Infrared spectra of complex organic molecules in astronomically relevant ice matrices. III. Methyl formate and its tentative solid-state detection

Abstract

Context. Infrared spectroscopy of star and planet forming regions is at the dawn of a new age with the upcoming James Webb Space Telescope (JWST). Its high resolution and unprecedented sensitivity allows us to probe the chemical complexity of planet forming regions, such as dense clouds, embedded protostars, and protoplanetary disks, both in the solid state and gas phase. In support of these observations, laboratory spectra are required to identify complex organic molecules in the ices that cover the dust grains in these regions.Aims. This study aims to provide the necessary reference spectra to firmly detect methyl formate (HCOOCH 3 ) in the different evolutionary stages of star and planet forming regions. Methyl formate is mixed in astronomically relevant matrices, and the peak positions, full width at half maximum, and relative band intensities are characterized for different temperatures to provide an analytical tool for astronomers. Methods. Methyl formate is deposited at 15 Kelvin on a cryogenically cooled infrared transmissive window under high-vacuum conditions. Specifically, methyl formate is deposited pure and mixed with CO, H 2 CO, CH 3 OH, H 2 O, and CO:H 2 CO:CH 3 OH combined. The sample is linearly heated until all solid-state constituents have desorbed. Throughout the experiment infrared spectra are acquired with a Fourier Transform Infrared spectrometer in the range from 4000-500 cm −1 (2.5-20 µm) at a spectral resolution of 0.5 cm −1 . Results. We present the characterization of five solid-state methyl formate vibrational modes in pure and astronomically relevant ice matrices. The five selected vibrational modes, namely the C=O stretch (5.804 µm), the C−O stretch (8.256 µm), CH 3 rocking (8.582 µm), O−CH 3 stretching (10.98 µm), and OCO deformation (13.02 µm), are best suited for a JWST identification of methyl formate. For each of these vibrational modes, and each of the mixtures the temperature versus spectra heatmaps, peak position versus full width at half maximum and relative band intensities are given. All spectra are publicly available on the Leiden Ice Database. Additionally, the acquired reference spectra of methyl formate are compared with archival Spitzer observations of HH 46. A tentative detection of methyl formate provides an upper limit to the column density of 1.7 × 10 17 cm −2 , corresponding to an upper limit relative to water of ≤ 2.2% and ≤ 40% with respect to methanol.

LIDA: The Leiden Ice Database for Astrochemistry

“…Infrared (IR) spectroscopy is a diagnostic tool used to characterize chemical structures of molecules and distinguish their functional groups (e.g., Coblentz 1905;Balkanski 1989). For this reason, a number of laboratories around the world have been focusing on providing laboratory-based IR data of interstellar ice analogs for a range of different ice compositions and temperatures (e.g., Hagen et al 1979;Strazzulla et al 1984;Schmitt et al 1989;Grim et al 1989;Hudgins et al 1993;Boudin et al 1998;Palumbo et al 1998;Schutte 1999;Muñoz Caro et al 2002;Öberg et al 2009;Pilling et al 2010;Vinogradoff et al 2015;Terwisscha van Scheltinga et al 2018;Urso et al 2020;Terwisscha van Scheltinga et al 2021;Rachid et al 2020Rachid et al , 2021Potapov et al 2021). IR spectra directly represent the molecular geometry of a molecule and as such can act as a molecular fingerprint.…”