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| EPoS Contribution |
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Complex Organic Molecule formation: grain surface of gas-phase chemistry?
Aurore Bacmann IPAG, Grenoble, FR | |
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Complex organic molecules (COMs) have long been observed in the warm regions of star forming regions surrounding nascent protostars, first in massive hot cores, and later in their low-mass counterparts. The large richness in chemical species displayed in the spectra of these regions has triggered the development of chemical models aiming at accounting for the observed chemical complexity. It was first proposed that simple molecules like e.g. methanol were injected into the gas phase when the icy mantles were evaporated during the protostellar phase. These species could then react in the gas via ion-molecule reactions followed by dissociative recombination to form COMs. Later experimental and theoretical work has however shown that these gas-phase processes were unable to produce sufficient amounts of COMs as observed. Alternative COM formation mechanisms were proposed in which atoms or diatomic molecules (e.g. CO, N) were hydrogenated on the grains during the pre-stellar phase. Heavy radicals formed on the grains can diffuse during the warm-up phase when the ice temperature reaches ~ 30 K to form COMs, which are subsequently released in the gas phase at higher temperatures. The recent discovery of COMs like methyl formate or dimethyl ether in pre-stellar cores, where gas and dust temperatures rarely exceed 10-15 K, casts doubt on this type of scenario, as the ice is never warm enough to enable the diffusion of radicals necessary to the formation of complex species. The question of COM formation has therefore to be investigated again: 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 hydrogenation of CO molecules in the ice (gas-phase synthesis of methanol is believed to be inefficient). Detection of COMs in another cold source, including the radical CH3O and the similar abundances found for these species has led to the speculation that they were probably formed on the grain surfaces. We present here observations of methanol and its precursors HCO, H2CO, CH3O in a sample of pre-stellar cores and derive their relative abundances. We find that the relative abundances are constant across the core sample. 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" volatiles CH3OH and H2CO, the reactive species HCO and CH3O might be "daughter" molecules directly produced in the gas-phase. | |
| Collaborators: A. Faure, IPAG, France |
Key publication
Suggested Sessions: Chemistry |