State-to-state chemistry and rotational excitation of CH+ in photon-dominated regions

9 pags., 8 figs.We present a detailed theoretical study of the rotational excitation of CH+ due to reactive and non-reactive collisions involving C+(2P), H2, CH+, H and free electrons. Specifically, the formation of CH+ proceeds through the reaction between C+(2P) and H2(νH2=1,2)⁠, while the collisional (de)excitation and destruction of CH+ is due to collisions with hydrogen atoms and free electrons. State-to-state and initial-state-specific rate coefficients are computed in the kinetic temperature range 10–3000 K for the inelastic, exchange, abstraction and dissociative recombination processes using accurate potential energy surfaces and the best scattering methods. Good agreement, within a factor of 2, is found between the experimental and theoretical thermal rate coefficients, except for the reaction of CH+ with H atoms at kinetic temperatures below 50 K. The full set of collisional and chemical data are then implemented in a radiative transfer model. Our non-LTE calculations confirm that the formation pumping due to vibrationally excited H2 has a substantial effect on the excitation of CH+ in photon-dominated regions. In addition, we are able to reproduce, within error bars, the far-infrared observations of CH+ towards the Orion Bar and the planetary nebula NGC 7027. Our results further suggest that the population of νH2=2 might be significant in the photon-dominated region of NGC 7027.This work has been supported by the Agence Nationale de la Recherche (ANR-HYDRIDES), contract ANR-12-BS05-0011-01 and by the CNRS national programme ‘Physico-Chimie du Milieu Interstellaire’. IFS acknowledges generous financial support from Région Haute-Normandie, the GRR Electronique, Energie et Matériaux, the ‘Fédération de Recherche Energie, Propulsion, Environnement’, the fund Bioengine and the LabEx EMC, via the project PicoLIBS. OR and AZ acknowledge the financial support of Ministerio de Economia e Innovación under grant FIS2014-52172-C2 and the European Research Council under ERC Grant Agreement n. 610256 (NANOCOSMOS). François Lique and Pierre Hily-Blant are acknowledged for useful discussions.Peer reviewe