AAS 199th meeting, Washington, DC, January 2002
Session 92. Circumstellar Material and Atmospheres: Cooler
Display, Wednesday, January 9, 2002, 9:20am-6:30pm, Exhibit Hall

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[92.15] Effects of Grain Size on the SED from Circumstellar Envelopes with Dust

A.C. Carciofi, J.E. Bjorkman (University of Toledo), A.M. Magalhães (University of São Paulo, Brazil)

Dust is associated with many stellar objects. In particular, it is found in the dense, cool outflows from evolved late-type stars, where most of the interstellar dust grains are formed. The presence of dust around a given source is usually detected via the modification of the stellar radiation as it traverses the dusty media. The emergent spectrum (SED) is given by the sum of the attenuated stellar spectrum and the thermal IR reemission by the grains. The parameters affecting the SED are the physical properties of the dust grains (composition, shape and size), the optical depth of the envelope, and its spatial distribution. Observationally determining these parameters is a crucial step toward understanding important astrophysical processes such as details of stellar evolution, mass loss mechanisms, grain formation processes, etc. In this paper, we introduce the concept of approximate scaling, which sets a useful framework that help us systematically explore the effects of dust grain size on the SED. Approximate scaling states that the IR SED of models with differing grain sizes will be approximately equal when the following conditions are met: i) the shape of the absorption efficiency factor, Q\lambda, of the different grain sizes are similar in the spectral region where most of the thermal radiation is emitted; ii) the fraction of the bolometric luminosity reprocessed by the dust in different models must be the same; and iii) the temperature of the dust grains in the inner region of the envelope is the same. We discuss the physical meaning of these conditions and the necessary requirements they impose. Finally, the observational and theoretical consequences of the approximate scaling are explored in detail.

The author(s) of this abstract have provided an email address for comments about the abstract: carciofi@usp.br

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