Discovery of interstellar NC4NH+: dicyanopolyynes are indeed abundant in space

Abstract

The previous detection of two species related to the non polar 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+, which is the protonated form of NC4N, 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+ by Marcelino et al. (2020) 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 +1.4 -0.6)e10 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.

Accepted for publication in A&A Letters

References in corpus (16)

• Discovery of benzyne, o-C6H4, in TMC-1 with the QUIJOTE line survey
• New molecules in IRC+10216: confirmation of C${}_5$S and tentative identification of MgCCH, NCCP, and SiH${}_3$CN
• Yebes 40 m radio telescope and the broad band NANOCOSMOS receivers at 7 mm and 3 mm for line surveys
• Discovery of HC4NC in TMC-1: A study of the isomers of HC3N, HC5N, and HC7N
• Discovery of fulvenallene in TMC-1 with the QUIJOTE line survey
• Space and laboratory discovery of HC3S+
• Discovery of HC3O+ in space: The chemistry of O-bearing species in TMC-1
• Tentative detection of HC5NH+ in TMC-1
• Discovery of the elusive thioketenylium, HCCS+, in TMC-1
• Discovery of the acetyl cation, CH3CO+, in space and in the laboratory
• Isocyanogen formation in the cold interstellar medium
• Discovery of C5H+ and detection of C3H+ in TMC-1 with the QUIJOTE line survey
• Discovery of a new molecular ion, HC7NH+, in TMC-1
• Cyanogen, cyanoacetylene, and acetonitrile in comet 67P and their relationship to the cyano radical
• Interstellar detection of the simplest aminocarbyne, H2NC: an ignored but abundant molecule
• A new protonated molecule discovered in TMC-1: HCCNCH+

Cited by in corpus (2)

• Carbon-Chain Chemistry in the Interstellar Medium
• A search for the three isomers of cyano-1,3-butadiene in TMC-1: Implications for bottom-up routes involving 1,3-butadiene