Fractal Grains and the Interstellar Extinction Curve
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**Session 92 -- Dust and Gas in the ISM**
*Oral presentation, Friday, January 14, 10:15-11:45, Salon VI Room (Crystal Gateway)*

## [92.07] Fractal Grains and the Interstellar Extinction Curve

*M. E. Fogel and C. M. Leung (Rensselaer)*
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.

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