Extinction Curves for Graphite Particles coated with Solid Hydrogen

Abstract

MANY attempts have been made recently to match the wavelength dependence of the observed interstellar extinction curves with calculations for theoretical models. These attempts (reviewed in refs. 1 and 2) have met with varying degrees of partial success. It is now generally recognized that a pure dielectric particle will not be able to explain the entire wavelength range of the extinction observations. In particular the very strictly linear portion (0.8µ⁻¹ < λ⁻¹ < 2.3µ⁻¹) of the mean extinction curves of Nandy^3,4 and the continued increase in extinction towards the far ultraviolet⁵ would seem to rule out homogeneous dielectric particles as a significant constituent of interstellar material. Calculations by Stecher and Donn⁶ and by ourselves⁷ have indicated that a size-distribution of pure graphite particles could produce general agreement with observations in the visible and ultraviolet spectral regions up to ∼ 2200 Å, but no agreement is possible further in the ultraviolet. This agreement includes the reproduction of an apparent hump in the observed extinction at ∼ 2400 Å—which Stecher and Donn have referred to as the “signature of graphite”. Of the grain models so far proposed the graphite core–dice mantle grain seems most favourable in being able to reproduce the detailed extinction curves in the visible spectral region as well as the overall features of the extinction up to ∼ 1500 Å (ref. 8). For such particles, however, the hump in the extinction at ∼ 2400 Å characteristic of graphite is washed out because of the effect of the outer ice mantle. If the structure in the extinction curve at the same wavelength is confirmed by future observations, the graphite core–ice mantle grain cannot be regarded as an entirely satisfactory model. A graphite component of the grains is, however, strongly indicated both by this ultraviolet feature as well as by the kink at λ⁻¹ = 2.3µ⁻¹ of the visible extinction observations, the latter being probably associated with a change in the refractive index of graphite. It is therefore interesting to explore the possibility that a graphite core with a dielectric mantle of refractive index different from that of ice may produce better agreement with the observations.