Graphite, polycyclic aromatic hydrocarbons, and the 2175Å extinction feature

We investigate the extinction produced by small carbon particles consisting of aromatic rings in configurations similar to those of polycyclic aromatic hydrocarbon (PAH) molecules, but with varying degrees of hydrogenation, ionization, and defects. Extinction produced by candidate particles is calculated using the discrete dipole array (DDA) formalism with optical constants similar to those of graphite modified to take these changes into account. Clusters of neutral molecules as well as mixed ion-neutral clusters are also investigated. We find that certain specific carbon structures such as a dehydrogenated coronene-like molecule, C-24, exhibit a plasmon-type resonance at the 2175 Angstrom wavelength of the interstellar extinction bump. However, we find that similar fits can be obtained using a distribution of molecule shape and sizes containing up to several hundred C atoms. These results suggest that interstellar "graphite" may be the high-mass component of the population of PAH seen in interstellar clouds. Such particles could derive from fragments ejected from carbonaceous grains in shocks.