The genealogical tree of ethanol: gas-phase formation of glycolaldehyde, acetic acid and formic acid

TL;DR

This paper introduces a novel gas-phase chemical pathway for synthesizing glycolaldehyde, acetic acid, and formic acid in interstellar environments, supported by quantum calculations and astrochemical modeling, challenging the grain-surface synthesis paradigm.

Contribution

It proposes the first gas-phase formation route for glycolaldehyde and revises existing chemical networks with new reactions and rate data, enhancing astrochemical models.

Findings

Predicted glycolaldehyde abundance matches observations in hot corinos and shock sites.

Model overestimates acetic acid and formic acid abundances, indicating incomplete reaction networks.

New reaction pathways improve understanding of complex organic molecule formation in space.

Abstract

Despite the harsh conditions of the interstellar medium, chemistry thrives in it, especially in star forming regions where several interstellar complex organic molecules (iCOMs) have been detected. Yet, how these species are synthesised is a mystery. The majority of current models claim that this happens on interstellar grain surfaces. Nevertheless, evidence is mounting that neutral gas-phase chemistry plays an important role. In this article, we propose a new scheme for the gas-phase synthesis of glycolaldehyde, a species with a prebiotic potential and for which no gas-phase formation route was previously known. In the proposed scheme, the ancestor is ethanol and the glycolaldehyde sister species are acetic acid (another iCOM with unknown gas-phase formation routes) and formic acid. For the reactions of the new scheme with no available data, we have performed electronic structure and kinetics calculations deriving rate coefficients and branching ratios. Furthermore, after a careful review of the chemistry literature, we revised the available chemical networks, adding and correcting several reactions related to glycolaldehyde, acetic acid and formic acid. The new chemical network has been used in an astrochemical model to predict the abundance of glycolaldehyde, acetic acid and formic acid. The predicted abundance of glycolaldehyde depends on the ethanol abundance in the gas phase and is in excellent agreement with the measured one in hot corinos and shock sites. Our new model overpredicts the abundance of acetic acid and formic acid by about a factor of ten, which might imply a yet incomplete reaction network.