  While astrochemical models are successful in reproducing many of the observed
interstellar species, they have been struggling to explain the observed
abundances of complex organic molecules. Current models tend to privilege grain
surface over gas phase chemistry in their formation. One key assumption of
those models is that radicals trapped in the grain mantles gain mobility and
react on lukewarm (>30 K) dust grains. Thus, the recent detections of methyl
formate (MF) and dimethyl ether (DME) in cold objects represent a challenge and
may clarify the respective role of grain surface and gas phase chemistry. We
propose here a new model to form DME and MF with gas phase reactions in cold
environments, where DME is the precursor of MF via an efficient reaction
overlooked by previous models. Furthermore, methoxy, a precursor of DME, is
also synthetized in the gas phase from methanol, which is desorbed by a
non-thermal process from the ices. Our new model reproduces fairy well the
observations towards L1544. It also explains, in a natural way, the observed
correlation between DME and MF. We conclude that gas phase reactions are major
actors in the formation of MF, DME and methoxy in cold gas. This challenges the
exclusive role of grain-surface chemistry and favours a combined grain-gas
chemistry.
