  During the formation of stars, the accretion of the surrounding material
toward the central object is thought to undergo strong luminosity outbursts,
followed by long periods of relative quiescence, even at the early stages of
star formation when the protostar is still embedded in a large envelope. We
investigated the gas phase formation and the recondensation of the complex
organic molecules (COMs) di-methyl ether and methyl formate, induced by sudden
ice evaporation processes occurring during luminosity outbursts of different
amplitudes in protostellar envelopes. For this purpose, we updated a gas phase
chemical network forming complex organic molecules in which ammonia plays a key
role. The model calculations presented here demonstrate that ion-molecule
reactions alone could account for the observed presence of di-methyl ether and
methyl formate in a large fraction of protostellar cores, without recourse to
grain-surface chemistry, although they depend on uncertain ice abundances and
gas phase reaction branching ratios. In spite of the short outburst timescales
of about one hundred years, abundance ratios of the considered species with
respect to methanol higher than 10 % are predicted during outbursts due to
their low binding energies relative to water and methanol that delay their
recondensation during the cooling. Although the current luminosity of most
embedded protostars would be too low to produce these complex species in hot
core regions that can be observable with current sub-millimetric
interferometers, previous luminosity outburst events would induce a formation
of COMs in extended regions of protostellar envelopes with sizes increasing by
up to one order of magnitude.
