Effects of CO2 on H2O band profiles and band strengths in mixed H2O:CO2 ices

Context. H2O is the most abundant component of astrophysical ices. In most lines of sight it is not possible to fit both the H2O 3 mu m stretching, the 6 mu m bending intensities with a single pure H2O spectrum. Recent Spitzer observations have revealed CO2 ice in high abundances and it has been suggested that CO2 mixed into H2O ice can affect the positions, shapes and relative strengths of the 3 mu m and 6 mu m bands. Aims. We investigate whether the discrepancy in intensity between H2O bands in interstellar clouds and star forming regions can be explained by CO2 mixed into the observed H2O ice affecting the bands differently. Methods. Laboratory infrared transmission spectroscopy is used to record spectra of H2O:CO2 ice mixtures at astrophysically relevant temperatures and composition ratios. Results. The H2O peak profiles and band strengths are significantly different in H2O:CO2 ice mixtures compared to pure H2O ice. The ratio between the strengths of the 3 mu m and 6 mu m bands drops linearly with CO2 concentration such that it is 50% lower in a 1:1 mixture compared to pure H2O ice. In all H2O: CO2 mixtures, a strong free-OH stretching band appears around 2.73 mu m, which can be used to put an upper limit on the CO2 concentration in the H2O ice. The H2O bending mode profile also changes drastically with CO2 concentration; the broad pure H2O band gives way to two narrow bands as the CO2 concentration is increased. This makes it crucial to constrain the environment of H2O ice to enable correct assignments of other species contributing to the interstellar 6 mu m absorption band. The amount of CO2 present in the H2O ice of B5:IRS1 is estimated by simultaneously comparing the H2O stretching and bending regions and the CO2 bending mode to laboratory spectra of H2O, CO2, H2O:CO2 and HCOOH.