Abstract
Context. The molecular composition of interstellar ice mantles is defined by gas-grain processes in molecular clouds, with the main components being H2O, CO, and CO2. Methanol (CH3OH) ice is detected towards the denser pre-stellar cores and star-forming regions, where large amounts of CO molecules freeze out and get hydrogenated on top of the icy grains. The thermal heating from nearby protostars can further change the ice structure and composition. Despite the several observations of icy features carried out towards molecular clouds and along the line of site of protostars, it is not yet clear if interstellar ices are mixed or if they have a layered structure. Aims. We aim to examine the effect of mixed and layered ice growth in dust grain mantle analogues, with specific focus on the position and shape of methanol infrared bands, so dedicated future observations could shed light on the structure of interstellar ices in different environments. Methods. Mixed and layered ice samples were deposited on a cold substrate kept at a temperature of 10 K using a closed-cycle cryostat placed in a vacuum chamber. The spectroscopic features were analysed by Fourier transform infrared spectroscopy. Different proportions of the most abundant four molecular species in ice mantles, namely H2O, CO, CO2, and CH3OH, were investigated, with a special attention placed on the analysis of the CH3OH bands. Results. We measure changes in the position and shape of the CH and CO stretching bands of CH3OH depending on the mixed or layered nature of the ice sample. Spectroscopic features of methanol are also found to change due to heating. Conclusions. A layered ice structure best reproduces the CH3OH band position recently observed towards a pre-stellar core and in star-forming regions. Based on our experimental results, we conclude that observations of CH3OH ice features in space can provide information about the structure of interstellar ices, and we expect the James Webb Space Telescope to put stringent constraints on the layered or mixed nature of ices in different interstellar environments, from molecular clouds to pre-stellar cores to protostars and protoplanetary discs.
Highlights
The interpretation of astronomical observations relies on the contribution provided by laboratory data and theoretical modelling
The spectroscopic features of interstellar ice analogues recorded in the laboratory at infrared wavelengths provide important information to shed light on the composition and physical state of the icy mantles of interstellar dust grains
We focus our attention on solid CH3OH features, as methanol has been detected in the gas phase of starless and pre-stellar cores (e.g. Tafalla et al 2004, 2006; Bizzocchi et al 2014; Chacón-Tanarro et al 2019; Scibelli & Shirley 2020) and its distribution was recently modelled by Vasyunin et al (2017)
Summary
The interpretation of astronomical observations relies on the contribution provided by laboratory data and theoretical modelling. Öberg et al (2011) showed that the most realistic picture of ice mantles covering the dust grains is a layered model, in which the chemical composition of the ice changes as the ice accretes on the dust surface following the changes in the gas-phase chemical composition. This produces changes of the observed ice. The raw spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc. The bombardment of icy mantles by energetic particles such as cosmic rays could already affect the ice structure (e.g. Leger et al 1985; Dartois et al 2020; Ivlev et al 2015) in the pre-stellar phase, while protostellar activity (thermal heating and shocks) may provide other mechanisms to mix a previously layered icy mantle onto dust grains
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