Infrared Spectra Of Complex Organic Molecules In Astronomically Relevant Ice Mixtures. V. Methyl cyanide (acetonitrile)

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 COMs in the ISM. Upcoming JWST observations of interstellar ices in star-forming regions will reveal IR features of frozen molecules with unprecedented resolution and sensitivity. To identify COM features in the JWST data, lab IR spectra of ices for conditions that simulate interstellar environments are needed.

This work presents FTIR spectra (500-4000 cm−1/20-2.5μm, with a resolution of 1 cm−1) of methyl cyanide (CH3CN, aka acetonitrile) mixed with H2O, CO, CO2, CH4, and NH3, at temperatures from 15-150 K. The refractive index of pure amorphous CH3CN ice at 15K and the band strength, peak position, and FWHM of selected IR bands are also measured.

These bands are: the CH3 sym stretching at 2940.9 cm−1, the CN stretching at 2252.2 cm−1, a combination of modes at 1448.3 cm−1 , the CH3 antisym def. at 1410 cm−1 , the CH3 sym def. at 1374.5 cm−1, and the CH3 rock at 1041.6 cm−1. The lab spectra of CH3CN are compared to observations of ices toward W33A and three low-mass YSOs.

Since an unambiguous identification of CH3CN 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. W.r.t 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.

Marina G. Rachid, Will Rocha, Harold Linnartz

Comments: 26 pages, 11 figures, accepted for publication in A&A
Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2207.12502 [astro-ph.GA] (or arXiv:2207.12502v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2207.12502
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Submission history
From: Marina Rachid
[v1] Mon, 25 Jul 2022 20:06:32 UTC (917 KB)
https://arxiv.org/abs/2207.12502
Astrobiology, Astrochemistry,