Letter to the Editor

Discovery of interstellar NC₄NH⁺: Dicyanopolyynes are indeed abundant in space^⋆

¹ Instituto de Física Fundamental, CSIC, Calle Serrano 123, 28006 Madrid, Spain
e-mail: marcelino.agundez@csic.es; jose.cernicharo@csic.es
² Observatorio Astronómico Nacional, IGN, Calle Alfonso XII 3, 28014 Madrid, Spain
³ Observatorio de Yebes, IGN, Cerro de la Palera s/n, 19141 Yebes, Guadalajara, Spain

Received: 17 November 2022
Accepted: 13 December 2022

Abstract

The previous detection of two species related to the nonpolar molecule cyanogen (NCCN), its protonated form (NCCNH⁺) and one metastable isomer (CNCN), in cold dense clouds supported the hypothesis that dicyanopolyynes are abundant in space. Here we report the first identification in space of NC₄NH⁺. This cation is the protonated form of NC₄N, which is the second member of the series of dicyanopolyynes after NCCN. The detection was based on the observation of six harmonically related lines within the Yebes 40m line survey of TMC-1 QUIJOTE. The six lines can be fitted to a rotational constant B = 1293.90840 ± 0.00060 MHz and a centrifugal distortion constant D = 28.59 ± 1.21 Hz. We confidently assign this series of lines to NC₄NH⁺ based on high-level ab initio calculations, which supports the previous identification of HC₅NH⁺ from the observation of a series of lines with a rotational constant 2 MHz lower than that derived here. The column density of NC₄NH⁺ in TMC-1 is (1.1 _−0.6^+1.4) × 10¹⁰ cm⁻², which implies that NC₄NH⁺ is eight times less abundant than NCCNH⁺. The species CNCN, previously reported toward L483 and tentatively in TMC-1, is confirmed in this latter source. We estimate that NCCN and NC₄N are present in TMC-1 with abundances a few times to one order of magnitude lower than HC₃N and HC₅N, respectively. This means that dicyanopolyynes NC−(CC)_n−CN are present at a lower level than the corresponding monocyanopolyynes HCC−(CC)_n−CN. The reactions of the radicals CN and C₃N with HNC arise as the most likely formation pathways to NCCN and NC₄N in cold dense clouds.