Laboratory formation and analysis of the materials comprising interstellar dust

Abstract

The optical properties of interstellar dust analogs are investigated. Thin, solid, amorphous films are deposited on a substrate through excimer laser ablation. This process is analogous to the vapour deposition of atoms and ions which is thought to occur in stellar outflows and the interstellar medium (ISM).

Refractive indices are calculated for typical polymeric hydrogenated amorphous carbon (HAC) flms. Thickness dependent void structure is shown to influence the resultant density of the substance. Refractive indicies are also determined for amorphous magnesium silicates in both the pyroxene (MgSiO3) and olivine (Mg2SiO4) composites. A plausible dust grain model is constructed which successfully reproduces the major features of the diffuse interstellar extinction.

The effects of thermal annealing and dehydrogenation of HAC are investigated. The resulting graphitized HAC is shown to exhibit a spectral signature which is commonly associated with gas phase polycyclic aromatic hydrocarbons (PAHs). Thermal emission from HAC is observed for the first time and sis shown to be an excellent match to various anomalous unidentified infrared (UIR) emitters. The emission from HAC closely resembles those rare sources labelled Type B UIT emitters in recent works by Geballe (1996) and Tokunaga (1996).

Near threshold laser ionization mass spectroscopy (LIMS) of HAC is shown to produce large ionized molecular clusters including fullerenes. The astrophysical implications of these results are discussed.