On modelling cosmic ray sputtering of interstellar grain ices

Abstract

In the interstellar medium (ISM), the formation of complex organic molecules (COMs) is largely facilitated by surface reactions. However, in cold dark clouds, thermal desorption of COMs is inefficient because of the lack of thermal energy to overcome binding energies to the grain surface. Non-thermal desorption methods are therefore important explanations for the gas-phase detection of many COMs that are primarily formed on grains. Here we present a new non-thermal desorption process: cosmic ray sputtering of grain ice surfaces based on water, carbon dioxide, and a simple mixed ice. Our model applies estimated rates of sputtering to the 3-phase rate equation model Nautilus-1.1, where this inclusion results in enhanced gas phase abundances for molecules produced by grain reactions such as methanol (CH3OH) and methyl formate (HCOOCH3). Notably, species with efficient gas phase destruction pathways exhibit less of an increase in models with sputtering compared to other molecules. These model results suggest that sputtering is an efficient, non-specific method of non-thermal desorption that should be considered as an important factor in future chemical models.

New interstellar laboratories in the molecular ring

“…One possibility is low-temperature sputtering of CH 3 OH off grains by, for example, CO 2 , models of which reproduced observed gas-phase CH 3 OH abundances in Taurus Molecular Cloud within a factor of 2.5. 48 In addition, grain–grain collisions resulting from turbulence have been offered as an explanation for gas-phase organics in cold regions. 49 X-ray photodesorption of CH 3 OH at 15 K in disks has also been demonstrated experimentally.…”

First observations of warm and cold methanol in Class 0/I proto-brown dwarfs

“…Species such as ions and radicals resulting from UV absorption that remain in the grain mantle can react further to form more complex species. Sputtering by strong jets and shock emission knots can spot raise the grain temperature and inject frozen intact methanol into the gas-phase (e.g., Draine 1995, Wakelam et al 2021, Paulive et al 2022).…”