On the nature of bipolar sources in dense molecular clouds

Abstract

An attempt is made to present a unified interpretation of anisotropic emission and outflow phenomena which are detected over a wide range of scales in the vicinity of young stellar objects embedded in dense molecular clouds. The evolution of an interstellar bubble in an anisotropic medium is considered, and the formation and propagation of jets, and their relation to the various bipolar phenomena observed in molecular clouds, are discussed. The possible role of magnetic fields in reproducing bipolar emission sources is examined. It is found that an isotropic stellar wind which expands in a medium with an anisotropic density distribution would give rise to an interstellar bubble which elongates in the direction of the external density gradient. Under certain conditions which are likely to prevail in protostellar environments, the elongating bubble could become unstable to the formation of de Laval nozzles. Once these are established, the outflow becomes channeled into two oppositely directed, supersonic jets.