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Abstract: Up to 90% of the chemical reactions during star formation occurs on icesurfaces, probably including the formation of complex organics. Only the mostabundant ice species are however observed directly by infrared spectroscopy.This study aims to develop an indirect observational method of ices based onnon-thermal ice desorption in the colder part of protostellar envelopes. Forthat purpose the IRAM 30m telescope was employed to observe two molecules thatcan be detected both in the gas and the ice, CH3 OH and HNCO, toward 4 low massembedded protostars. Their respective gas-phase column densities are determinedusing rotational diagrams. The relationship between ice and gas phaseabundances is subsequently determined. The observed gas and ice abundances spanseveral orders of magnitude. Most of the CH3OH and HNCO gas along the lines ofsight is inferred to be quiescent from the measured line widths and the derivedexcitation temperatures, and hence not affected by thermal desorption close tothe protostar or in outflow shocks. The measured gas to ice ratio of ~10-4agrees well with model predictions for non-thermal desorption under coldenvelope conditions and there is a tentative correlation between ice and gasphase abundances. This indicates that non-thermal desorption products can serveas a signature of the ice composition. A larger sample is however necessary toprovide a conclusive proof of concept.



Author: Karin I. Oberg, Sandrine Bottinelli, Ewine F. van Dishoeck

Source: https://arxiv.org/







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