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 \mu m emission .
Previously reported ^ { 13 } CO , C ^ { 17 } O and H _ { 2 } CO line observations are reproduced through detailed Monte Carlo line radiative transfer models .
It is found that the ^ { 13 } CO 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 ^ { 13 } CO 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 C ^ { 17 } O 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 \approx 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 H _ { 2 } CO lines are successfully reproduced in the model of an increased ISRF with constant abundances of 3–5 \times 10 ^ { -10 } .