Interstellar Dust—Reply to Duley's Criticisms

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DULEY' second1 criticism of our hypothesis of graphite-iron-silicate grain mixtures2,3 contains further errors and misrepresentations of our statements. We wish to refer here to a few crucial facts which seem to have been overlooked by Duley1,4 and which invalidate his contention that the interstellar extinction curve does not provide additional evidence to support either graphite or silicate grains. The suggestion that a fit to the observed extinction curve on the basis of graphite, iron and silicate grains is confined to the adoption of the special set of values 0.06, 0.02 and 0.15 µm for their respective mean radii is incorrect. In our original paper2 we stated clearly that r (graphite)=0.045–0.07 µm, r (iron)≤0.02 µm and r (silicate)=0.15–0.18 µm are the requirements necessary to obtain good agreement with the observations. A wide range of relative proportions of the three grain species and several different types of size distributions2,3,5,6 have been shown to provide satisfactory fits to the available astronomical data. It should be stressed that the optical constants of graphite are such that the observed optical extinction curve and the ultraviolet hump at λ2200 A could be reproduced for a wide range of particle sizes. Iron and silicates in our model play a less important part at optical wavelengths so that constraints on their sizes and proportions relative to graphite are less rigorously specified by the optical extinction data.