1999ApJ...525L.113J

We have used the Keck 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 ∼3".3, corresponding to 3.0x10¹⁷ cm. We estimate that the star was losing ∼4x10^–4 M_☉.yr^–1 until about 1800 yr ago, when the mass loss slowed dramatically. During the past ∼10⁴ yr, the star has lost ∼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_∞ deviates by +20% from the average in the southern hemisphere and -20% from the average in the northern hemisphere. The mass-loss rate M{dot}(θ) may have been inversely correlated with the outflow velocity so that the momentum outflow p{dot} was isotropic during the mass-loss phase. It also seems that M{dot}_totalV_∞ 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.