Interstellar Obscuration

Abstract

AN accurate extinction curve with a standard error of ± 0.008^m which has been derived for a region in the direction of Cygnus (looking along the spiral arm) from spectrophotometric observations at the Royal Observatory, Edinburgh¹, shows a pronounced discontinuity in slope at 4300 Å. This feature as well as recent rocket observations² rule out the possibility that the interstellar dust is made up of iron or ice particles. Hoyle and Wickramasinghe³ have suggested that graphite particles, formed on the surface of carbon stars, could be ejected into interstellar space by radiation pressure, and Wickramasinghe⁴ finds that such carbon particles may become covered with ice mantles. The wave-length 4300 Å is close to that at which the refractive index of graphite particles begins to change, the extinction of graphite being remarkably close to a 1/λ law in the region 0.8 ⩽ λ⁻¹ ⩽ 2.3 and thereafter deviating from such a law more or less according to particle size⁵. Nandy and Wickramasinghe⁶ have shown that the observed extinction curve for Cygnus can be well reproduced by the assumption of graphite cores with radii less than ∼ 0.06μ covered with ice mantles.