THE ORIGIN OF COMPLEX ORGANIC MOLECULES IN PRESTELLAR CORES

Complex organic molecules (COMs) have been detected in a variety of environments, including cold prestellar cores. Given the low temperature of these objects, these last detections challenge existing models. We report here new observations towards the prestellar core L1544. They are based on an unbiased spectral survey of the 3 mm band at the IRAM-30 m telescope, as part of the Large Program ASAI. The observations allow us to provide the full census of the oxygen bearing COMs in this source. We detected tricarbon monoxide, methanol, acetaldehyde, formic acid, ketene, and propyne with abundances varying from 5 x 10(-11) to 6 x 10(-9). The non-LTE analysis of the methanol lines shows that they are likely emitted at the border of the core, at a radius of similar to 8000 AU where T similar to 10 K and n(H2) similar to 2 x 10(4) cm(-3). Previous works have shown that water vapour is enhanced in the same region because of the photodesorption of water ices. We propose that a non-thermal desorption mechanism is also responsible for the observed emission of methanol and COMs from the same layer. The desorbed oxygen and a tiny amount of desorbed methanol and ethene are enough to reproduce the abundances of tricarbon monoxide, methanol, acetaldehyde and ketene measured in L1544. These new findings open the possibility that COMs in prestellar cores originate in a similar outer layer rather than in the dense inner cores, as previously assumed, and that their formation is driven by the non-thermally desorbed species.