The Prevalence and Implications of Sharp Outer Edges of Disks in Preplanetary Nebulae

Mark Morris & Raghvendra Sahai

UCLA, USA

Many preplanetary nebulae appear to have an optically thick concentration of dust in or near their equatorial plane, manifested as a dark "waist" in the typically bipolar nebula. In many cases, these waists can apparently be attributed to unbound material flowing relatively slowly out of the system. Some, however, may be bound disks. In either case, the presence of an equatorial dust concentration extending to fairly large radii is noticeable when it is projected against the background lobe of the bipolar, partially obscuring only that lobe. In many systems in which the projection angle is suitable, observations show that the obscuration by the equatorial "disk" ends abruptly at some radius, where one can see a clear discontinuity in the surface brightness along the curved outer edge of the disk. That is, there appears to be a sharp radial cutoff in the dust distribution. In a few cases, the outer edge of the disk shows an excess brightness, as if it is preferentially illuminated. These sharp outer edges to the dust distributions have profound implications. For unbound, outflowing equatorial dust concentrations, the sharp discontinuity would correspond to an extremely abrupt onset of the heavy mass loss, which severely constrains the nature of the mass-loss mechanism. For bound disks, on the other hand, the radial cutoff raises the possibility of a dynamical interaction with a planet or a companion star. Alternatively, the cutoff might reveal that the mechanism for forming a bound disk cannot provide the orbiting material with more angular momentum than a certain critical value. Dynamical studies of the gas associated with the dust in these systems will be important for sorting out these possibilities.

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