  [Abridged] Ethylene oxide and its isomer acetaldehyde are important complex
organic molecules because of their potential role in the formation of amino
acids. Despite the fact that acetaldehyde is ubiquitous in the interstellar
medium, ethylene oxide has not yet been detected in cold sources. We aim to
understand the chemistry of the formation and loss of ethylene oxide in hot and
cold interstellar objects (i) by including in a revised gas-grain network some
recent experimental results on grain surfaces and (ii) by comparison with the
chemical behaviour of its isomer, acetaldehyde. We test the code for the case
of a hot core. The model allows us to predict the gaseous and solid ethylene
oxide abundances during a cooling-down phase prior to star formation and during
the subsequent warm-up phase. We can therefore predict at what temperatures
ethylene oxide forms on grain surfaces and at what temperature it starts to
desorb into the gas phase. The model reproduces the observed gaseous abundances
of ethylene oxide and acetaldehyde towards high-mass star-forming regions. In
addition, our results show that ethylene oxide may be present in outer and
cooler regions of hot cores where its isomer has already been detected. Despite
their different chemical structures, the chemistry of ethylene oxide is coupled
to that of acetaldehyde, suggesting that acetaldehyde may be used as a tracer
for ethylene oxide towards cold cores.

Solar and Stellar Astrophysics