Interstellar Dust—Graphite, Iron and Silicate

Abstract

WE suggested recently¹ that the observed interstellar extinction curve may be accounted for by a mixture of graphite, iron and silicate grains. Duley² has criticized our suggestion, dismissing the coincidence between our theoretical curves and the observations as fortuitous. We wish here to point out certain errors in his comments and also to present additional evidence in favour of our model. To begin with, we wish to point out that Duley has misquoted us in stating that we used a constant refractive index (n-ik) for iron at wavenumbers 4 µm⁻¹<λ<7µm⁻¹. We explicitly stated in our paper that we assume a constant conductivity (σ=2nk/λ), not constant n and k. The validity of this assumption for transition metals (which include iron) is evident in the data available for nickel over the energy range 3–12 eV (ref. 3 and see also the caption to Fig. 1).