Reactive Desorption of CO Hydrogenation Products under Cold Pre-stellar Core Conditions

Abstract

The astronomical gas-phase detection of simple species and small organic molecules in cold pre-stellar cores, with abundances as high as $\sim$$10^{-8}-10^{-9}$ n${}_{\text{H}}$, contradicts the generally accepted idea that at $10$ K, such species should be fully frozen out on grain surfaces. A physical or chemical mechanism that results in a net transfer from solid-state species into the gas phase offers a possible explanation. Reactive desorption, i.e., desorption following the exothermic formation of a species, is one of the options that has been proposed. In astronomical models, the fraction of molecules desorbed through this process is handled as a free parameter, as experimental studies quantifying the impact of exothermicity on desorption efficiencies are largely lacking. In this work, we present a detailed laboratory study with the goal of deriving an upper limit for the reactive desorption efficiency of species involved in the CO-H${}_2$CO-CH${}_3$OH solid-state hydrogenation reaction chain. The limit for the overall reactive desorption fraction is derived by precisely investigating the solid-state elemental carbon budget, using reflection absorption infrared spectroscopy and the calibrated solid-state band-strength values for CO, H${}_2$CO and CH${}_3$OH. We find that for temperatures in the range of $10$ to $14$ K, an upper limit of $0.24\pm 0.02$ for the overall elemental carbon loss upon CO conversion into CH${}_3$OH. This corresponds with an effective reaction desorption fraction of $\le$$0.07$ per hydrogenation step, or $\le$$0.02$ per H-atom induced reaction, assuming that H-atom addition and abstraction reactions equally contribute to the overall reactive desorption fraction along the hydrogenation sequence. The astronomical relevance of this finding is discussed.

arXiv comment9 pages, 7 figures

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