Shock focusing and jet collimation in young stars

We study the collimation of a initially uncollimated outflow from a young star by its environment, a cloud core with a toroidal density distribution. It is shown that when cooling is unimportant the shape of the inner shock and the internal structure of the bubble will collimate the outflow and it is shown that in some cases cooling is unimportant. The effects of the focusing by an elliptical shock are studied. Numerical simulations show that when cooling is important, the collimation is actually much better and interesting combinations of energy- and momentum-conserving flow occur.