Context. It is nowadays clear that a rich organic chemistry takes place in protostellar regions. However, the processes responsible for it, that is, the dominant formation routes to interstellar complex organic molecules, are still a source of debate. Two paradigms have been evoked: the formation of these molecules on interstellar dust mantles and their formation in the gas phase from simpler species previously synthesised on the dust mantles.Aims. In the past, observations of protostellar shocks have been used to set constraints on the formation route of formamide (NH2CHO), exploiting its observed spatial distribution and comparison with astrochemical model predictions. In this work, we follow the same strategy to study the case of acetaldehyde (CH3CHO).Methods. To this end, we used the data obtained with the IRAM-NOEMA interferometer in the framework of the Large Program SOLIS to image the B0 and B1 shocks along the L1157 blueshifted outflow in methanol (CH3OH) and acetaldehyde line emission.Results. We imaged six CH3OH and eight CH3CHO lines which cover upper-level energies up to similar to 30 K. Both species trace the B0 molecular cavity as well as the northern B1 portion, that is, the regions where the youngest shocks (similar to 1000 yr) occurred. The CH3OH and CH3CHO emission peaks towards the B1b clump, where we measured the following column densities and relative abundances: 1.3 x 10(16) cm(-2) and 6.5 x 10(-6) (methanol), and 7 x 10(13) cm(-2) and 3.5 x 10(-8) (acetaldehyde). We carried out a non-LTE (non-Local Thermodinamic Equilibrium) Large Velocity Gradient (LVG) analysis of the observed CH3OH line: the average kinetic temperature and density of the emitting gas are T-kin similar to 90 K and n(H2) similar to 4 x 10(5) cm(-3), respectively. The CH3OH and CH3CHO abundance ratio towards B1b is 190, varying by less than a factor three throughout the whole B0-B1 structure.Conclusions. Comparison of astrochemical model predictions with the observed methanol and acetaldehyde spatial distribution does not allow us to distinguish whether acetaldehyde is formed on the grain mantles or in the gas phase, as its gas-phase formation, which is dominated by the reaction of ethyl radical (CH3CH2) with atomic oxygen, is very fast. Observations of acetaldehyde in younger shocks, for example those of similar to 10(2) yr old, and/or of the ethyl radical, whose frequencies are not presently available, are necessary to settle the issue.
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