The effect of a strong external radiation field on protostellar envelopes in Orion

¹ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street MS42, Cambridge, MA 02138, USA e-mail: jjorgensen@cfa.harvard.edu
³ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Rd, Victoria, BC, V9E 2E7, Canada
⁴ Department of Physics & Astronomy, University of Victoria, Victoria, BC, V8P 1A1, Canada
⁵ Department of Physics and Astronomy, Denison University, Granville, OH 43023, USA

Received: 7 March 2005
Accepted: 5 December 2005

Abstract

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 μm 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 10³ 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 ≈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 3–5  10^-10.