The formation of bipolar sources in dense molecular clouds

A unified interpretation of the formation of bipolar molecular outflows and optical jets associated with young stellar objects is attempted. Models involving the collimation of an isotropic wind are discussed applying constraints on the pressure distribution in the formation environment of the form $$p\propto r^{-n}$$. The de Laval nozzle mechanism, in which the shocked gas remains hot, is limited to high-velocity outflows. The oblique-shock model, in which the shocked gas cools rapidly, is shown to result in well-collimated twin jets for $$1\lt n \lt2$$. A dense circumstellar disc is required to support the flow configuration. The possible presence of one-sided flow patterns is demonstrated. Bipolar nebula structures are created when dense sheets are ejected for n ∼ 0. Therefore, the model links the nature of the stellar environment to the form in which material is recycled to the molecular cloud.