2006A&A...451..723V

The spectra of pure, mixed and layered CO and CO₂ ices have been studied systematically under laboratory conditions using Transmission-Absorption Fourier Transform infrared spectroscopy. This work provides improved resolution spectra (0.5cm^–1) of the CO₂ bending and asymmetric stretching mode, as well as the CO stretching mode, extending the existing Leiden database (http://www.strw.leidenuniv.nl/~lab/databases/ ) of laboratory spectra to match the spectral resolution reached by modern telescopes and to support the interpretation of the most recent data from the Spitzer Space Telescope. It is shown that mixed and layered CO and CO₂ ices exhibit very different spectral characteristics, which depend critically on thermal annealing and can be used to distinguish between mixed, layered and thermally annealed CO-CO₂ ices. CO only affects the CO₂ bending mode spectra in mixed ices below 50K under the current experimental conditions, where it exhibits a single asymmetric band profile in intimate mixtures. In all other ice morphologies the CO₂ bending mode shows a double peaked profile, similar to that observed for pure solid CO₂. Conversely, CO₂ induces a blue-shift in the peak-position of the CO stretching vibration, to a maximum of 2142cm^–1 in mixed ices, and 2140-2146cm^–1 in layered ices. As such, the CO₂ bending mode puts clear constraints on the ice morphology below 50K, whereas beyond this temperature the CO₂ stretching vibration can distinguish between initially mixed and layered ices. This is illustrated for the low-mass young stellar object HH46, where the laboratory spectra are used to analyse the observed CO and CO₂ band profiles and try to constrain the formation scenarios of CO₂.