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

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.