Dust Formation and Inhomogeneous Mass Loss from AGB Stars

Abstract

We examine the flow from asymptotic giant branch (AGB) stars when along a small solid angle the optical depth due to dust is very large. We consider two types of flows. In the first, small cool spots are formed on the surface of slowly rotating AGB stars. During the last AGB phase when mass loss rate is high, the dust shields the region above it from the stellar radiation. This leads to both further dust formation in the shaded region, and the convergence of the stream toward the shaded region. A concentration of magnetic cool spots toward the equator will lead to a density contrast of up to a few between the equatorial and polar directions. This process can explain the positive correlation between high mass loss rate and a larger departure from sphericity in progenitors of elliptical planetary nebulae. In the second type of flow, the high density in the equatorial plane is formed by a binary interaction, where the secondary star is close to, but outside the AGB envelope. The shielding of the radiation by dust results in a very slow and dense flow in the equatorial plane. We suggest this flow as an alternative explanation for the equatorial dense matter found at several hundred astronomical units around several post-AGB binary systems.Comment: 15 pages + 2 figures (postscript), to be published in MNRA