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J. B. Tartar, A. K. Speck (University of Missouri), M. Elitzur (University of Kentucky), M. Meixner (Space Telescope Science Institute), M. Nenkova (University of Kentucky)
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. Using far-infrared (120 and 180\mum) observations obtained from the Infrared Space Observatory (ISO), we have discovered extremely large (parsec-sized) dust shells around the well-known post-AGB star, the Egg Nebula (Speck, Meixner & Knapp 2001). The radial profile of this dust shell suggest that episodic mass loss has occurred with mass-loss enhancements on timescales corresponding to theoretical predictions of thermal pulses on the AGB. We present preliminary results of modeling this dust shell around the Egg Nebula using a version of the 1-d radiative transfer code DUSTY. Such modeling allows the determination of the density distribution of the dust around the nebula as a function of radius. However, modeling such large dust shells is not trivial. Previous studies of very large circumstellar shells showed that most of the outer shell is heated by the interstellar radiation field (ISRF) rather than the central star. Therefore using radiative transfer models with only the central star heating the dust is unrealistic. Furthermore, where the circumstellar shell ploughs into the surrounding interstellar medium may lead to a pile up of material at the outer edge of the dust shell. Increased mass-loss towards the end of the AGB is also expected. Mass-loss episodes are modeled as enhancements in the density distribution.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.