The IRAM-30 m line survey of the Horsehead PDR. III. High abundance of complex (iso-)nitrile molecules in UV-illuminated gas

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