Optical , UV , and X - Ray Scattering by Interstellar Dust Grains

Scattering and absorption properties at optical, ultraviolet, and X-ray wavelengths are calculated for an interstellar dust model consisting of carbonaceous grains and amorphous silicate grains. The widely-used Henyey-Greenstein phase function provides a good approximation for the scattering phase function at wavelengths between ∼0.4 and 1µm, but fails to fit the calculated phase functions at shorter wavelengths. For λ < 0.24µm, the Henyey-Greenstein phase function has an absolute error > 50%, and should not be used for accurate modelling in the vacuum ultraviolet. Polarization as a function of scattering angle is calculated for selected wavelengths from the infrared to the vacuum ultraviolet. The calculations employ realistic dielectric functions with structure near X-ray absorption edges, with resulting features in absorption, scattering, and extinction. Differential scattering cross sections are calculated for energies between 0.3 and 10 keV. The median scattering angle is given as a function of energy, and simple but accurate approximations are found for the X-ray scattering properties of the dust mixture, as well as for the angular distribution of the scattered X-ray halo for dust with simple density distributions. Observational estimates of the X-ray scattering optical depth are compared to model predictions. Observations of X-ray halos to test interstellar dust grain models are best carried out using extragalactic point sources.