Dust as interstellar catalyst

II. How chemical desorption impacts the gas

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
e-mail: cazaux@astro.rug.nl
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ LERMA, UMR 8112 du CNRS, Observatoire de Paris et Université de Cergy Pontoise, 5 mail Gay-Lussac, 95031 Cergy-Pontoise Cedex, France
⁴ Max-Planck Institute for Extraterrestrial Physics, Giessenbachstr. 1, 85741 Garching, Germany

Received: 13 August 2015
Accepted: 28 October 2015

Abstract

Context. Interstellar dust particles, which represent 1% of the total mass, are recognized to be very powerful interstellar catalysts in star-forming regions. The presence of dust can have a strong impact on the chemical composition of molecular clouds. While observations show that many species that formed onto dust grains populate the gas phase, the process that transforms solid state into gas phase remains unclear.

Aims. The aim of this paper is to consider the chemical desorption process, i.e. the process that releases solid species into the gas phase, in astrochemical models. These models allow determining the chemical composition of star-forming environments with an accurate treatment of the solid-phase chemistry.

Methods. In paper I we derived a formula based on experimental studies with which we quantified the efficiencies of the chemical desorption process. Here we extend these results to astrophysical conditions.

Results. The simulations of astrophysical environments show that the abundances of gas-phase methanol and H₂O₂ increase by four orders of magnitude, whereas gas-phase H₂CO and HO₂ increase by one order of magnitude when the chemical desorption process is taken into account. The composition of the ices strongly varies when the chemical desorption is considered or neglected.

Conclusions. We show that the chemical desorption process, which directly transforms solid species into gas-phase species, is very efficient for many reactions. Applied to astrophysical environments such as ρ Oph A, we show that the chemical desorption efficiencies derived in this study reproduce the abundances of observed gas-phase methanol, HO₂, and H₂O₂, and that the presence of these molecules in the gas shows the last signs of the evolution of a cloud before the frost.