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 C18O emission. The flow (which appears to originate from FIR5) consists of many clumps of gas, with sizes $\lesssim {10}^{15}$ m and velocity dispersions ~ 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 ~ 2°. 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 $\text{H}_{2}\, \upsilon = 1-0\,S(1)$ 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.