Investigation of the Ultraviolet Interstellar Extinction Curve

The effect of the interstellar medium (ISM) on ultraviolet radiation is striking; there is a prominent extinction feature (a “bump”) near 2175 A (4.6 mm ). Since its discovery, the carrier of this feature has been linked to graphite dust grains. However, a study of several Galactic lines of sight using International Ultraviolet Explorer (IUE) data (E. L. Fitzpatrick & D. Massa 1988, ApJ, 328, 734) showed that the peak of this feature was tightly confined (less than 1% deviation) to 4.6 mm , while the average width of the feature was found to be about 1.0 mm 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 position. Therefore, graphite grains of a single shape and size cannot produce the observed feature. Other carriers have been proposed, but none can satisfy the lack of correlation between the peak and the width of the feature as noted in Fitzpatrick & Massa. 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. Most models currently in the literature focus on comparison to the Galactic average extinction curve, whereas the interesting and challenging details of this feature occur in the differences between Galactic sight lines. Our work consistently strives to meet all of the observational parameters. The major conclusions of this study are as follows: 1. New, representative optical constants for astronomical graphite can be developed using the method described by J. S. Mathis (1994, ApJ, 422, 176), so that the narrowest UV bump observed in the ISM can be reproduced. Using these properties, realistic extinction features can be generated for combinations of graphite particles of various shapes. The resulting extinction features have stationary peak positions but cannot attain the full range of widths observed in the ISM. Also, there is a correlation between peak position and width. Whereas these particles can make a significant contribution to the 2175 A feature, they cannot be the sole carriers. 2. We have demonstrated the ability of shape probability distributions to impose symmetry on the extinction profile. One or more shape distributions may exist in the ISM and would restrict the shape of the profile. Because of their asymmetric extinction profiles, grains of extreme shape (very prolate or oblate) are not favored to be the carrier of the 2175 A feature, which has implications for the mechanisms behind the polarization observed in the feature along two Galactic sight lines. 3. Using optical properties for amorphous carbon, graphite, coals, and vacuum, we determined that carbonaceous grains with core-mantle or composite structures cannot be the sole carriers of the UV feature. However, using the continuous distribution of oscillators (CDO) method (D. K. Lynch 1996, ApJ, 467, 894), optical constants for graphite in various degrees of amorphization can be constructed that can generate extinction features that satisfy all of the observational criteria which have been established for the 2175 A feature. The CDO model is similar to the model used by Mathis (1994) in that the dielectric function is represented by a Lorentzian oscillator. This model becomes more useful over a broader range of frequencies for multiple oscillators. For a distribution of different oscillators, the dielectric function may be represented as a linear superposition of the individual dielectric functions. The foundation of the CDO approximation is that the arrangement of atoms in an amorphous material is similar to the crystalline structure, with the differences, such as interatomic separations and bond angles, having a range of values represented by a statistical distribution. Therefore, a crystalline material would have plasma frequencies and damping constants represented by narrow distributions, and more amorphous materials would be represented by broader distributions. In our study, the resonance frequency was set at 2.94 mm 1 with a possible range of 0.2 mm . We then varied the damping constant in the range 0.800–1.383. By doing so, we could reproduce the UV bump along the lines of sight contained in Fitzpatrick & Massa. The CDO model is the only model in our study and in the existing astronomical literature that can satisfy all of the observational constraints. We feel that this method is an important step in the right direction for further discussion of the 2175 A feature.