1997A&A...318..595S

Molecular jets announce the successful birth of a protostar. We develop here a model for the jets and their environments, adapting a multi-dimensional hydrocode to follow the molecular-atomic transitions of hydrogen. We examine powerful outflows into dense gas. The cocoon which forms around a jet is a very low density cavity of atomic gas. These atoms originate from strong shocks which dissociate the molecules. The rest of the molecules are either within the jet or swept up into very thin layers. Pulsed jets produce wider cavities and molecular layers which can grow onto resolvable jet knots. Three-dimensional simulations produce shocked molecular knots, distorted and multiple bow shocks and arclike structures. The resemblance of simulated images of the 1-0S(1) H₂ emission to recently observed deeply embedded outflows in HH211, HH212, HH288 and L1634 is discussed. Spectroscopic and excitation properties of the hydrogen molecules and CO maps are calculated. In the infrared, strong emission is seen from shocks within the jet (when pulsed) as well as from discrete regions along the cavity walls. Excitation, as measured by line ratios, is not generally constant. Broad double-peaked, shifted emission lines are predicted. Low-J CO emission is limb-brightened but spread over the whole outflow region. Some of these signatures are shown to depend on the chosen jet conditions. We find that three-dimensional calculations are essential for numerical simulations of strong cooling jets.