A radical route to interstellar propylene formation

TL;DR

This paper proposes a new grain-driven chemical mechanism involving rapid high-density gas-phase reactions following ice sublimation to explain the high abundance of propylene in molecular clouds, linking it to source age and activity.

Contribution

It introduces a novel model for interstellar propylene formation that bypasses complex surface chemistry, emphasizing rapid gas-phase reactions after ice sublimation.

Findings

Model explains high propylene abundances in certain sources.

High-density gas-phase chemistry is key to complex organic molecule formation.

Source age and dynamical activity influence propylene presence.

Abstract

Complex organic molecules, such as propylene (CH3CHCH2), are detected in molecular clouds (such as TMC1) with high fractional abundances (~2.10^{-9}, relative to hydrogen) that cannot be explained by gas-phase chemical reactions under normal dark cloud conditions. To obtain such high abundances requires an efficient grain-driven chemistry to be operating, coupled with effective desorption of the complex organics back into the gas-phase. We propose that the mechanism that we have previously described; rapid high density gas-phase chemistry in the gas released following sudden, total, ice mantle sublimation - can explain the high abundances, without recourse to ill-defined surface chemical pathways. Our model provides a natural explanation for why it is that some sources harbour high abundances of propylene, whilst others do not; based on the age and level of dynamical activity within the source (which affects the ice composition) and the chemical composition of the ambient gas.