CO2 infrared spectra on silicate dust grain analogs: Implications for JWST observations

TL;DR

This study investigates CO2 infrared spectra on silicate dust analogs, revealing spectral features and behaviors relevant for interpreting JWST observations of molecular clouds.

Contribution

It provides new laboratory IR spectra of CO2 on silicate dust analogs, linking laboratory results to JWST observations and identifying spectral markers for CO2 diffusion.

Findings

CO2 IR profile on silicates differs from metal surfaces, matching observations.

Presence of CO and CH4 facilitates CO2 crystallization at lower temperatures.

Split in 13CO2 IR feature indicates onset of diffusion, matching JWST data.

Abstract

Carbon dioxide is one of the three most abundant species within the ice mantles around dust grains inside molecular clouds. Since a substantial amount of interstellar grains is made of siliceous materials, we have studied the infrared profile of CO2 deposited on top of a bare and ice-coated amorphous silicate (MgFeSiO4) film using reflection absorption infrared spectroscopy (RAIRS). In contrast to a metal surface, the CO2 IR profile shows a relaxation of the metal surface selection rule in the presence of the bare MgFeSiO4 dust grain analog, which brings the IR profile closer to the observational spectra while maintaining the sensitivity of RAIRS. Experiments with the underlying CO and CH4 ices show that their presence facilitates structural changes toward crystalline ice for the deposited CO2 at much lower temperatures than on the polar ice layers. Warming-up experiments of CO2 showed that it tends to stay on the silicate surface for much longer than on the gold surface without the silicate layer. We noticed for the first time a split in the 13CO2 IR feature on the pure or ice-covered silicate grain as a marker for the onset of diffusion. The laboratory 13CO2 profile then closely resembles recent JWST observations of this feature around young and embedded protostars, suggesting that it can be linked to the observed feature.