Particle Shapes and the 2175 A Feature

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The Galactic interstellar extinction curve exhibits a pronounced feature at 2175 ?. 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 to what extent these characteristics may be reproduced by invoking a shape distribution of prolate and oblate graphite particles. The graphite constants calculated by Draine and Lee do not yield the narrowest 2175 ? features that have been observed. We have recalculated the dielectric function of astronomical graphite such that a spherical grain of radius 50 ? produces a feature with a peak at x0 = 4.6 ?m-1 and a width ? = 0.8 ?m-1. Using grains in the Rayleigh approximation, we have calculated extinction curves for combinations of prolate, spherical, and oblate grain shapes and successfully reproduce the stationary nature of the peak. In addition, such shape variations do give a standard deviation in the width of the feature of ~1/2 the observed standard deviation. However, the absolute values of the calculated widths fall short by ~10% compared with observed widths. The observed lack of correlation between peak position and width is not reproduced by the shape variations considered here.