Evaporative cooling of icy interstellar grains

Abstract

Context. Evaporative (sublimation) cooling of icy interstellar grains occurs when the grains have been suddenly heated by a cosmic-ray (CR) particle or other process. It results in thermal desorption of icy species, affecting the chemical composition of interstellar clouds. Aims. We investigate details on sublimation cooling, obtaining necessary knowledge before this process is considered in astrochemical models. Methods. We employed a numerical code that describes the sublimation of molecules from an icy grain, layer by layer, also considering a limited diffusion of bulk-ice molecules toward the surface before they sublimate. We studied a grain, suddenly heated to peak temperature T, which cools via sublimation and radiation. Results. A number of questions were answered. The choice of grain heat capacity C has a limited effect on the number of sublimated molecules N, if the grain temperature T > 40 K. For grains with different sizes, CR-induced desorption is most efficient for rather small grains with a core radius of a ≈ 0.02 μm. CR-induced sublimation of CO2 ice can occur only from small grains if their peak temperature is T > 80 K and there is a lack of other volatiles. The presence of H2 molecules on grain surface hastens their cooling and thus significantly reduces N for other sublimated molecules for T ≤ 30 K. Finally, if there is no diffusion and subsequent sublimation of bulk-ice molecules (i.e., sublimation occurs only from the surface layer), sublimation yields do not exceed 1–2 monolayers and, if T > 50 K, N does not increase with increasing T. Conclusions. Important details regarding the sublimation cooling of icy interstellar grains were clarified, which will enable a proper consideration of this process in astrochemical modeling.