PHOTON HEATING OF ENVELOPES AROUND YOUNG STELLAR OBJECTS - AN EXPLANATION FOR CO J=6-5 EMISSION

We propose that the narrow (CO)-C-12 and (CO)-C-13 J = 6-5 emission ob served toward many low-mass young stellar objects is produced in molec ular material in the circumstellar envelope, which is heated by the 10 ,000 K radiation field generated in the inner part of the accretion di sk. Ultraviolet photons traveling through the biconical cavity evacuat ed by the bipolar outflow are scattered by dust grains present in the low-density material in the cavity. These photons are not energetic en ough to photodissociate H-2 and CO, but can heat the envelope surround ing the cavity. The temperature structure and the CO excitation of thi s photon-dominated region are computed using two-dimensional Monte Car lo methods. It is found that the material is heated up to a few hundre d K close to the cavity wall, and that the observed low-velocity mid-J CO emission can be well explained by our model for a wide range in en velope density and stellar luminosity. Emergent CO spectra are compare d to observations of the embedded low-mass YSO IRAS 04361+2547 (TMR-1) .