Title:
Surface chemistry in photodissociation regions
Authors:
Esplugues, G. B.; Cazaux, S.; Meijerink, R.; Spaans, M.; Caselli, P.
Affiliation:
AA(Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands esplugues@astro.rug.nl), AB(Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands), AC(Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands), AD(Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands), AE(Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748, Garching, Germany)
Publication:
Astronomy & Astrophysics, Volume 591, id.A52, 18 pp. (A&A Homepage)
Publication Date:
06/2016
Origin:
EDP Sciences
Astronomy Keywords:
astrochemistry, ISM: abundances, photon-dominated region
DOI:
10.1051/0004-6361/201528001
Bibliographic Code:
2016A&A...591A..52E
Abstract
Context. The presence of dust can strongly affect the chemical composition of
the interstellar medium. We model the chemistry in photodissociation regions
(PDRs) using both gas-phase and dust-phase chemical reactions.
Aims: Our aim is to determine the chemical compositions of the interstellar
medium (gas/dust/ice) in regions with distinct (molecular) gas densities that are
exposed to radiation fields with different intensities.
Methods: We have significantly improved the Meijerink PDR code by including
3050 new gas-phase chemical reactions and also by implementing surface
chemistry. In particular, we have included 117 chemical reactions occurring on
grain surfaces covering different processes, such as adsorption, thermal
desorption, chemical desorption, two-body reactions, photo processes, and
cosmic-ray processes on dust grains.
Results: We obtain abundances for different gas and solid species as a function
of visual extinction, depending on the density and radiation field. We also analyse
the rates of the formation of CO2 and H2O ices in different environments. In
addition, we study how chemistry is affected by the presence/absence of ice
mantles (bare dust or icy dust) and the impact of considering different desorption
probabilities.
Conclusions: The type of substrate (bare dust or icy dust) and the probability of
desorption can significantly alter the chemistry occurring on grain surfaces,
leading to differences of several orders of magnitude in the abundances of gas-
phase species, such as CO, H2CO, and CH3OH. The type of substrate, together
with the density and intensity of the radiation field, also determine the threshold
extinction to form ices of CO2 and H2O. We also conclude that H2CO and
CH3OH are mainly released into the gas phase of low, far-ultraviolet illuminated
PDRs through chemical desorption upon two-body surface reactions, rather than
through photodesorption.