Hydrodynamic collimation of YSO jets

We present the results of numerical hydrodynamic models for the collimation of outflows from young stellar objects. We show that the presence of a toroidal environment can lead to efficient formation of jets and bipolar outflows from initially uncollimated central winds. The interaction between the wind and the environment leads to two types of collimation, one which is dominated by radiative cooling effects, and one which works when cooling is less efficient. We describe the two types of jets as they appear in the simulations and we suggest a description for the long term evolution of these structures in more realistic time-dependent wind sources.