Infrared spectra of complex organic molecules in astronomically relevant ice mixtures

Context. The increasing sensitivity and resolution of ground-based telescopes have enabled the detection of gas-phase complex organic molecules (COMs) across a variety of environments. Many of the detected species are expected to form on the icy surface of interstellar grains and transfer later into the gas phase. Therefore, icy material is regarded as a primordial source of complex molecules in the interstellar medium. Upcoming James Webb Space Telescope (JWST) observations of interstellar ices in star-forming regions will reveal infrared (IR) features of frozen molecules with unprecedented resolution and sensitivity. To identify COM features in the JWST data, laboratory IR spectra of ices for conditions that simulate interstellar environments are needed. Aims. This work provides laboratory mid-IR spectra of methyl cyanide (CH3CN, also known as acetonitrile) ice in its pure form and mixed with known interstellar molecules at cryogenic temperatures. The spectroscopic data presented in this work will support the interpretation of JWST ice observations and are made available to the community through the Leiden Ice Database for Astrochemistry (LIDA). Methods. Fourier transform IR spectroscopy is used to record the mid-IR spectra (500–4000 cm−1/20–2.5 µm, with a resolution of 1 cm−1 ) of methyl cyanide (acetonitrile, CH3 CN) mixed with H2O, CO, CO2, CH4, NH3, H2O:CO2, and H2O:CH4:NH3, at temperatures ranging from 15 to 150 K. The refractive index (at 632.8 nm) of pure amorphous CH3CN ice at 15 K and the band strength of selected IR transitions are also measured. Results. We present a variety of reference mid-IR spectra of frozen CH3CN that can be compared to astronomical ice observations. The peak position and full width at half maximum (FWHM) of six absorption bands of frozen methyl cyanide in its pure form and mixed ices, at temperatures between 15–150 K, are characterized. These bands are the following: the CH3 symmetric stretching at 2940.9 cm−1 (3.400 µm), the CN stretching at 2252.2 cm−1 (4.440 µm), a peak resulting from a combination of different vibrational modes at 1448.3 cm−1 (6.905 µm), the CH3 antisymmetric deformation at 1410 cm−1 (7.092 µm), the CH3 symmetric deformation at 1374.5 cm−1 (7.275 um), and the CH3 rock vibration at 1041.6 cm−1 (9.600 um). Additionally, the apparent band strength of these vibrational modes in mixed ices is derived. The laboratory spectra of CH3CN are compared to observations of interstellar ices toward W33A and three low-mass Young Stellar Objects (YSO). Since an unambiguous identification of solid methyl cyanide toward these objects is not possible, upper limits for the CH3CN column density are determined as ≤2.4 × 1017 molecules cm−2 for W33A and 5.2 × 1016, 1.9 × 1017, and 3.8 × 1016 molecules cm−2 for EC92, IRAS 03235, and L1455 IRS3, respectively. With respect to solid H2O, these values correspond to relative abundances of 1.9, 3.1, 1.3, and 4.1%, for W33A, EC92, IRAS 03235, and L1455 IRS3, respectively