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

Abstract

We present an extensive review of gas phase reactions producing methylacetylene and propene showing that these relatively abundant unsaturated hydrocarbons cannot be synthesized through gas-phase reactions. We explain the formation of propene and methylacetylene through surface hydrogenation of C 3 depleted onto interstellar ices, C 3 being a very abundant species in the gas phase.

Temperature dependent product yields for the spin forbidden singlet channel of the C(3P) + C2H2 reaction

“…Dissociative electronic recombination reactions of c-/l-C 3 H 2 + and c-/l-C 3 H 3 + drive the production of c-/l-C 3 H and c-/l-C 3 H 2 as these processes involve substantially higher fluxes than the C + C 2 H 2 reaction. In a similar manner to the simulated C 3 H 6 abundance [18], the predicted presence of a barrier for the O + C 3 reaction [19] is critical for c-/l-C 3 H and c-/l-C 3 H 2 abundances.…”

“…As C 3 is thought to be unreactive with both atomic nitrogen [17] and oxygen [18,19] which are predicted to be present at high abundances in dense interstellar clouds, such high yields could inhibit complex molecule formation in current simulations. Indeed, both l/c-C 3 H produced by channels (1a) and (1b) are currently considered to react rapidly with both of these atomic species [17].…”

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.…”

“…This finding is in contrast with the results form Miettinen et al (2006), but is based on a much larger number of sources. Therefore, if methyl acetylene is formed onto dust grains, it needs efficient routes of formation at low temperatures, such as the progressive hydrogenation reactions of C 3 proposed by Hickson et al (2016), as well as desorption mechanisms able to release a significant quantity of CH 3 CCH already for T ∼ 20 K, to explain the scales and temperatures traced. Because no clear jump in abundance is seen in hot cores for this species also when the mantle is sublimated, only a small quantity of methyl acetylene must remain in the ice when a temperature of ∼ 110 K is reached, either due to reprocessing or nearly complete sublimation at lower temperatures.…”

Protoplanetary discs: sensitivity of the chemical composition to various model parameters

“…The gas-phase chemical network is derived from kida.uva.2014 (Wakelam et al 2015a), which contains all relevant and common processes for the gas-phase chemistry (unimolecular reactions, such as photo-dissociation and ionisation by direct UV photons and comic-ray induced UV photons, bimolecular reactions such as ion-neutral and neutralneutral reactions, and electronic recombination). This network has been updated from more recent publications from Wakelam et al (2015b), Loison et al (2016), Hickson et al (2016), Vidal et al (2017b), and Loison et al (2017). CO, H 2 , and N 2 self-shielding are computed using tabulated values from Lee et al (1996), Visser et al (2009), and Li et al (2013) respectively.…”