The ultraviolet extinction from interstellar graphitic onions

Abstract

Carbon particles in interstellar dust cause absorption of starlight at certain wavelengths. Kroto and McKay1 have suggested that such particles have the form of quasi-icosahedral spiral shells, with the hexagonal graphitic planes perpendicular to the radius, like the layers of an onion. Using the discrete dipole approximation (DDA)2 I have calculated the ultraviolet extinction curves of small spherical particles with the tensor dielectric constant of graphite but with the c-axis always running parallel to the radius vector. The feature at a wavelength of 2,200 A is doubled in width and moved to a wavelength about 80 A shorter in comparison to the parameters derived from the usual Mie theory approximation for a mixture of particles with scalar dielectric constants. For larger particles, the peak shifts to longer wavelengths, but for a radius of ∼300 A which puts the peak at 2,200 A (the same position as the mean interstellar absorption peak3), the width of absorption by quasi-icosahedral particles is too large to fit the data.