Isocyanogen formation in the cold interstellar medium

¹ IRAP, Université de Toulouse, CNRS, UPS, CNES, 31400 Toulouse, France
e-mail: cvastel@irap.omp.eu
² Institut des Sciences Moléculaires (ISM), CNRS, Université de Bordeaux, 351 cours de la Libération, 33400 Talence, France
³ Laboratoire d’astrophysique de Bordeaux, Université de Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁴ CNRS, IPAG, Université Grenoble Alpes, 38000 Grenoble, France

Received: 3 January 2019
Accepted: 23 March 2019

Abstract

Context. Cyanogen (NCCN) is the simplest member of the dicyanopolyynes group, and has been proposed as a major source of the CN radical observed in cometary atmospheres. Although not detected through its rotational spectrum in the cold interstellar medium, this very stable species is supposed to be very abundant.

Aims. The chemistry of cyanogen in the cold interstellar medium can be investigated through its metastable isomer, CNCN (isocyanogen). Its formation may provide a clue on the widely abundant CN radical observed in cometary atmospheres.

Methods. We performed an unbiased spectral survey of the L1544 proto-typical prestellar core, using the IRAM-30 m and have analysed, for this paper, the nitrogen chemistry that leads to the formation of isocyanogen. We report on the first detection of CNCN, NCCNH⁺, C₃N, CH₃CN, C₂H₃CN, and H₂CN in L1544. We built a detailed chemical network for NCCN/CNCN/HC₂N₂⁺ involving all the nitrogen bearing species detected (CN, HCN, HNC, C₃N, CNCN, CH₃CN, CH₂CN, HCCNC, HC₃N, HNC₃, H₂CN, C₂H₃CN, HCNH⁺, HC₃NH+) and the upper limits on C₄N, C₂N. The main cyanogen production pathways considered in the network are the CN + HNC and N + C₃N reactions.

Results. The comparison between the observations of the nitrogen bearing species and the predictions from the chemical modelling shows a very good agreement, taking into account the new chemical network. The expected cyanogen abundance is greater than the isocyanogen abundance by a factor of 100. Although cyanogen cannot be detected through its rotational spectrum, the chemical modelling predicts that it should be abundant in the gas phase and hence might be traced through the detection of isocyanogen. It is however expected to have a very low abundance on the grain surfaces compared to HCN.