Evaporative cooling of icy interstellar grains

Abstract

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 N 2 ) 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 CH 4 and even CO 2 , 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 (H 2 O) 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.

Systematic investigation of CO2 : NH3 ice mixtures using mid-IR and VUV spectroscopy – part 2: electron irradiation and thermal processing

“…This corresponds to the practices within the literature which use more refractory molecules as 'plugs' to prevent the desorption of more volatile molecules. 21,22 We also observed that less conversion of ammonium carbamate to carbamic acid occurred in the e-irradiated CO 2 : NH 3 mixtures upon thermal processing between 150-200 K. For example, the non-irradiated CO 2 : NH 3 2 : 1, 1 : 1, 1 : 3 & 1 : 10 mixtures showed an increase in the C]O and C-O stretch in Fig. 5 of RJ20, whereas, very little change was observed in Fig.…”

“…An overall comparison of the ratios from the e-irradiated CO 2 : NH 3 mixtures with the non-irradiated mixtures of RJ20 shows that the temperatures at which segregation of the The desorption temperatures of CO 2 and NH 3 were generally higher when subjected to electron irradiation compared to the desorption temperatures of CO 2 and NH 3 from the nonirradiated study of RJ20. Refractory material overlaying a more volatile material can elevate desorption temperatures due to trapping of molecules beneath the refractory layer 21,22 and products formed from e-irradiation likely formed a more refractory layer in our ice mixtures.…”

ALCHEMI Finds a “Shocking” Carbon Footprint in the Starburst Galaxy NGC 253

“…Hasegawa & Herbst (1993) estimated that the maximum dust temperature due to the cosmic-ray heating of the dust is 70 K, but this temperature can be different for the case of NGC 253, where some dust is already warm. With a higher maximum dust temperature, the evaporation rate of ice species can be enhanced (e.g., Kalvāns & Kalnin 2020). This dependence of desorption due to cosmic-ray heating on the dust temperature should be further explored with theoretical studies.…”

Towards a better understanding of ice mantle desorption by cosmic rays

“…Some studies have addressed the physics of the interaction between the cosmic ray particles and the dust grains, e.g. Ivlev et al (2015a) considered a model of localised spot-heating driven explosions, whilst Kalvans & Kalnin (2020a) is the only other investigation that has considered the time-dependence of the coupled cooling, and layer-by-layer evaporative cooling of different molecular species. That study tracked the cooling profile, the composition of the ices and the evaporative yields for several species.…”