A FAST CO JET IN ORION-B

We report 12-arcsec resolution CO observations of the fast unipolar molecular jet in Orion B together with maps of the ambient cloud as traced by CS and (CO)-O-18 emission. The flow (which appears to originate from FIR5) consists of many clumps of gas, with sizes less-than-or-similar-to 10(15) m and velocity dispersions approximately 6 km s-1, which fill an almost cylindrical lobe; these clumps are probably formed locally through hydrodynamic instabilities rather than being bullets of protostellar ejecta. The flow is centrally condensed and, at the highest projected velocities (-45 km s-1), an unresolved jet of material is detected with an opening angle approximately 2-degrees. The lower velocity outflowing gas forms a sheath around this jet. This narrow CO feature is as highly collimated as the optical jets seen in other systems, and its termination has been detected in the H-2 upsilon = 1-0 S(1) line. The system is thus very similar to many optical jet/HH objects seen in less obscured star-forming regions, and it is suggested that the fast highly collimated CO is being entrained in the neutral high Mach number jet which is driving the molecular outflow.