MIDINFRARED IMAGES OF THE POSTASYMPTOTIC GIANT BRANCH STAR HD 161796

We present ground-based images in the 10 mum window of the post-asympt otic giant branch (AGB) star HD 161796. The images show an almost circ ular ring of emission at 12.5 and at 10.5 mum, with brightness enhance ments consistent with dust in a toroidal distribution. The ring is bri ghter on the southeast than the northwest side. Combining the measured diameter of this ring with results of a radiative transfer code for c ircumstellar dust shells, we have determined uniquely a distance (1200 pc) and luminosity (3600 L.) for this source which imply that it must be a Population II star, in accord with earlier abundance determinati ons. These images imply that the star left the AGB only 240 years ago, and that it did so with a final phase of enormously enhanced, equator ially concentrated mass loss.