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

¹ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France
² CNRS, IPAG, 38000 Grenoble, France

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 CH₃O 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, H₂CO, CH₃O in a sample of prestellar cores and derive their relative abundances. We find that the relative abundances HCO:H₂CO:CH₃O:CH₃OH are constant across the core sample, close to 10:100:1:100. Our results also show that the amounts of HCO and CH₃O are consistent with a gas-phase synthesis of these species from H₂CO and CH₃OH 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 CH₃OH, and possibly H₂CO, the reactive species HCO and CH₃O might be daughter molecules directly produced in the gas-phase.