2014MNRAS.443.2612S

Molecular jets have been discovered in large numbers, spread throughout star formation regions. They can usually be traced back to embedded driving protostars. We here investigate a squadron of such molecular hydrogen jets in the DR21/W75N region through echelle spectroscopy of the near-infrared v = 1-0 S(1) emission line centred at 2.122 µm. We detect 79 components, a number of which possess radial velocities in excess of 80 km/s. The majority of the components exhibit blueshifts. The regions closer to DR21 exhibit more blue-shifted components suggesting that extinction is important across individual flows and is higher near DR21. We provide a classification scheme for the resulting collection of position-velocity diagrams, including other published data. One prominent class is associated with pairs of shocks well separated in radial velocity. We use hydrodynamic simulations with molecular cooling and chemistry to show that these are associated with Mach discs and bow shocks. We also employ a steady-state bow shock model to interpret other revealing position-velocity diagrams. We consider mechanisms which can generate vibrationally excited hydrogen molecules moving at speeds well beyond the breakdown speed permitted for shock excitation. We conclude that the molecules have formed within the jets well before being excited by internal shocks triggered by impacts with the ambient clouds. We also note the relatively high number of high blue-shifted radial velocity components and argue that these must be associated with high-density molecular jets from Class 0 protostars which are obscured unless we are selectively viewing within a conical cavity containing the jet.