The deceleration of giant Herbig-Haro flows

It has been recently discovered that spatially separated Herbig-Haro object s, once considered unrelated, are linked within a chain that may extend for parsecs in either direction of the embedded protostar, forming a giant Her big-Haro jet. Presently, several dozen of these giant flows have been detec ted, and the best documented example, the HH 34 system, shows a systematic velocity decrease with distance on either side of the source. In this paper , we have modeled giant jets by performing fully three-dimensional simulati ons of overdense, radiatively cooling jets modulated with long-period (P si milar to several hundred years) and large-amplitude sinusoidal velocity var iability at injection (Delta upsilon similar to mean jet flow velocity). Al lowing them to travel over a distance well beyond the source, we have found that multiple traveling pulses develop, and their velocity indeed falls of f smoothly and systematically with distance. This deceleration is fastest i f the jet is pressure-confined, in which case the falloff in velocity is ro ughly consistent with the observations. The deceleration occurs as momentum is transferred by gas expelled sideways from the traveling pulses. The sim ulation of a pressure-confined, steady state jet with similar initial condi tions to those of the pulsed jet shows that the flow in this case experienc es acceleration. This result is thus an additional indication that the prim ary source of deceleration in the giant flows cannot be attributed to braki ng of the jet head against the external medium.