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 Å is doubled in width and moved to a wavelength about 80 Å 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 Å which puts the peak at 2,200 Å (the same position as the mean interstellar absorption peak³), the width of absorption by quasi-icosahedral particles is too large to fit the data.