The acceleration of molecular hydrogen to high velocities in time-dependent fast J-shocks

It is a well known but puzzling result that zones within star formation regions sometimes show molecular hydrogen emission at very high (∼100 km s−1) velocities. These kinds of observations are somewhat difficult to explain because non-magnetized, J-type shock waves of velocities above ∼20 km s−1 mostly dissociate the molecules present in the preshock medium, and therefore produce almost no H2 emission. We quantify this result by presenting models of steady shock waves moving into a molecular environment, which show that the H2 molecules are indeed dissociated in the immediate postshock region for higher shock velocities. We argue that the total destruction of molecules by high-velocity shocks is a direct result of the assumption of an instantaneous ‘turning on’ of the flow that is generally done in computing shock models. We present models in which a shock wave gradually accelerates over a period of ∼1000 yr as would be expected, for example, from the ‘turning on’ of an outflow from a young star. We find that such shock waves are indeed able to accelerate significant masses of molecular material to velocities of ∼100 km s−1, and are a plausible explanation for widely observed high-velocity H2 emission.