The gas phase origin of complex organic molecules precursors in prestellar cores

Abstract

Complex organic molecules (COMs) have long been observed in the warm regions surrounding nascent protostars. The recent discovery of oxygen-bearing COMs like methyl formate or dimethyl ether in prestellar cores (Bacmann et al. [2]), where gas and dust temperatures rarely exceed 10–15 K, has challenged the previously accepted models according to which COM formation relied on the diffusion of heavy radicals on warm (∼30 K) grains. Following these detections, new questions have arisen: do non-thermal processes play a role in increasing radical mobility or should new gas-phase routes be explored? The radicals involved in the formation of the aforementioned COMs, HCO and CH3 O represent intermediate species in the grain-surface synthesis of methanol which proceeds via successive hydrogenations of CO molecules in the ice. We present here observations of methanol and its grain-surface precursors HCO, H2 CO, CH3 O in a sample of prestellar cores and derive their relative abundances. We find that the relative abundances HCO:H2 CO:CH3 O:CH3 OH are constant across the core sample, close to 10:100:1:100. Our results also show that the amounts of HCO and CH3 O are consistent with a gas-phase synthesis of these species from H2 CO and CH3 OH via radical-neutral or ion-molecule reactions followed by dissociative recombinations. Thus, while grain chemistry is necessary to explain the abundances of the parent volatile CH3 OH, and possibly H2 CO, the reactive species HCO and CH3 O might be daughter molecules directly produced in the gas-phase.