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

Abstract

We present a detailed theoretical study of the rotational excitation of CH + due to reactive and nonreactive collisions involving C + ( 2 P), H 2 , CH + , H and free electrons. Specifically, the formation of CH + proceeds through the reaction between C + ( 2 P) and H 2 (ν 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 H 2 has a substantial effect on the excitation of CH + in photon-dominated regions. In addition, we are able to reproduce, ⋆ alexandre.faure@univ-grenoble-alpes.fr. Europe PMC Funders GroupAuthor Manuscript Mon Not R Astron Soc. Author manuscript; available in PMC 2017 July 06. Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts within error bars, the far-infrared observations of CH + toward the Orion Bar and the planetary nebula NGC 7027. Our results further suggest that the population of ν H 2 = 2 might be significant in the photon-dominated region of NGC 7027.

Rise of the intensity of interstellar CH+ in the spectrum of AE Aur

“…The CH 4,300 Å line shows only a very limited variability if any since 2016 (Figure ). The paths of CH and CH + species formation are different (Faure et al ; Gerin et al ) and thus one phenomenon is not necessarily followed by another one. This seems to be the case, depicted in Figure and Figure .…”

Formation of interstellar SH+from vibrationally excited H2: Quantum study of S++ H2⇄ SH++ H reaction and inelastic collision

“…Indeed, laboratory experiments showed that for CH + , reaction (1) becomes exothermic and fast if H 2 (v ≥ 1) (Hierl et al 1997). State-to-state rate constants for this reaction have have been calculated from quantum calculations (Zanchet et al 2013b) and PDR and excitation models using these data predict CH + abundances and rotational line intensities close to the observed ones (e.g., Agúndez et al 2010;Godard & Cernicharo 2012;Nagy et al 2013;Faure et al 2017;Joblin et al 2018). In addition, mapping observations of the Orion molecular cloud have revealed spatial correlation between the [C ii] 158 µm emission (from ionized carbon C + ), the IR H 2 (≥1) emission, and the CH + ( j = 1 − 0) rotational emission (Goicoechea et al 2019).…”

Dissociative Recombination of CH+ Molecular Ion Induced by Very Low Energy Electrons

“…CH + is easily destroyed by reactions with electrons and hydrogen atoms and also by reactions with H 2 molecules. The inelastic collisions with these dominant species, are no faster than the destructive reactive processes [16], thus, inelastic collisions can never fully equilibrate the rotational population of CH + , leading to strong deviations of the level populations from local thermodynamic equilibrium.…”

“…where N + i and v + i denote respectively, the initial rotational and vibrational levels of the ground-state of the molecular ion. The present work is devoted to give the theoretical and computational details on the rate coefficients reported in Reference [16], by presenting the methodology used in obtaining the cross sections of the dissociative recombination of CH + , by taking into account in full detail the rotational effects.…”