Gas-grain interaction in the interstellar medium: thermal accommodation and trapping

A numerical model is developed for calculating thermal accommodation coefficients α/sub T/ and trapping fractions f/sub t/ for gases incident upon solid surfaces. The numerical method is especially designed for astrophysical applications in that it treats economically and with moderate accuracy ( +- 20%) the dependences of α/sub T/ and f/sub t/ on finite and different surface and gas temperatures for a large number of gas-surface combinations. Graphs are presented of the dependence of α/sub T/ and f/sub t/ on interstellar gas temperatures in the range of 10 K< or =T/sub g/< or =104 K and grain temperatures in the range of 10 K< or =T/sub s/< or =103 K, assuming the current estimates of the gas-surface physical parameters such as the composition and the Debye temperature of the grain material, the repulsive range of the surface potential, and the gas-grain adsorption energy. The dependences of α/sub T/ and f/sub t/ on the physical parameters are presented graphically, so that α/sub T/ and f/sub t/ can readily be calculated as knowledge of these parameters improves