Icy molecule desorption in interstellar grain collisions

TL;DR

This study explores how collisions between interstellar grains can cause ice mantle fragments to release complex organic molecules into the gas phase, potentially explaining observed abundances in cold starless cores.

Contribution

It introduces a simple model showing that grain collisions can significantly enhance gas-phase COMs, a mechanism not thoroughly examined before.

Findings

Turbulence-induced collisions can increase COM abundances by several orders of magnitude.

Longer timescales (up to 1 Myr) allow for sustained high COM levels.

Low initial methanol ice abundance can still lead to observed COM levels.

Abstract

Observations of gaseous complex organic molecules (COMs) in cold starless and prestellar cloud cores require efficient desorption of the COMs and their parent species from icy mantles on interstellar grains. With a simple astrochemical model, we investigate if mechanical removal of ice fragments in oblique collisions between grains in two size bins (0.01 and 0.1 micron) can substantially affect COM abundances. Two grain collision velocities were considered - 10 and 50 meters per second, corresponding to realistic grain relative speeds arising from ambipolar diffusion and turbulence, respectively. From the smaller grains, the collisions are assumed to remove a spherical cap with height equal to 1/3 and 1 ice mantle thickness, respectively. We find that the turbulence-induced desorption can elevate the gas-phase abundances of COMs by several orders of magnitude, reproducing observed COM abundances within an order of magnitude. Importantly, the high gaseous COM abundances are attained for long time-scales of up to 1 Myr and for a rather low methanol ice abundance, common for starless cores. The simple model, considering only two grain size bins and several assumptions, demonstrates a concept that must be tested with a more sophisticated approach.