| As astronomers, we are dependent on the light that comes to us from stars, nebulae, and galaxies. This light, however, does not travel to us unchanged. The interstellar medium can polarize, scatter, and extinguish light during its journey to our detectors. The effect of the interstellar medium on light in the ultraviolet wavelength region is particularly interesting; there is a prominent extinction feature (a "bump") near 4.6 mu m--1. Since its discovery, the carrier of this feature has been attributed to graphite dust grains. However, a study of several lines of sight using International Ultraviolet Explorer (IUE) data showed that the peak of this feature was tightly confined (less than 1% deviation) to 4.6 mum--1, while the average width of the feature was found to be about 1.0 mum--1 with approximately 10% deviation. Solid state physics requires that, for a particle of a particular size, the width of the extinction profile cannot be changed without also changing the peak. Therefore, graphite grains of a single shape and size cannot produce the feature. Other carriers have been proposed, but none can satisfy the observed lack of correlation between the peak and the width of the feature. As these observational constraints called into question the model of small graphite spheres as the carrier of the 2175 A feature, possible solutions were proposed. This study systematically investigates these proposed modifications to the graphite hypothesis. This research determines that changes in shape and internal grain structure have some effect, but that the observed behaviors can only be reproduced by varying the optical constants of graphite along different lines of sight. |
