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

The astronomical gas-phase detection of simple species and small organic molecules in cold pre-stellar cores, with abundances as high as ~10-10 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 to derive 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 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.