Astrochemistry at work in the L1157-B1 shock: acetaldehyde formation

The formation of complex organic molecules (COMs) in protostellar environments is a hotly debated topic. In particular, the relative importance of the gas phase processes as compared to a direct formation of COMs on the dust grain surfaces is so far unknown. We report here the first high-resolution images of acetaldehyde (CH$_3$CHO) emission towards the chemically rich protostellar shock L1157-B1, obtained at 2 mm with the IRAM Plateau de Bure interferometer. Six blueshifted CH$_3$CHO lines with $E_{\rm u}$ = 26-35 K have been detected. The acetaldehyde spatial distribution follows the young ($\sim$ 2000 yr) outflow cavity produced by the impact of the jet with the ambient medium, indicating that this COM is closely associated with the region enriched by iced species evaporated from dust mantles and released into the gas phase. A high CH$_3$CHO relative abundance, 2-3 $\times$ 10$^{-8}$, is inferred, similarly to what found in hot-corinos. Astrochemical modelling indicates that gas phase reactions can produce the observed quantity of acetaldehyde only if a large fraction of carbon, of the order of 0.1%, is locked into iced hydrocarbons.