Numerical simulations of highly collimated protostellar outflows the effects of relative density

We present three-dimensional hydrodynamic simulations of jets as a model for protostellar outflows. We investigate molecular jets which are initially heavier, equal or lighter than a uniform ambient molecular medium, as well as a ballistic atomic jet, with the aim of distinguishing the resulting structures and relating them to various proposed protostellar evolutionary stages. We modify the ZEUS numerical code, to include time-dependent molecular hydrogen chemistry, a limited equilibrium C and O chemistry, and a detailed cooling function. We find highly focussed and accelerated flow patterns for outflows driven by molecular jets, caused by the combined strong cooling, small imposed jet shear and precession. We also find shoulders in the interface with associated shocks visible in our simulated near-infrared H 2 images. The shoulder location relative to the front of the bow shock distinguishes the relative density. Apart from this, the outflow structures are quite similar provided the jet is molecular. The ratio of jet power to H2 1-0 S(1) line luminosity (increasingly required to interpret observations), is generally in the range 80-600. Sub-millimetre CO properties, including a velocity-position and velocity-channel diagram; are presented. We compare mass-velocity relationships derived directly and via the simulated CO data: significant systematic differences are uncovered. For the future, we identify fine-scale structure in the rotational CO 2-1 and CO 14-13 rotational lines which can be resolved with the millimetre array ALMA and the Herschel (FIRST) Observatory. We identify highly collimated outflows in the near-infrared that can be interpreted by this model.