Formation of complex organic molecules in cold objects: the role of gas phase reactions

TL;DR

This paper presents a new gas-phase chemical model explaining the formation of complex organic molecules like methyl formate and dimethyl ether in cold interstellar environments, challenging the traditional grain-surface chemistry dominance.

Contribution

The study introduces a gas-phase reaction pathway for complex organic molecule formation in cold environments, supported by observational data and explaining correlations between molecules.

Findings

The model reproduces observed abundances in L1544.

Gas-phase reactions significantly contribute to complex molecule formation.

Correlation between DME and MF explained by the model.

Abstract

While astrochemical models are successful in reproducing many of the observed interstellar species, they have been struggling to explain the observed abundances of complex organic molecules. Current models tend to privilege grain surface over gas phase chemistry in their formation. One key assumption of those models is that radicals trapped in the grain mantles gain mobility and react on lukewarm (>30 K) dust grains. Thus, the recent detections of methyl formate (MF) and dimethyl ether (DME) in cold objects represent a challenge and may clarify the respective role of grain surface and gas phase chemistry. We propose here a new model to form DME and MF with gas phase reactions in cold environments, where DME is the precursor of MF via an efficient reaction overlooked by previous models. Furthermore, methoxy, a precursor of DME, is also synthetized in the gas phase from methanol, which is desorbed by a non-thermal process from the ices. Our new model reproduces fairy well the observations towards L1544. It also explains, in a natural way, the observed correlation between DME and MF. We conclude that gas phase reactions are major actors in the formation of MF, DME and methoxy in cold gas. This challenges the exclusive role of grain-surface chemistry and favours a combined grain-gas chemistry.