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)e10 cm-2 in
TMC-1 and (3.9+/-1.8)e10 cm-2 in L483, which translate to fairly low fractional
abundances relative to H2, in the range (1-10)e-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 1e-4, which implies that the abundance of
cyanogen in dark clouds could be as high as (1-10)e-8 relative to H2, i.e.,
comparable to that of other abundant nitriles such as HCN, HNC, and HC3N.
