On the chemical ladder of esters Detection and formation of ethyl formate in the W51 e2 hot molecular core

Context. In recent years, the detection of organic molecules with increasing complexity and potential biological relevance is opening the possibility to understand the formation of the building blocks of life in the interstellar medium. One of the families of molecules of substantial astrobiological interest are the esters. The simplest ester, methyl formate (CH3OCHO), is rather abundant in star-forming regions. The next step in the chemical complexity of esters is ethyl formate, C2H5OCHO. Despite the increase in sensitivity of current telescopes, the detection of complex molecules with more than ten atoms such as C2H5OCHO is still a challenge. Only two detections of this species have been reported so far, which strongly limits our understanding of how complex molecules are formed in the interstellar medium. New detections towards additional sources with a wide range of physical conditions are crucial to differentiate between competing chemical models based on dust grain surface and gas-phase chemistry. Aims. We have searched for ethyl formate towards the W51 e2 hot molecular core, one of the most chemically rich sources in the Galaxy and one of the most promising regions to study prebiotic chemistry, especially after the recent discovery of the P-O bond, key in the formation of DNA. Methods. We have analyzed a spectral line survey towards the W51 e2 hot molecular core, which covers 44 GHz in the 1, 2 and 3 mm bands, carried out with the IRAM 30 m telescope. Results. We report the detection of the trans and gauche conformers of ethyl formate. A local thermodynamic equilibrium analysis indicates that the excitation temperature is 78 +/- 10 K and that the two conformers have similar source-averaged column densities of (2.0 +/- 0 .3) x 10(-16) cm(-2) and an abundance of similar to 10(-8). We compare for the first time the observed molecular abundances of ethyl formate with different competing chemical models based on grain surface and gas-phase chemistry. Conclusions. We propose that grain-surface chemistry may have a dominant role in the formation of ethyl formate (and other complex organic molecules) in hot molecular cores, rather than reactions in the gas phase.