Compact disks inside pre-planetary nebulae

Disks are known to exist around evolved, mass-losing Asymptotic giant branch (AGB) and post-AGB stars and are suspected to play a fundamental role in the shapes of planetary nebulae (PNe). Compact disks have been found in some bipolar PNe, but their role in the shaping process remains unknown. Compact Keplerian disks are found to be common around post-AGB binaries; however, these objects may never develop into PNe as no nebulae are observed. Collimated nebulae shining by reflected light or shock ionisation surround another group of post-AGB stars, known as pre-PNe. Pre-PNe are thought to be the immediate precursors to non-spherical PNe. We have begun the first systematic survey of pre-PNe, with the Very Large Telescope Interferometer (VLTI). We seek to detect disks left behind by the shaping process and compare these disks to those around other post-AGB stars and PNe. At present we have observed 8 pre-PNe, with several showing evidence of disks in their inner-circumstellar regions. We are using MC3D to conduct detailed radiative transfer models of these disks. We are deriving parameters such as inner and outer radii, scale-height, mass, and inclination.