Context : 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 \pm 4.4 ) \times 10 ^ { 10 } cm ^ { -2 } in TMC-1 and ( 3.9 \pm 1.8 ) \times 10 ^ { 10 } cm ^ { -2 } in L483 , which translate to fairly low fractional abundances relative to H _ { 2 } , in the range ( 1- 10 ) \times 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 \sim 10 ^ { -4 } , which implies that the abundance of cyanogen in dark clouds could be as high as ( 1- 10 ) \times 10 ^ { -8 } relative to H _ { 2 } , i.e. , comparable to that of other abundant nitriles such as HCN , HNC , and HC _ { 3 } N .

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