New developments of the coal model of interstellar dust

Abstract

No individual, isolated molecule has yet been found to exhibit simultaneously all the IR, so-called unidentified, bands (UIBs) emitted by carbon-rich nebulae. By contrast, coal or, rather, its main, insoluble, organic constituent, termed kerogen, which is a solid material, exhibits all these bands as well as the accompanying continuum. We report here new experiments which show that coal also carries the π-resonance associated with the UV absorption band (2175 Å). All these features of coal can be understood on the basis of its known composition (mainly C, H, O) and complex and inhomogeneous structure (disordered stacks of graphitic planes, linked by oxygen bridges and carrying various hydrocarbon functional groups), which depend only on its age, not on its place of origin. Even the large variations in the relative intensities of the UIBs from nebula to nebula are paralleled by the coal behaviour and can be understood in terms of H and O concentrations.

We have also measured the spectral emissivity of coal grains, which is very high in the visible and near IR, and quite low in the mid- and far-IR. In addition, the emissivity increases as a function of temperature. As a result, the equilibrium temperature of coal grains, in a typical nebula, should be relatively low and weakly sensitive to the radiation field intensity, 200–500 K, which is precisely the required temperature range for the emitted spectrum to match nebular spectra. Thus, in most instances, non-equilibrium heating by single, UV photons should not be necessary.