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 ∼10 −8 −10 −9 n 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 ± 0.02 for the overall elemental carbon loss upon CO conversion into CH 3 OH. 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.

Reactivity of formic acid (HCOOH) with H atoms on cold surfaces of interstellar interest

“…On the other hand, we do not find any clear signature of COMs (except for methanol) formed via HCOOH hydrogenation. However, some CO-bearing products are subject to reactive desorption through the exothermic hydrogen-abstraction surface reactions, as has been demonstrated in previous laboratory experiments (Chuang et al 2018;Minissale et al 2016b), astrochemical models (Vasyunin & Herbst 2013), and recent theoretical studies (Morisset et al 2019). This chemical desorption effect (Dulieu et al 2013) should be taken into account in the evaluation of the product formation of the hydrogenation HCOOH+H reaction, in addition to other factors, such as the molecular ice environment that can stabilize some intermediates (HCOOH 2 *, HCOO*, *COOH, HO-CO*), and even the geometrical orientations of the dissymmetrical HCOOH molecules and their conformers (cis and trans) toward specific directions favoring or enabling H/OH abstraction surface reactions.…”

Why does ammonia not freeze out in the centre of pre-stellar cores?

“…Here we adopt the approach of Garrod et al (2007), in which exothermic surface reactions lead to desorption with a probability of ∼1%. Recent investigations (Dulieu et al 2013;Minissale et al 2016;Chuang et al 2018) have shown that the efficiency of chemical desorption may vary significantly depending on the reaction and type of surface. These results have already been incorporated in chemical models (Vasyunin et al 2017).…”

O2 signature in thin and thick O2−H2O ices

“…The gasphase O 2 abundance in the model by Vasyunin et al (2017) is mainly controlled by the reactive desorption mechanism, whose efficiency under various conditions is currently a matter of debate (see e.g. Minissale et al 2016;Chuang et al 2018;He et al 2017;Oba et al 2018). Thus, gas-phase abundance of O 2 will be the subject of further theoretical and experimental studies.…”