Interstellar Grains

Overview Submicron solid particles are dispersed through interstellar gas. These interstellar grains absorb and scatter light, thus shielding some regions from ultraviolet radiation, but also limiting our ability to detect photons which have been emitted by astronomical objects. Dust grains reradiate absorbed energy in the in-frared, thus contributing to the overall emission spectrum of astronomical systems ranging from dusty disks around stars to ultraluminous starburst galaxies. A naked-eye view of the sky from a dark site on a clear summer night reveals dramatic dark patches in the Milky Way. These dark regions are not due to a deficiency of stars – they are instead the result of obscuration by dust clouds interposed between the Earth and distant stars. The obscuration tends to be greater at shorter wavelengths; as a result, the light reaching us from distant, obscured stars is " reddened ". This reddening by interstellar dust can be understood as arising from scattering and absorption by a population of interstellar sub-micron dust grains. The grain population spans a range of sizes, from molecules 1 containing only tens of atoms to particles as large as ∼ 0.3 µm, containing ∼ 10 10 atoms. Most of the grain mass appears to be due to two types of solid, in approximately equal amounts: (1) amorphous silicate mineral, and (2) carbonaceous material. A number of elements – including silicon and iron – are primarily in solid form in the interstellar medium. Approximately 2/3 of the interstellar carbon in diffuse clouds is in solid form (see INTERSTELLAR 1 Since all grains are " molecules " , it is natural to consider small molecules as the small-size end of the overall grain population. It is of course important to characterize the wavelength-dependent interstellar extinction so that astronomical observations can be " corrected " for the obscuring effects of dust. In addition, the infrared emission from dust grains provides a valuable probe of dense regions , and the dust grains themselves play important roles in interstellar chemistry (shielding from ultraviolet radiation, and catalyzing the formation of H 2), interstellar gas dynamics (radiation pressure forces on dust grains, and coupling of charged dust grains to magnetic fields) and heating and cooling of interstellar gas. Dust grains are central to many problems in modern astrophysics. Observational Evidence: Summary There are many different astronomical phenomena which both reveal the existence of inter-stellar dust grains, and provide information allowing us …