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The exact size distribution of interstellar grains is unknown. By examining interstellar extinction curves, various grain properties, including sizes and compositions, can be inferred, grain models are then constructed and compared with relavent observations. This was the approach taken by Mathis, Rumpl, and Nordsieck (1977, ApJ , 217 , 425-433). This MRN grain model assumed a power law size distribution of interstellar grains composed of graphite and olivine (or silicate). A successful fit was obtained for a power law index of 3.5, with grain sizes ranging from $0.005 \mu m$ to $0.25 \mu m$. In fitting the interstellar extinction curve, the MRN model assumes spherical grains.
Polarization measurements of interstellar dust grains rule out the possiblility of purely spherical grains. The harsh environments in the outflows of evolved stars and subsequent processing more likely will lead to irregularly shaped dust paticles. We investigate the effect of irregular grain shape in fitting the interstellar extinction curve. In our model we assume grain aggregates generated by a cluster-cluster fractal growth process. The resulting fractal grain is a non-spherical, non-compact object. The extinction cross sections are then calculated using the discrete dipole approximation. We examine the effect these fractal grains have on the fitting of the interstellar extinction curve including the $\lambda2175$ and the far UV rise. In particular, it is found that, compared to spherical grains, fractal grains tend to produce a broader $\lambda2175$ feature. We also explore the parameters of the MRN model to derive a new power law size distribution for fractal dust grains.
This research has been partially supported by NASA grants NAGW-2817 and NAGW-3144.
