The Dust Ring around AC Herculis

We have used the Keck I telescope to obtain images at 11.7 and 18.7 μm of AC Her, a post-asymptotic giant branch spectroscopic binary. The source is resolved into two spots separated by about 0 6, which can be explained as emission from particles associated with an edge-on dust ring with a radius of approximately 300 AU that is oriented approximately north-south. We propose that the ring is mainly composed of particles that are larger than 200 μm in radius that are orbiting the binary, while the infrared emission that we observe is emitted by small grains with sizes less than 1 μm that are not gravitationally bound and, instead, are flowing out in a wind from the ring. We propose that collisions among the larger ring particles produce the smaller wind particles. Whether most of the 10 30 g of dust orbiting AC Her ultimately is expelled in a wind or coalesces into macroscopic objects is unknown.