Volume 633, January 2020
|Number of page(s)||11|
|Section||Interstellar and circumstellar matter|
|Published online||17 January 2020|
Evaporative cooling of icy interstellar grains
I. Basic characterization
Engineering Research Institute “Ventspils International Radio Astronomy Center” of Ventspils University of Applied Sciences,
Accepted: 5 December 2019
Context. While radiative cooling of interstellar grains is a well-known process, little detail is known about the cooling of grains with an icy mantle that contains volatile adsorbed molecules.
Aims. We explore basic details for the cooling process of an icy grain with properties relevant to dark interstellar clouds.
Methods. Grain cooling was described with the help of a numerical code considering a grain with an icy mantle that is structured in monolayers and containing several volatile species in proportions consistent with interstellar ice. Evaporation was treated as first-order decay. Diffusion and subsequent thermal desorption of bulk-ice species was included. Temperature decrease from initial temperatures of 100, 90, 80, 70, 60, 50, 40, 30, and 20 K was studied, and we also followed the composition of ice and evaporated matter.
Results. We find that grain cooling occurs by partially successive and partially overlapping evaporation of different species. The most volatile molecules (such as N2) first evaporate at the greatest rate and are most rapidly depleted from the outer ice monolayers. The most important coolant is CO, but evaporation of more refractory species, such as CH4 and even CO2, is possible when the former volatiles are not available. Cooling of high-temperature grains takes longer because volatile molecules are depleted faster and the grain has to switch to slow radiative cooling at a higher temperature. For grain temperatures above 40 K, most of the thermal energy is carried away by evaporation. Evaporation of the nonpolar volatile species induces a complete change of the ice surface, as the refractory polar molecules (H2O) are left behind.
Conclusions. The effectiveness of thermal desorption from heated icy grains (e.g., the yield of cosmic-ray-induced desorption) is primarily controlled by the thermal energy content of the grain and the number and availability of volatile molecules.
Key words: molecular processes / dust, extinction / astrochemistry
© ESO 2020
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.