2011A&A...528A.118G

Infrared observations show the presence of icy mantles along the line of sight toward young stellar objects (YSOs), where a temperature gradient is expected and indirectly observed. In this environment, icy mantles are affected by ion and UV irradiation. Laboratory experiments show that molecules are formed after irradiation of icy mixtures. However, most of the experiments done so far have been performed in the temperatures range of 10-20K. To extend previous work we irradiated some icy mixtures, namely H₂O:CO=10:1, H₂O:CH₄=4:1, and H₂O:CO₂=3:1 at two different temperatures (12K and 40 or 60K) to study the effects of temperature on the synthesis of molecules and the decrease in their parent species after ion irradiation. The experiments were performed in a high-vacuum chamber (P<10^–7mbar), where icy samples were irradiated with 30keV He⁺ ions and analyzed by a FTIR spectrophotometer. Infrared spectra of the samples were recorded after various steps of irradiation. We found that the temperature affects the behavior of the volatile species (i.e., CO and CH₄) during irradiation. As a consequence, the production of molecular species is generally more prevalent at 12K than at either 40 or 60K, while the decrease in their parent volatile species is faster at high temperature. We conclude that the behavior of each species depends on the value of its sublimation temperature with respect to the temperature of the sample. If this latter is higher than the sublimation temperature of a given species, then the effects of thermal desorption compete with those due to irradiation.