Abstract
Context. The occurrence of complex organic molecules (COMs) in the gas phase at low temperature in the dense phases of the interstellar medium suggests that a non-thermal desorption mechanism is at work because otherwise, COMs should condense within a short timescale onto dust grains. Vacuum ultraviolet (VUV) photodesorption has been shown to be much less efficient for complex organic molecules, such as methanol, because mostly photoproducts are ejected. The induced photolysis competes with photodesorption for large COMs, which considerably lowers the efficiency to desorb intact molecules. Aims. We pursue an experimental work that has already shown that water molecules, the dominant ice mantle species, can be efficiently sputtered by cosmic rays. We investigate the sputtering efficiency of complex organic molecules that are observed either in the ice mantles of interstellar dense clouds directly by infrared spectroscopy (CH3OH), or that are observed in the gas phase by millimeter telescopes (CH3COOCH3) and that could be released from interstellar grain surfaces. Methods. We irradiated ice films containing complex organic molecules (methanol and methyl acetate) and water with swift heavy ions in the electronic sputtering regime. We monitored the infrared spectra of the film as well as the species released to the gas phase with a mass spectrometer. Results. We demonstrate that when methanol or methyl acetate is embedded in a water-ice mantle exposed to cosmic rays, a large portion is sputtered as an intact molecule, with a sputtering yield close to that of the main water-ice matrix. This must be even more true for the case of more volatile ice matrices, such as those that are embedded in carbon monoxide. Conclusions. Cosmic rays penetrating deep into dense clouds provide an efficient mechanism to desorb complex organic molecules. Compared to the VUV photons, which are induced by the interaction of cosmic rays, a large portion desorb as intact molecules with a proportion corresponding to the time-dependent bulk composition of the ice mantle, the latter evolving with time as a function of fluence due to the radiolysis of the bulk.
Highlights
The observed relatively high abundance of complex organic molecules in dense and pre-stellar phases of the interstellar medium (ISM) is puzzling (e.g. Lefloch et al 2018; Soma et al 2018; Ceccarelli et al 2017; López-Sepulcre et al 2017; Jørgensen et al 2016; Jiménez-Serra et al 2016; Öberg et al 2014; Vasyunina et al 2014; Bacmann et al 2012, and references therein)
Pure CH3OH, thin and thick films Before we investigated potentially more relevant astrophysical methanol/ice abundance mixtures, we performed pure methanol ice-irradiation experiments for which we varied the film thicknesses to determine among other parameters the methanol destruction cross section under fixed ion-beam conditions
We conclude that for complex organic molecules (COMs), whose abundance is generally a small fraction of the main ice-matrix component, a large portion of intact molecules are desorbed by cosmic rays and with a sputtering efficiency that is close to that of the ice matrix
Summary
The observed relatively high abundance of complex organic molecules in dense and pre-stellar phases of the interstellar medium (ISM) is puzzling (e.g. Lefloch et al 2018; Soma et al 2018; Ceccarelli et al 2017; López-Sepulcre et al 2017; Jørgensen et al 2016; Jiménez-Serra et al 2016; Öberg et al 2014; Vasyunina et al 2014; Bacmann et al 2012, and references therein). Lefloch et al 2018; Soma et al 2018; Ceccarelli et al 2017; López-Sepulcre et al 2017; Jørgensen et al 2016; Jiménez-Serra et al 2016; Öberg et al 2014; Vasyunina et al 2014; Bacmann et al 2012, and references therein). If these molecules are formed on the surface of dust grains, an effective desorption mechanism must be invoked for their release in the gas phase. Competition with other channels during formation (e.g. Minissale et al 2016) is of consequence at the surface of the grains in the desorption of the smaller species
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