  We discuss the effects of an enhanced interstellar radiation field (ISRF) on
the observables of protostellar cores in the Orion cloud region. Dust radiative
transfer is used to constrain the envelope physical structure by reproducing
SCUBA 850 micron emission. Previously reported 13CO, C17O and H2CO line
observations are reproduced through detailed Monte Carlo line radiative
transfer models. It is found that the 13CO line emission is marginally
optically thick and sensitive to the physical conditions in the outer envelope.
An increased temperature in this region is needed in order to reproduce the
13CO line strengths and it is suggested to be caused by a strong heating from
the exterior, corresponding to an ISRF in Orion 10^3 times stronger than the
"standard" ISRF. The typical temperatures in the outer envelope are higher than
the desorption temperature for CO. The C17O emission is less sensitive to this
increased temperature but rather traces the bulk envelope material. The data
are only fit by a model where CO is depleted, except in the inner and outermost
regions where the temperature increases above 30-40 K. The fact that the
temperatures do not drop below approximately 25 K in any of the envelopes
whereas a significant fraction of CO is frozen-out suggest that the
interstellar radiation field has changed through the evolution of the cores.
The H2CO lines are successfully reproduced in the model of an increased ISRF
with constant abundances of 3-5x10^{-10}.
