Episodic Explosions in Interstellar Ices

TL;DR

This paper proposes a cyclic model where explosive sublimation of interstellar ice mantles leads to gas-phase chemistry, forming large organic molecules in dark clouds, including in quiescent regions, which complements existing solid-state formation theories.

Contribution

It introduces a novel cyclic mechanism involving ice mantle explosions and subsequent gas-phase chemistry for organic molecule formation in dark clouds.

Findings

Significant abundances of large molecules can form post-explosion.

Large molecules are predicted in quiescent molecular clouds.

The mechanism complements existing solid-state chemistry models.

Abstract

We present a model for the formation of large organic molecules in dark clouds. The molecules are produced in the high density gas-phase that exists immediately after ice mantles are explosively sublimated. The explosions are initiated by the catastrophic recombination of trapped atomic hydrogen. We propose that, in molecular clouds, the processes of freeze-out onto ice mantles, accumulation of radicals, explosion and then rapid (three-body) gas-phase chemistry occurs in a cyclic fashion. This can lead to a cumulative molecular enrichment of the interstellar medium. A model of the time-dependent chemistries, based on this hypothesis, shows that significant abundances of large molecular species can be formed, although the complexity of the species is limited by the short expansion timescale in the gas, immediately following mantle explosion. We find that this mechanism may be an important source of smaller organic species, such as methanol and formaldehyde, as well as precursors to bio-molecule formation. Most significantly, we predict the gas-phase presence of these larger molecular species in quiescent molecular clouds and not just dynamically active regions, such as hot cores. As such the mechanism that we propose complements alternative methods of large molecule formation, such as those that invoke solid-state chemistry within activated ice mantles.