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Complex organic molecules (COMs) have been detected in a variety ofenvironments, including cold prestellar cores. Given the low temperature ofthese objects, these last detections challenge existing models. We report herenew observations towards the prestellar core L1544. They are based on anunbiased spectral survey of the 3mm band at the IRAM-30m telescope, as part ofthe Large Program ASAI. The observations allow us to provide the full census ofthe oxygen bearing COMs in this source. We detected tricarbon monoxide,methanol, acetaldehyde, formic acid, ketene, and propyne with abundancesvarying from 5e-11 to 6e-9. The non-LTE analysis of the methanol lines showsthat they are likely emitted at the border of the core, at a radius of ~8000 AUwhere T~10 K and nH2~2e4 cm-3. Previous works have shown that water vapour isenhanced in the same region because of the photodesorption of water ices. Wepropose that a non-thermal desorption mechanism is also responsible for theobserved emission of methanol and COMs from the same layer. The desorbed oxygenand a tiny amount of desorbed methanol and ethene are enough to reproduce theabundances of tricarbon monoxide, methanol, acetaldehyde and ketene measured inL1544. These new findings open the possibility that COMs in prestellar coresoriginate in a similar outer layer rather than in the dense inner cores, aspreviously assumed, and that their formation is driven by the non-thermallydesorbed species.

The origin of complex organic molecules in prestellar cores