AAS 207th Meeting, 8-12 January 2006
Session 182 Stellar Winds, Circumstellar Matter, and Activity
Poster, Thursday, 9:20am-4:00pm, January 12, 2006, Exhibit Hall

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[182.15] Spitzer/MIPS Infrared Imaging of the Extremely Extended Circumstellar Dust Shell of HD 161796.

A. K. Speck (University of Missouri - Columbia), T. Ueta (NASA Ames Research Center), R. Stencel (University of Denver), MIRIAD Collaboration

Evolved intermediate mass stars are major contributors to the interstellar medium. However, the mechanisms by which they do this are not well understood. 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, circumstellar shells of AGB and post-AGB stars contain the fossil record of their mass loss, and therefore have the potential to verify many aspects of stellar evolution. IRAS and ISO data indicate that huge dust shells exist around many such objects, extending several parsecs from the central star. Furthermore, some of these large dust shells show evidence for mass-loss variations that correlate with evolutionary changes in the star itself. Previous observations lacked the sensitivity and spatial resolution to investigate the full extent and detailed structure of these large dust shells. Using Spitzer/MIPS's unique sensitivity and mapping capabilities, we have obtained a 160\mum image of the very extended dust shell around post-AGB star HD161796, which confirms that it exhibits weak extended emission out to a radius of several hundred arcseconds. We present preliminary studies of this observation and compare to previous FIR observations of this and other post-AGB stars. From this study we will be able to (a) constrain the mass of the progenitor star; (b) test theories of stellar evolution and mass-loss mechanisms; (c) determine the effect of dust chemistry on mass loss (and therefore on stellar evolution).

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