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. 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 reference spectra to firmly detect icy methyl formate in the different stages of star and planet forming regions. Methyl formate is mixed in astronomically relevant matrices, and the peak positions, FWHMs, and relative band intensities are characterized for different temperatures to provide an analytical tool for astronomers. Methods. Methyl formate is deposited at 15 K 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. Throughout the experiment infrared spectra are acquired with a FTIR spectrometer in the range from 4000-500 cm$^{-1}$ (2.5-20 $\mu$m) 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, C$-$O stretch, CH$_3$ rocking, O$-$CH$_3$ stretching, and OCO deformation, are best suited for a JWST identification of methyl formate. For each of these vibrational modes, and each of the mixtures the TvS heatmaps, peak position versus FWHM, and relative band intensities are given. Additionally, the acquired reference spectra of methyl formate are compared with Spitzer observations of HH 46. A tentative detection of methyl formate provides an upper limit to the column density of $1.7\times10^{17}$ cm$^{-2}$, corresponding to an upper limit relative to water of $\leq 2.2\%$ and $\leq 40\%$ with respect to methanol.