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

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.

Reaction dynamics of oxygen atoms with unsaturated hydrocarbons from crossed molecular beam studies: primary products, branching ratios and role of intersystem crossing

“…In the modelling of cold/warm interstellar objects the reactions of O( 3 P) with simple organic molecules (including small unsaturated hydrocarbons) or radicals need to be included, as they affect the molecular abundances. Very recently, the results of our studies on some of the discussed reactions have been introduced in models which simulate the chemistry of the interstellar objects by Occhiogrosso et al [6]. Until recently [148], however, limited and simplistic information about these reactions were included in the models.…”

“…For instance, the reaction between atomic oxygen and the CH 3 OCH 2 radical is the only viable route of methyl formate formation in the gas phase [147]. The effects of this more accurate chemical network representing the reactions of atomic oxygen with several unsaturated hydrocarbons on both warm and cold regions of the ISM has been examined [6]. Very recently, the results of our studies on some of the discussed reactions have been introduced in models which simulate the chemistry of the interstellar objects by Occhiogrosso et al [6].…”

ATLASGAL-selected massive clumps in the inner Galaxy

“…A strong debate is ongoing in literature about how CH 3 CCH is formed, focusing on whether or not surface chemistry is necessary to ex-plain the observed abundances of this species (e.g. Occhiogrosso et al 2013;Hickson et al 2016). We find that the detection rate of CH 3 CCH increases with evolutionary class, but we find no correlation between the temperature traced by methyl acetylene and its abundance, even when considering only sources with distances below 6 kpc, which would indicate a progressively more efficient evaporation from the grains due to the warm-up.…”

Methylacetylene (CH3CCH) and propene (C3H6) formation in cold dense clouds: A case of dust grain chemistry

“…It should be noted that Occhiogrosso et al recently published an astrochemical model for hydrocarbons, including CH 3 CCH, in star-forming regions (Occhiogrosso et al 2013). In their study, they used the UMIST 2006 network with some reactions from the online KIDA database and provide in the paper a list of reactions and rate coefficients to be used (in their Table 4).…”