An improved chemical scheme for the reactions of atomic oxygen and simple unsaturated hydrocarbons - implications for star-forming regions

TL;DR

This paper presents an improved chemical model for reactions between atomic oxygen and small unsaturated hydrocarbons, revealing how environmental conditions influence hydrocarbon abundance and suggesting new radicals to include in astrochemical databases.

Contribution

The study introduces an upgraded reaction scheme for atomic oxygen and hydrocarbons, highlighting the impact of environment on hydrocarbon destruction and growth, and proposing new radical species for astrochemical models.

Findings

Atomic oxygen degrades hydrocarbons to CO and precursor species in hot cores.

Hydrocarbon growth is favored in O-poor, C-rich environments.

Hydrocarbon column densities match observed values in the 10^13-10^15 cm^-2 range.

Abstract

Recent laboratory experiments have demonstrated that, even though contribution from other reaction channels cannot be neglected, unsaturated hydrocarbons easily break their multiple C-C bonds to form CO after their interactions with atomic oxygen. Here we present an upgraded chemical modelling including a revision of the reactions between oxygen atoms and small unsaturated hydrocarbons for different astrochemical environments. A first conclusion is that towards hot cores/corinos atomic oxygen easily degrades unsaturated hydrocarbons directly to CO or to its precursor species (such as HCCO or HCO) and destroys the double or triple bond of alkenes and alkynes. Therefore, environments rich in atomic oxygen at a relatively high temperature are not expected to be rich in large unsaturated hydrocarbons or polycyclic aromatic hydrocarbons. On the contrary, in O-poor and C-rich objects, hydrocarbon growth can occur to a large extent. In addition, new radical species, namely ketyl and vinoxy radicals, generated from other reaction channels can influence the abundances of hydrocarbons towards hot cores. We, therefore, suggest they should be included in the available databases. Hydrocarbon column densities are calculated in the 10(13)-10(15) cm(-2) range, in good agreement with their observed values, despite the small number of data currently published in the literature.