2013A&A...557A.101G

Complex (iso-)nitrile molecules, such as CH₃CN and HC₃N, are relatively easily detected in our Galaxy and in other galaxies. We aim at constraining their chemistry through observations of two positions in the Horsehead edge: the photo-dissociation region (PDR) and the dense, cold, and UV-shielded core just behind it. We systematically searched for lines of CH₃CN, HC₃N, C₃N, and some of their isomers in our sensitive unbiased line survey at 3, 2, and 1#1. We stacked the lines of C₃N to improve the detectability of this species. We derived column densities and abundances through Bayesian analysis using a large velocity gradient radiative transfer model. We report the first clear detection of CH₃NC at millimeter wavelength. We detected 17 lines of CH₃CN at the PDR and 6 at the dense core position, and we resolved its hyperfine structure for 3 lines. We detected 4 lines of HC₃N, and C₃N is clearly detected at the PDR position. We computed new electron collisional rate coefficients for CH₃CN, and we found that including electron excitation reduces the derived column density by 40% at the PDR position, where the electron density is 1-5#1. While CH₃CN is 30 times more abundant in the PDR (2.5x10^–10) than in the dense core (8x10^–12), HC₃N has similar abundance at both positions (8x10^–12). The isomeric ratio CH₃NC/CH₃CN is 0.15±0.02. The significant amount of complex (iso-)nitrile molecule in the UV illuminated gas is puzzling as the photodissociation is expected to be efficient. This is all the more surprising in the case of CH₃CN, which is 30 times more abundant in the PDR than in the dense core. In this case, pure gas phase chemistry cannot reproduce the amount of CH₃CN observed in the UV-illuminated gas. We propose that CH₃CN gas phase abundance is enhanced when ice mantles of grains are destroyed through photo-desorption or thermal-evaporation in PDRs, and through sputtering in shocks.