Infrared spectra of complex organic molecules in astronomically relevant ice mixtures

Abstract

Context. Complex organic molecules (COMs) have been largely identified through their characteristic rotational transitions in the gas of interstellar and circumstellar regions. Although these species are formed in the icy mantles that cover dust grains, the most complex species that has been unambiguously identified in the solid-phase to date is methanol (CH3OH). With the upcoming launch of the James Webb Space Telescope (JWST), this situation may change. The higher sensitivity, spectral and spatial resolution of the JWST will allow for the probing of the chemical inventory of ices in star-forming regions. In order to identify features of solid-state molecules in astronomical spectra, laboratory infrared spectra of COMs within astronomically relevant conditions are required. This paper is part of a series of laboratory studies focusing on the infrared spectra of frozen COMs embedded in ice matrices. These reflect the environmental conditions in which COMs are thought to be found. Aims. This work is aimed at characterizing the infrared features of acetone mixed in ice matrices containing H2O, CO2, CO, CH4, and CH3OH for temperatures ranging between 15 K and 160 K. Changes in the band positions and shapes due to variations in the temperature, ice composition, and morphology are reported. This work also points out the IR features that are considered the best promising tracers when searching for interstellar acetone-containing ices. Methods. Acetone-containing ices were grown at 15 K under high-vacuum conditions and infrared (IR) spectra (500–4000 cm−1/20–2.5 μm, 0.5 cm−1 resolution) in transmission mode were recorded using a Fourier transform infrared spectrometer. Spectra of the ices at higher temperatures are acquired during the heating of the sample (at a rate of 25 K h−1) up to 160 K. The changes in the infrared features for varying conditions were analyzed. Results. A large set of IR spectra of acetone-containing ices is presented and made available as a basis for interpreting current and future infrared astronomical spectra. The peak position and full width at half maximum of selected acetone bands have been measured for different ice mixtures and temperatures. The bands that are best suitable for acetone identification in astronomical spectra are: the C=O stretch mode, around 1710.3 cm−1 (5.847 μm), that lies in the 1715–1695 cm−1 (5.83–5.90 μm) range in the mixed ices; the CH3 symmetric deformation, around 1363.4 cm−1 (7.335 μm) that lies in the 1353–1373 cm−1 (7.28–7.39 μm) range in the mixed ices; and the CCC asymmetric stretch, around 1228.4 cm−1 (8.141 μm), that lies in the 1224–1245 cm−1 (8.16–8.03 μm) range in the mixed ices. The CCC asymmetric stretch band also exhibits potential as a remote probe of the ice temperature and composition; this feature is the superposition of two components that respond differently to temperature and the presence of CH3OH. All the spectra are available through the Leiden Ice Database.