Hydrodynamical models of outflow collimation in young stellar objects

In this paper we explore the physics of time-dependent hydrodynamic collimation of jets from young stellar objects (YSOs). Using parameters appropriate to YSOs, we have carried out high-resolution hydrodynamic simulations modeling the interaction of a central wind with an environment characterized by a toroidal density distribution which has a moderate opening angle of theta(rho) approximate to 90 degrees. The results show that for all but low values of the equator-to-pole density contrast the wind/environment interaction produces strongly collimated supersonic jets. The jet is composed of shocked wind gas. Using analytical models of wind-blown bubble evolution, we show that the scenario studied here should be applicable to YSOs and can, in principle, initiate collimation on the correct scales (R less than or similar to 100 AU). Comparison of our simulations with analytical models demonstrates that the evolution seen in the simulations is a mix of wind-blown bubble and jet dynamics. The simulations reveal a number of time-dependent nonlinear features not anticipated in previous analytical studies. These include: a prolate wind shock; a chimney of cold swept-dragged into the bubble cavity; a plug of dense material between the jet and bow ABSTRACT up ambient material shocks. We find that the collimation of the jet occurs through both de Laval nozzles and focusing of the wind via the prolate wind shock. Using an analytical model for shock focusing we demonstrate that a prolate wind shock can, by itself, produce highly collimated supersonic jets.