  Aims: The production of saturated organic molecules in hot cores and corinos
is not well understood. The standard approach is to assume that, as
temperatures heat up during star formation, methanol and other species
evaporate from grain surfaces and undergo a warm gas-phase chemistry at 100 K
or greater to produce species such as methyl formate, dimethyl ether, and
others. But a series of laboratory results shows that protonated ions, typical
precursors to final products in ion-molecule schemes, tend to fragment upon
dissociative recombination with electrons rather than just ejecting a hydrogen
atom. Moreover, the specific proposed reaction to produce protonated methyl
formate is now known not to occur at all. Methods: We utilize a gas-grain
chemical network to probe the chemistry of the relatively ignored stage of hot
core evolution during which the protostar switches on and the temperature of
the surrounding gas and dust rises from 10 K to over 100 K. During this stage,
surface chemistry involving heavy radicals becomes more important as surface
hydrogen atoms tend to evaporate rather than react. Results: Our results show
that complex species such as methyl formate, formic acid, and dimethyl ether
can be produced in large abundance during the protostellar switch-on phase, but
that both grain-surface and gas-phase processes help to produce most species.
The longer the timescale for protostellar switch-on, the more important the
surface processes.
