Young bipolar nebulae

Abstract

Young bipolar nebulae are those rather rare exceptions among deeply embedded pre-main sequence objects driving bipolar outflows, which are amenable to detailed optical studies — by that means they provide unique information about this whole class of objects. The aim of this review is to work out, what their observations have told us about their structure and about the winds of young stars and their interaction with circumstellar matter. While the emphasis lies on the discussion of the optical observations, the large body of infrared and radio data is also 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 observational results about disks and winds of T Tauri stars are considered, which shed new light also on the essential structural elements of young bipolar nebulae. After the description of some peculiar cases, the common 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 molecular 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 stars and their massive circumstellar disks. Finally, current theoretical concepts about the mechanisms driving the winds are discussed, and present ideas about the evolution of bipolar nebulae are described. The Appendix includes a catalog of young bipolar nebulae.