| dc.description |
AN accurate extinction curve with a standard error of ± 0.008<sup>m</sup> which has been derived for a region in the direction of Cygnus (looking along the spiral arm) from spectrophotometric observations at the Royal Observatory, Edinburgh<sup>1</sup>, shows a pronounced discontinuity in slope at 4300 â «. This feature as well as recent rocket observations<sup>2</sup> rule out the possibility that the interstellar dust is made up of iron or ice particles. Hoyle and Wickramasinghe<sup>3</sup> have suggested that graphite particles, formed on the surface of carbon stars, could be ejected into interstellar space by radiation pressure, and Wickramasinghe<sup>4</sup> finds that such carbon particles may become covered with ice mantles. The wave-length 4300 â « 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 î º λ<sup>â 1</sup> î º 2.3 and thereafter deviating from such a law more or less according to particle size<sup>5</sup>. Nandy and Wickramasinghe<sup>6</sup> have shown that the observed extinction curve for Cygnus can be well reproduced by the assumption of graphite cores with radii less than â ¼ 0.06μ covered with ice mantles. |
