The effects of irregularities of internal structure in determining the ultraviolet properties of interstellar grains

Abstract

Whereas data for the extinction of starlight in the visible show the interstellar grains must be partially hollow, data in the ultraviolet show the vesicular interiors must be irregular. In particular, the absorbing materials responsible for excess extinction over a band centred at ∼2175 Å are required to be concentrated in irregularly distributed chromophores with dimensions of the order of or less than one-tenth of the whole grain.

The laboratory measurements of absorptions produced by microorganisms agree very closely with astronomical observations for a large number of early-type stars. Since the interior structures of microorganisms are indeed highly irregular, the laboratory measurements made with microorganisms suspended in a fluid, can reasonably be transferred to microorganismsin vacuo.

The characteristic dimension for the scattering of visible light by rod-like grains has in the past been taken to be rod diameters. An alternate interpretation with the characteristic dimension taken instead to be rod-oengths may turn out to have advantages in respect of data in the ultraviolet, as well as agreeing in scale with bacteria-like objects found in meteorites, and possibly also, in particles entering the Earth's atmosphere from the zodiacal cloud.