Enstatite Grains and the 2200 Å Interstellar Extinction Feature

Abstract

THE discovery of a hump in the interstellar extinction curve at λ≃2200Å by Stecher¹ and the subsequent confirmation and extension of this observation (refs. 2, 3 and R. C. Bless at the Fourteenth IAU General Assembly, Brighton, 1970) have led to an extensive discussion of the role of graphite particles in interstellar space. A band centred at the same wavelength occurs in the theoretical extinction curves for graphite particles over a wide range of particle sizes. The band in graphite arises from a transition of π-electrons to the conduction band of graphite. Fig. 1 shows the rocket ultraviolet extinction data for several stars and Fig. 2 shows theoretical graphite extinction curves for size distributionswith median sizes r_c=0.04−0.07 µm, where n(r)dr stands for the number density of grains in the radius range r, r+dr. The, results in Fig. 2 are not sensitive to the particular form of n(r). It has been shown that the shape and wavelength location of this absorption band is similar to the observed band for a wide range of single sizes and size distributions⁴. This independence (or very weak dependence) on size is a satisfactory feature of the graphite model because the astronomical data available for several stars in widely separated galactic regions all exhibit the same central wavelength for the band. Although a similar feature occurs in some organic materials, notably in coals and hydrocarbon compounds containing unsubstituted benzene rings, their abundance in space is scarcely likely to prove a serious rival to graphite.