Non-thermal desorption of complex organic molecules

E Dartois,M Chabot,A Domaracka,H Rothard,P Boduch,A Bacmann

doi:10.1051/0004-6361/201936934

Abstract

Aims. Methanol ice is embedded in interstellar ice mantles present in dense molecular clouds. We aim to measure the sputtering efficiencies starting from different ice mantles of varying compositions experimentally, in order to evaluate their potential impact on astrochemical models. The sputtering yields of complex organic molecules is of particular interest, since few mechanisms are efficient enough to induce a significant feedback to the gas phase. Methods. We irradiated ice film mixtures made of methanol and carbon dioxide of varying ratios with swift heavy ions in the electronic sputtering regime. We monitored the evolution of the infrared spectra as well as the species released to the gas phase with a mass spectrometer. Methanol (12C) and isotopically labelled 13C-methanol were used to remove any ambiguity on the measured irradiation products. Results. The sputtering of methanol embedded in carbon dioxide ice is an efficient process leading to the ejection of intact methanol in the gas phase. We establish that when methanol is embedded in a carbon-dioxide-rich mantle exposed to cosmic rays, a significant fraction (0.2–0.3 in this work) is sputtered as intact molecules. The sputtered fraction follows the time-dependent bulk composition of the ice mantle, the latter evolving with time due to the radiolysis-induced evolution of the bulk. If methanol is embedded in a carbon dioxide ice matrix, as the analyses of the spectral shape of the CO2 bending mode observations in some lines of sight suggest, the overall methanol sputtering yield is higher than if embedded in a water ice mantle. The sputtering is increased by a factor close to the dominant ice matrix sputtering yield, which is about six times higher for pure carbon dioxide ice when compared to water ice. These experiments are further constraining the cosmic-ray-induced ice mantle sputtering mechanisms important role in the gas-phase release of complex organic molecules from the interstellar solid phase.

Highlights

Cosmic rays pervade dense clouds and protostellar discs, the resident sites of interstellar ice mantles
We measured the sputtering yield of methanol embedded in a carbon dioxide ice matrix at 10 K irradiated by swift heavy ions in the electronic regime of energy deposition, simulating experimentally interstellar cosmic rays
We conclude that a large fraction of intact molecules are desorbed by cosmic rays with a sputtering yield close to that of the carbon dioxide ice matrix

Summary

Introduction

Cosmic rays pervade dense clouds and protostellar discs, the resident sites of interstellar ice mantles. They are a source of radiochemistry for these solids, in addition to photolysis from induced secondary vacuum ultraviolet (VUV) photons. They are at the origin of a sputtering process releasing ice grain mantle species and products in the gas phase. This must be considered as a desorption mechanism to be properly quantified to examine to what extent this sputtering in the electronic regime influences the astrochemical balance in dense regions of the interstellar medium.

Methods

Results

Discussion

Conclusion