Probing non-polar interstellar molecules through their protonated form: Detection of protonated cyanogen (NCCNH⁺)^⋆

¹ Instituto de Ciencia de Materiales de Madrid, CSIC, C/ Sor Juana Inés de la Cruz 3, 28049 Cantoblanco, Spain
e-mail: marcelino.agundez@icmm.csic.es
² Centro Nacional de Tecnologías Radioastronómicas y Aplicaciones Geoespaciales (CNTRAG), Observatorio de Yebes (IGN), Spain
³ INAF, Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
⁴ LERMA, Observatoire de Paris, PSL Research University, CNRS, UMR 8112, place Janssen, 92190 Meudon Cedex, France
⁵ Observatorio Astronómico Nacional (OAN), Calle Alfonso XII, No 3, 28014 Madrid, Spain
⁶ LERMA, Observatoire de Paris, École Normale Supérieure, PSL Research University, CNRS, UMR 8112, 75014 Paris, France
⁷ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 St. Martin d’Héres, France

Received: 1 June 2015
Accepted: 23 June 2015

Abstract

Cyanogen (NCCN) is the simplest member of the series of dicyanopolyynes. It has been hypothesized that this family of molecules can be important constituents of interstellar and circumstellar media, although the lack of a permanent electric dipole moment prevents its detection through radioastronomical techniques. Here we present the first solid evidence of the presence of cyanogen in interstellar clouds by detection of its protonated form toward the cold dark clouds TMC-1 and L483. Protonated cyanogen (NCCNH⁺) has been identified through the J = 5−4 and J = 10−9 rotational transitions using the 40 m radiotelescope of Yebes and the IRAM 30 m telescope. We derive beam-averaged column densities for NCCNH⁺ of (8.6 ± 4.4) × 10¹⁰ cm^-2 in TMC-1 and (3.9 ± 1.8) × 10¹⁰ cm^-2 in L483, which translate into fairly low fractional abundances relative to H₂, in the range (1–10) × 10^-12. The chemistry of protonated molecules in dark clouds is discussed, and it is found that, in general terms, the abundance ratio between the protonated and non-protonated forms of a molecule increases with increasing proton affinity. Our chemical model predicts an abundance ratio NCCNH⁺/NCCN of ~10^-4, which implies that the abundance of cyanogen in dark clouds could be as high as (1–10) × 10^-8 relative to H₂, i.e., comparable to that of other abundant nitriles such as HCN, HNC, and HC₃N.