LABORATORY SPECTRA OF THE CO2 BENDING-MODE FEATURE IN INTERSTELLAR ICE ANALOGUES SUBJECT TO THERMAL PROCESSING

The infrared absorption features of solid carbon dioxide have been detected by space-based observatories in nearly all lines of sight probing the dense interstellar medium (ISM). It has also been shown that the absorption feature of solid CO2 near 658 cm(-1) (15.2 mu m) should be a sensitive indicator of the physical conditions of the ice (e. g., temperature and composition). However, the profile structure of this feature is not well understood, and previous laboratory studies have concentrated on a limited range of temperatures and compositions for comparisons with observed spectra from both the Infrared Space Observatory and the Spitzer Space Telescope. In the laboratory study described here, the infrared spectra of ices bearing H2O, CH3OH, and CO2 have been measured with systematically varying compositions and temperatures that span the range of the values expected in the ISM. The mid-IR spectra (lambda = 2.5-25 mu m) were measured for 47 different ice compositions at temperatures ranging from similar to 5.5 K to evaporation, at 5K intervals.