2 ± 0.7 Hz. The assignment of the carrier to iminopentadienylidene was achieved through examining the possible candidates at a high level of theoretical ab initio calculations. Motivated by the good agreement between the observed B and the calculated value for HNC5, we searched for it in the laboratory and observed the transitions J = 5−4 to 7−6. The derived rotational and distortion constants are 1361.74998 ± 0.00040 MHz and 26.5 ± 5.5 Hz, respectively. Hence, we solidly conclude that the carrier of the lines found in TMC-1 is HNC5. The calculated dipole moment for this species is 7.7 D and the derived column density is (1.3 ± 0.2) × 1010 cm−2. We used the new QUIJOTE data to improve previous observations of HC4NC and found that the abundance ratio HC4NC/HNC5 is 10 ± 2. The abundance ratio of HC5N and its two isomers HC4NC and HNC5 is 500 ± 80 and 5100 ± 800, respectively. These abundance ratios are higher by a factor of ∼10 than those of the equivalent isomers of HC3N. Chemical models reproduce the observed abundances reasonably well when a chemistry similar to that of the smaller species C3HN isomers is adopted. The formation of HNC5 and HC4NC arises from the dissociative recombination with electrons of the cations HC5NH+ and HC4NCH+.
Key words: astrochemistry / line: identification / molecular data / ISM: molecules / ISM: individual objects: TMC-1
© The Authors 2024
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