Evaporative cooling of icy interstellar grains II. Key parameters. (arXiv:2007.04720v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Kalvans_J/0/1/0/all/0/1">Juris Kalvāns</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kalnin_J/0/1/0/all/0/1">Juris Roberts Kalnin</a>
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 > 40K. For grains
with different sizes, CR-induced desorption is most efficient for rather small
grains with a core radius of a ~ 0.02 micron. CR-induced sublimation of CO2 ice
can occur only from small grains if their peak temperature is T > 80K 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 < 30K. 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 >
50K, 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.
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 > 40K. For grains
with different sizes, CR-induced desorption is most efficient for rather small
grains with a core radius of a ~ 0.02 micron. CR-induced sublimation of CO2 ice
can occur only from small grains if their peak temperature is T > 80K 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 < 30K. 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 >
50K, 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.
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