Astrochemical Modeling of Propargyl Radical Chemistry in TMC-1

TL;DR

This study refines astrochemical models of the propargyl radical in TMC-1, improving abundance predictions and shedding light on aromatic molecule formation in dark molecular clouds.

Contribution

It updates the chemical network in astrochemical models to better account for propargyl radical chemistry, aligning predictions closer to observations.

Findings

Propargyl radical abundance increased by 0.5 order of magnitude in models.

Predicted C4H3N isomer abundances are within an order of magnitude of observations.

Resonance-stabilized radicals play a key role in PAH formation in dark clouds.

Abstract

Recent detections of aromatic species in dark molecular clouds suggest formation pathways may be efficient at very low temperatures and pressures, yet current astrochemical models are unable to account for their derived abundances, which can often deviate from model predictions by several orders of magnitude. The propargyl radical, a highly abundant species in the dark molecular cloud TMC- 1, is an important aromatic precursor in combustion flames and possibly interstellar environments. We performed astrochemical modeling of TMC-1 using the three-phase gas-grain code NAUTILUS and an updated chemical network, focused on refining the chemistry of the propargyl radical and related species. The abundance of the propargyl radical has been increased by half an order of magnitude compared to the previous GOTHAM network. This brings it closer in line with observations, but it remains underestimated by two orders of magnitude compared to its observed value. Predicted abundances for the chemically related C4H3N isomers within an order of magnitude of observed values corroborate the high efficiency of CN addition to closed-shell hydrocarbons under dark molecular cloud conditions. The results of our modeling provide insight into the chemical processes of the propargyl radical in dark molecular clouds and highlight the importance of resonance-stabilized radicals in PAH formation.