AAS 205th Meeting, 9-13 January 2005
Session 59 ISM: Dust and Molecules
Poster, Tuesday, January 11, 2005, 9:20am-6:30pm, Exhibit Hall

## [59.11] Interstellar Extinction and Polarization by Graphite-Silicate Clusters

E. T. Johnson, B. T. Draine (Princeton University)

The geometry of interstellar dust continues to be uncertain. In some models, intertellar grains are assumed to homogeneous spheres, with a suitable mixture of sizes and compositions in order to reproduce observations of of absorption and scattering (e.g., Weingartner & Draine 2001, or Zubko et al.\ 2004).

However, it is often thought that the larger interstellar grains may be formed by agglomeration of smaller particles, with the resulting cluster'' being of nonuniform composition and having a fluffy'' geometry. The optical properties of such fluffy'' grains have sometimes been estimated using effective medium theory'' or other approximations, but it is now possible to directly calculate scattering and absorption using the discrete dipole approximation (Draine & Flatau 1994).

We construct candidate clusters by random ballistic agglomeration of small graphite and silicate spheres, and calculate their scattering and absorption cross sections using the discrete dipole approximation code DDSCAT 6.x (Draine & Flatau 2004).

We consider a model for interstellar dust consisting of very small grains plus clusters built by ballistic agglomeration with a suitable size distribution, and we test the model by trying to reproduce the observed wavelength dependence of interstellar extinction and polarization.

This research was supported in part by NSF grants AST-0216105 and AST-0406883.

\noindent References:

\noindent Draine, B.T., & Flatau, P.J. 1994, JOSA, A11, 1491\hfill\break Draine, B.T., & Flatau, P.J. 2004, http://arxiv.org/abs/astro-ph/0409262\hfill\break Weingartner, J.C., & Draine, B.T. 2001, ApJ, 548, 296\hfill\break Zubko, V., Dwek, E., & Arendt, R.G. 2004, ApJS, 152, 211\hfill\break

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© 2004. The American Astronomical Society.