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B.M. Mchunu, A.K. Speck (University of Missouri-Columbia, Columbia), M. Meixiner (University of Illinois at Urbana/Champaign)
Thermally pulsing asymptotic giant branch (AGB) stars suffer mass loss which leads to the formation of a circumstellar shell of gas and dust. At the end of the AGB phase, mass-loss stops and the circumstellar shell begins to drift away from the star. If the velocity of the AGB wind has been relatively constant, then dust furthest from the star represents the oldest mass loss, while material closer to the star represents more recent mass loss. Hence, the history of mass loss during the AGB phase is imprinted on the dust shell of the post-AGB envelope. By studying the distribution of matter in these circumstellar shells we can gain a better understanding of the mass-loss processes involved in the evolution of these stars. We present far-infrared (IR) ISOPHOT images of the extremely extended dust emission from the circumstellar shells of 13 -O rich AGB stars.
All these objects appear to have very bright central point sources together with extremely extended (radius ~300"-500") dust shells with brightnesses of a fraction of the peak brightness of the point source. Assuming constant expansion velocities, ages for these dust shells are of the order are derived. For most of these objects infrared intensities fall off smoothly with radius as one would expect for constant or steadily increasing mass-loss rates. However, several show evidence for episodic mass-loss enhancements. We discuss the possible causes of the different radial density profiles implied by the observations.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.