General constraints on the average scattering characteristics of interstellar grains in the ultraviolet

Abstract

The sizes and optical properties of interstellar grains broadly determined by the full interstellar extinction law extending from infrared to ultraviolet wavelengths lead to rather strict constraints on the average scattering characteristics of grains in the ultraviolet. The presence of relatively large (radii ∼0.1–0.2, μm) grains implied by the visual extinction is the key to determining the maximum and minimum values of the average albedo |$\overline \alpha$| and the scattering asymmetry |$\overline g$| in the ultraviolet. Additional restrictions on the values of |$\overline \alpha$| and |$\overline g$| are derived from the most basic physical properties of the small particle population. This procedure produces a range of possible pairs of |$\overline \alpha$| and |$\overline g$| at each UV wavelength, which in turn provides a basis for critical evaluation of grain models and results derived from direct measures of scattered light.