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

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 through the 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 40m radiotelescope of Yebes and the IRAM 30m 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 to 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.