Accretion Disks in Pre-Planetary Nebulae

A number of planetary nebulae (PNs) exhibit collimated, high-velocity outflows or jets. These hydrodynamical structures cannot be easily accommodated within the classical models of the evolution of post-asymptotic giant branch 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 presence of accretion disks, which would presumably set the conditions for the 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-envelope evolution, and explores the expected basic physical structure of such disks. The results of the analysis show substantial restrictions on the composition of binary systems that can form a disk upon accretion onto the primary. Typically, it is found that for a primary asymptotic giant branch (AGB) core of 0.6 M_solar and envelope mass of 2-3 M_solar, secondaries with M₂>~0.08 M_solar and initial separation a_i>~200 R_solar will not lead to RLOF. For systems that do lead to RLOF, this is achieved at orbital separations <2 R_solar. We also find that dynamically stable mass transfer from secondaries with M₂>~0.08 M_solar does not lead to disk formation, since the circularization radius lies below the surface of the AGB core. Only lower mass companions, after a dynamically unstable mass transfer process, may lead to disk formation. Under reasonable simplifying assumptions, we estimate the resulting accretion disk properties and evolution and discuss their potential role in driving collimated outflows.