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Abstract
Abstract We present experimental constraints on the insertion of oxygen atoms into methane to form methanol in astrophysical ice analogs. In gas-phase and theoretical studies this process has previously been demonstrated to have a very low or nonexistent energy barrier, but the energetics and mechanisms have not yet been characterized in the solid state. We use a deuterium UV lamp filtered by a sapphire window to selectively dissociate O2 within a mixture of O2:CH4 and observe efficient production of CH3OH via O(1D) insertion. CH3OH growth curves are fit with a kinetic model, and we observe no temperature dependence of the reaction rate constant at temperatures below the oxygen desorption temperature of 25 K. Through an analysis of side products we determine the branching ratio of ice-phase oxygen insertion into CH4: ∼65% of insertions lead to CH3OH, with the remainder leading instead to H2CO formation. There is no evidence for CH3 or OH radical formation, indicating that the fragmentation is not an important channel and that insertions typically lead to increased chemical complexity. CH3OH formation from O2 and CH4 diluted in a CO-dominated ice similarly shows no temperature dependence, consistent with expectations that insertion proceeds with a small or nonexistent barrier. Oxygen insertion chemistry in ices should therefore be efficient under low-temperature ISM-like conditions and could provide an important channel to complex organic molecule formation on grain surfaces in cold interstellar regions such as cloud cores and protoplanetary disk midplanes.
Highlights
Complex organic molecules (COMs) have been detected towards star-forming regions at all stages of evolution, including molecular clouds, protostellar hot cores, envelopes, and outflows, and protoplanetary disks (e.g. Blake et al 1987; Bottinelli et al 2004; Arce et al 2008; Oberg et al 2010, 2015)
There is no evidence for CH3 or OH radical formation, indicating that the fragmentation is not an important channel and that insertions typically lead to increased chemical complexity
Oxygen insertion chemistry in ices should be efficient under low-temperature interstellar medium (ISM)-like conditions, and could provide an important channel to complex organic molecule formation on grain surfaces in cold interstellar regions such as cloud cores and protoplanetary disk midplanes
Summary
Complex organic molecules (COMs) have been detected towards star-forming regions at all stages of evolution, including molecular clouds, protostellar hot cores, envelopes, and outflows, and protoplanetary disks (e.g. Blake et al 1987; Bottinelli et al 2004; Arce et al 2008; Oberg et al 2010, 2015). It is of great interest to understand the rich chemistry that feeds the formation and destruction of these molecules in the interstellar medium (ISM) in order to constrain the chemical inventories available for pre-biotic chemistry as solar systems develop. Observations of COMs towards very cold interstellar environments such as prestellar cores (Oberg et al 2010; Bacmann et al 2012; Cernicharo et al 2012, e.g.) indicate that a cold pathway to complex molecule formation must be active. Repeated hydrogenation of CO has been shown to be efficient and leads to the production of the stable molecules H2CO and CH3OH (Watanabe & Kouchi 2002; Fuchs et al 2009): CO −−H→ HCO −−H→ H2CO −−H→ H3CO −−H→ CH3OH (1)
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