MOMENTUM-TRANSFER BY ASTROPHYSICAL JETS

We have used three dimensional smoothed particle hydrodynamical simula tions to study the basic physical properties of the outflow that is cr eated by a protostellar jet in a dense molecular cloud. The dynamics o f the jet/cloud interaction is strongly affected by the cooling in the shocked gas behind the bow shock at the head of the jet. We show that this cooling is very rapid, with the cooling distance of the gas much less than the jet radius. Thus, although ambient gas is initially dri ven away from the jet axis by the high thermal pressure of the post-sh ock gas, rapid cooling reduces the pressure and the outflow subsequent ly evolves in a momentum-conserving snowplow fashion. The velocity of the ambient gas is high in the vicinity of the jet head, but decreases rapidly as more material is swept up. Thus, this type of outflow prod uces extremely high-velocity clumps of post-shock gas which resemble t he features seen in outflows. We have investigated the transfer of mom entum from the jet to the ambient medium as a function of the jet para meters. We show that a low Mach number (less-than-or-equal-to 6) jet s lows down rapidly because it entrains ambient material along its sides . On the other hand, the beam of a high Mach number jet is separated f rom the ambient gas by a low-density cocoon of post-shock gas, and thi s jet transfers momentum to the ambient medium principally at the bow shock. In high Mach number jets, as those from young stellar objects, the dominant interaction is therefore at the bow shock at the head of the jet.