On the Absorption and Emission of Light by Interstellar Grains

The Kramers-Kronig relations are applied to the interstellar medium, which for this purpose is considered to be a vacuum sparsely populated by spheroidal grains. An integral over the spectrum of the observed extinction is thereby related to the static dielectric constant of the medium. This yields at once a lower bound Ofl the fraction of the volume occupied by the grains. The same method is used to discuss the far-infrared ernissivity of grains. The low-frequency emissivity of a grain cannot be arbitrarily in- creased by the inclusion of low-frequency oscillators An upper bound on the attainable emissivity is derived, and from it a lower bound is derived on the equilibrium temperature of grains of a given size and shape. I. INTRODUCTIO