AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 71. Between the Stars II: The ISM, Galactic and Extragalactic
Display, Wednesday, June 2, 1999, 10:00am-6:30pm, Southwest Exhibit Hall

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[71.08] Shape and Optical Constant Effects on the 2175Å\ Feature

L.M. Will, P.A. Aannestad (ASU)

The galactic interstellar extinction curve exhibits a pronounced feature at 2175Å. The most striking characteristic of this feature is the relatively fixed nature of its central wavelength, while the width of the feature may vary. Small graphite particles are often proposed as the carrier of this UV bump. However, graphite grains of a single size and shape cannot reproduce the observed width and peak characteristics simultaneously. Here, we investigate the extent to which these characteristics may be reproduced by manipulating the optical constants and invoking shape distributions.

The graphite constants calculated by Draine & Lee (ApJ,285,99) do not yield the narrowest 2175Åfeatures observed. We have calculated three sets of optical constants for astronomical graphite such that, in each case, a different shape (sphere, prolate(b/a=3/2), & oblate(b/a=3/2)) produces a feature with a peak at 4.6 \mu m-1 and \gamma = 0.8. Using grains in the Rayleigh approximation, we have calculated extinction curves for combinations of prolate, spherical, and oblate grain shapes, and have successfully reproduced the stationary nature of the peak. In addition, such shape variations do give variations in the width of the feature whose relative magnitudes match the observations. However, the absolute values of the calculated widths fall short by approximately 10 percent. The effect that these results may have on polarization studies of the 2175Åfeature will be discussed.

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