Measurements of Interstellar Extinction

Abstract

The results of interstellar extinction measurements from the near IR to the far-UV are reviewed. The average interstellar extinction curve for the diffuse cloud medium exhibits a nearly linear rise (in l/λ) from 1 µm⁻¹ to the 2.25 µm⁻¹ “knee” in extinction where the slope changes. In the UV there is a pronounced extinction bump near 4.6 µm⁻¹ (2175 Å) followed by a broad minimum and a steep rise in extinction to the shortest wavelengths for which measurements exist. For wavelengths shortward of about 5500 Å, the interstellar extinction curve exhibits considerable variation in shape from one sight line to another. In addition to strength variations, the width (FWHM) of the 2175 Å extinction bump has been observed to vary by more than a factor of two from 360 to 770 Å with the average width being 480 Å. In contrast, the central position of the feature only varies from 2110 Å to 2195 Å with the average central position being 2175 Å. The most extreme variations in extinction are found at far-UV wavelengths where E(λ–V)/E(B–V) has been found to range from 3 to 12.5 at 1250 Å. In the last few years significant progress has been made in determining various empirical relationships among extinction parameters in the different wavelength regimes and in determining how extinction curve shape changes are influenced by the interstellar environment in which the dust resides. Those relationships are discussed.