Discovery of Interstellar Isocyanogen (CNCN): Further Evidence that Dicyanopolyynes are Abundant in Space

Abstract

It is thought that dicyanopolyynes could be potentially abundant interstellar molecules, although their lack of dipole moment makes it impossible to detect them through radioastronomical techniques. Recently, the simplest member of this chemical family, cyanogen (NCCN), was indirectly probed for the first time in interstellar space through the detection of its protonated form toward the dense clouds L483 and TMC-1. Here we present a second firm evidence of the presence of NCCN in interstellar space, namely the detection of the metastable and polar isomer isocyanogen (CNCN). This species has been identified in L483 and tentatively in TMC-1 by observing various rotational transitions in the λ 3 mm band with the IRAM 30m telescope. We derive beam-averaged column densities for CNCN of 1.6 × 10¹² cm^-2 in L483 and 9 × 10¹¹ cm^-2 in TMC-1, which imply fractional abundances relative to H₂ in the range (5 - 9) × 10^-11. While the presence of NCCN in interstellar clouds seems out of doubt owing to the detection of NCCNH⁺ and CNCN, putting tight constraints on its abundance is still hampered by the poor knowledge of the chemistry that links NCCN with NCCNH⁺ and especially with CNCN. We estimate that NCCN could be fairly abundant, in the range 10^-9-10^-7 relative to H₂, as other abundant nitriles like HCN and HC₃N.

Keywords: ISM: clouds; ISM: molecules; astrochemistry; line: identification; radio lines: ISM.

Figure 1

Lines of CNCN observed toward L483 with the IRAM 30m telescope. Lines are detected at 8-14σ confidence levels. The J = 11 − 10 line at 113.8 GHz is not detected due to the more limited sensitivity of the data at these frequencies.

Figure 2

IRAM 30m spectrum obtained toward TMC-1 by stacking four lines, the J = 8 − 7 through J = 11 − 10, of CNCN. The line is detected at a 5-6σ confidence level.

Figure 3

Main formation (neutral in green and ionic in blue) and destruction (in red) pathways for NCCN and CNCN. The low-temperature kinetics of these reactions is very poorly constrained.