K-J Chuang, G Fedoseev, D Qasim, S Ioppolo, EF van Dishoeck, H Linnartz
The astronomical gas-phase detection of simple species and small organic molecules in cold pre-stellar cores, with abundances as high as ∼1e−8–1e−9 nH, 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–H2CO–CH3OH 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, H2CO and CH3OH. We find that for temperatures in the range of 10 to 14 K, an upper limit of 0.24 ± 0.02 for the overall elemental carbon loss upon CO conversion into CH3OH. This corresponds with an effective reaction desorption fraction of 0.07 per hydrogenation step, or 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.
2018, The Astrophysical Journal, 853(2), 102
Full-text URL: http://iopscience.iop.org/article/10.3847/1538-4357/aaa24e/meta