Abstract
We investigate the extinction produced by small carbon particles consisting of aromatic rings in con- -gurations similar to those of polycyclic aromatic hydrocarbon (PAH) molecules, but with varying degrees of hydrogenation, ionization, and defects. Extinction produced by candidate particles is calcu- lated using the discrete dipole array (DDA) formalism with optical constants similar to those of graphite modi-ed to take these changes into account. Clusters of neutral molecules as well as mixed ion-neutral clusters are also investigated. We -nd that certain speci-c carbon structures such as a dehydrogenated coronene-like molecule, exhibit a plasmon-type resonance at the 2175 wavelength of the inter- C 24 , stellar extinction bump. However, we -nd that similar -ts can be obtained using a distribution of mol- ecule shape and sizes containing up to several hundred C atoms. These results suggest that interstellar graphite II 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. Subject headings: dust, extinction E ISM: molecules E ultraviolet: ISM
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