Accretion disks in pre-planetary nebulae

A number of planetary nebulae (PNs) exhibit collimated, high-velocity outfl ows or jets. These hydrodynamical structures cannot be easily accommodated within the classical models of the evolution of post-asymptotic giant branc h stars, and understanding them has become a topical problem in PN research . One way to explain the existence of jets in PNs has been to invoke the pr esence of accretion disks, which would presumably set the conditions for th e collimation and driving of the outflows. This work investigates in detail the type of binary systems that are likely to lead to Roche-lobe overflow (RLOF) and the formation of accretion disks as a consequence of common-enve lope evolution, and explores the expected basic physical structure of such disks. The results of the analysis show substantial restrictions on the com position of binary systems that can form a disk upon accretion onto the pri mary. Typically, it is found that for a primary asymptotic giant branch (AG B) core of 0.6 M. and envelope mass of 2-3 M., secondaries with M-2 greater than or similar to 0.08 M. and initial separation a(i) greater than or sim ilar to 200 R. will not lead to RLOF. For systems that do lead to RLOF, thi s is achieved at orbital separations <2 R.. We also find that dynamically s table mass transfer from secondaries with M-2 greater than or similar to 0. 08 M. does not lead to disk formation, since the circularization radius lie s below the surface of the AGE core. Only lower mass companions, after a dy namically unstable mass transfer process, may lead to disk formation. Under reasonable simplifying assumptions, we estimate the resulting accretion di sk properties and evolution and discuss their potential role in driving col limated outflows.