Extended Co(7-6) Emission from Warm Gas in Orion

Abstract

We mapped quiescent 807 GHz CO(7-6) emission from Orion along a strip in RA extending from 0.7 pc west to 1.2 pc east of Ori theta 1C. The lines arise in warm gas with T > 40 K. The line brightness temperature is > 160 K in the di- rection of theta 1C, more than twice the dust temperature, and still exceeds 35 K more than a parsec east of theta 1C. The lines are narrow, with a max- imum velocity width of 7 km s^-1 near theta 1C and decreasing to 1.5-3 km s^-1 at the map boundaries. The density of the emitting gas is > 10^4 cm^-3 and the column density exceeds 10^21 cm^-2. The correlation of the bright, narrow CO(7-6) lines with 158 micron [C II] emission suggests that over the entire region mapped, the narrow CO lines arise in warm photo-dissociation regions ex- cited by UV photons from the Trapezium cluster. Although the Trapezium stars lie in front of the Orion A molecular cloud, not all of the warm gas is at the cloud surface. To the east of theta 1C the CO(7-6) lines split into two velo- city components (also seen in 13CO(2-1) emission) which persist over several arcminutes. Since only one of these components can be on the surface, the other must arise from a dense, UV-illuminated clump or filament within the molecular cloud. Comparison of the quiescent CO(7-6) emission to CO(7-6) ob- served in a cross-map of the energetic Orion KL outflow shows that the lumino- sity of shock-excited CO(7-6) emission in Orion is only a few percent of the luminosity of the widespread quiescent CO(7-6) emission.