1999A&A...343..953V

Very young protostars eject collimated jets of molecular gas. Although the protostars themselves are hidden, some of their properties are revealed through the jet dynamics. We here model velocity shear, precession, pulsation and spray within dense jets injected into less-dense molecular clouds. We investigate the Hammer Jet, for which extreme velocity variations as well as strong ripping and spray actions are introduced. A three dimensional ZEUS-type hydrodynamics code, extended with molecular physics, is employed. Jet knots, previously shown to be compact in simulations of smoother jets, now appear as prominent bow shocks in H₂ and as bullets in CO emission lines. High proper motions are predicted in the jet. In the lobes we uncover wide tubular low-velocity CO structures with concave bases near the nozzle. Proper motion vectors in the lobes delineate a strong accelerated flow away from the head with some superimposed turbulent-like motions. The leading bow is gradually distorted by the hammer blows and breaks up into mini-bow segments. The H₂ emission line profiles are wide and twin-peaked over much of the leading bow. On comparison with the simulations, we identify observed outflows driven by various dynamical types of jet. Shear is essential to produce the jet bows, spray or precession to widen the outflows and hammer blows to generate knotty jets. We identify the proper motions of maser spots with the pattern speed of density peaks in the inner jet and shell.