YOUNG BIPOLAR NEBULAE

Young bipolar nebulae are those rather rare exceptions among deeply em bedded pre-main sequence objects driving bipolar outflows, which are a menable to detailed optical studies - by that means they provide uniqu e information about this whole class of objects. The aim of this revie w is to work out, what their observations have told us about their str ucture and about the winds of young stars and their interaction with c ircumstellar matter. While the emphasis lies on the discussion of the optical observations, the large body of infrared and radio data is als o considered. First, four of the best studied examples, S 106, Cep A, R Mon, and L 1551 IRS 5, are analyzed in depth. Then, recent observati onal results about disks and winds of T Tauri stars are considered, wh ich shed new light also on the essential structural elements of young bipolar nebulae. After the description of some peculiar cases, the com mon properties characterizing the whole class of young bipolar nebulae are worked out. We are led to a unified picture of the bipolar winds causing both, the broad optically bright reflection lobes and molecula r outflows, as well as the highly collimated optical jets observed in some cases on their polar axes; structure and dynamics of the bipolar winds are determined by the close interaction between the central star s and their massive circumstellar disks. Finally, current theoretical concepts about the mechanisms driving the winds are discussed, and pre sent ideas about the evolution of bipolar nebulae are described. The A ppendix includes a catalog of young bipolar nebulae.