State-to-state chemistry and rotational excitation of CH+ in photon-dominated regions
- 11 April 2017
- journal article
- research article
- Published by Oxford University Press (OUP) in Monthly Notices of the Royal Astronomical Society
- Vol. 469 (1), 612-620
- https://doi.org/10.1093/mnras/stx892
Abstract
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 H|$_2(\nu _{\rm H_2} = 1, 2)$|, 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 |$\nu _{\rm H_2} = 2$| might be significant in the photon-dominated region of NGC 7027.Funding Information
- European Research Council (610256)
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