[Previous] | [Session 59] | [Next]
L. M. Will (Arizona State Univ. & Mesa Community College)
Astronomers 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 2175 Å. Since its discovery, the carrier of this feature frequently has been attributed to graphite grains. However, observational constraints have called into question the model of small graphite spheres as the carrier of the ultraviolet bump. The peak wavelength of the feature is tightly confined to 2175 Å, while the width of the feature varies along different Galactic lines of sight. Graphite grains of a single size and shape cannot produce this behavior. Other carriers (such as amorphous carbon and PAHs) have been proposed, but none have satisfied the observed lack of correlation between the peak wavelength and the width of the feature.
This research systematically investigates proposed modifications (shape variations, core-mantle or composite grain structure, adjustments of optical constants) to the graphite hypothesis. We determine that changes in grain shape and internal structure have a significant effect on the shape and position of the 2175 Å\ feature. However, all of the observed behaviors can only be reproduced by varying the optical constants of graphite along different lines of sight in the Galaxy.
This work was funded by Arizona State University and the Arizona NASA Space Grant Consortium.
The author(s) of this abstract have provided an email address for comments about the abstract: lisa.will@asu.edu