Infrared spectra of complex organic molecules in astronomically relevant ice matrices

I. Acetaldehyde, ethanol, and dimethyl ether

¹ Raymond and Beverly Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: jeroentvs@strw.leidenuniv.nl; linnartz@strw.leidenuniv.nl
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Universities Space Research Association, Stratospheric Observatory for Infrared Astronomy, NASA Ames Research Center, Moffett Field, CA 94035, USA
⁴ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Dr., Honolulu, HI 98622, USA
⁵ Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbackstr. 1, 85748 Garching, Germany

Received: 26 September 2017
Accepted: 27 November 2017

Abstract

Context. The number of identified complex organic molecules (COMs) in inter- and circumstellar gas-phase environments is steadily increasing. Recent laboratory studies show that many such species form on icy dust grains. At present only smaller molecular species have been directly identified in space in the solid state. Accurate spectroscopic laboratory data of frozen COMs, embedded in ice matrices containing ingredients related to their formation scheme, are still largely lacking.

Aim. This work provides infrared reference spectra of acetaldehyde (CH₃CHO), ethanol (CH₃CH₂OH), and dimethyl ether (CH₃OCH₃) recorded in a variety of ice environments and for astronomically relevant temperatures, as needed to guide or interpret astronomical observations, specifically for upcoming James Webb Space Telescope observations.

Methods. Fourier transform transmission spectroscopy (500–4000 cm⁻¹/20–2.5 μm, 1.0 cm⁻¹ resolution) was used to investigate solid acetaldehyde, ethanol and dimethyl ether, pure or mixed with water, CO, methanol, or CO:methanol. These species were deposited on a cryogenically cooled infrared transmissive window at 15 K. A heating ramp was applied, during which IR spectra were recorded until all ice constituents were thermally desorbed.

Results. We present a large number of reference spectra that can be compared with astronomical data. Accurate band positions and band widths are provided for the studied ice mixtures and temperatures. Special efforts have been put into those bands of each molecule that are best suited for identification. For acetaldehyde the 7.427 and 5.803 μm bands are recommended, for ethanol the 11.36 and 7.240 μm bands are good candidates, and for dimethyl ether bands at 9.141 and 8.011 μm can be used. All spectra are publicly available in the Leiden Database for Ice.