Discovery of interstellar NC<sub>4</sub>NH<sup>+</sup>: Dicyanopolyynes are indeed abundant in space
M. Agúndez, C. Cabezas, N. Marcelino, R. Fuentetaja, B. Tercero, P. de Vicente, J. Cernicharo
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 NC 4 NH + . This cation is the protonated form of NC 4 N, 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 NC 4 NH + based on high-level ab initio calculations, which supports the previous identification of HC 5 NH + from the observation of a series of lines with a rotational constant 2 MHz lower than that derived here. The column density of NC 4 NH + in TMC-1 is (1.1 −0.6 +1.4 ) × 10 10 cm −2 , which implies that NC 4 NH + 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 NC 4 N are present in TMC-1 with abundances a few times to one order of magnitude lower than HC 3 N and HC 5 N, 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 C 3 N with HNC arise as the most likely formation pathways to NCCN and NC 4 N in cold dense clouds.