The detached dust shell around the massive star HD 179821

We have used the Keek I telescope to resolve at three mid-IR wavelengths the emission from HD 179821 (= RAFGL 2343), a G-type supergiant of perhaps 30 M. with a detached dust shell. The shell is very approximately circular in shape with an inner diameter of similar to 3 ".3, corresponding to 3.0 x 10(17) cm. We estimate that the star was losing similar to 4 x 10(-4) M. yr(-1) until about 1800 yr ago, when the mass loss slowed dramatically. During the past similar to 10(4) yr, the star has lost similar to 10% of its initial mass. The star lies about 0 ".35 off center and is closer to the brighter, northern hemisphere of the nebula, which can be explained if the outflow velocity V-infinity deviates by +20% from the average in the southern hemisphere and -20% from the average in the northern hemisphere. The mass-loss rate (M) over dot (theta) may have been inversely correlated with the outflow velocity so that the momentum outflow p was isotropic during the mass-loss phase. It also seems that MtotalVinfinity was within a factor of 2 of L-*/c, where L-* is the current luminosity of the star; the mass loss may have been driven by radiation pressure. These results may help characterize the asymmetric circumstellar winds into which supernova explosions propagate.