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A. Li, B.T. Draine (Princeton University Observatory)
A substantial fraction -- ~1/3 -- of the observed emission from interstellar dust in diffuse clouds is at wavelengths \lambda\lesssim 50\mum. This short-wavelength component of the emission is attributed to relatively small grains which undergo ``temperature spikes" following single-photon heating events. The observed emission contains valuable information constraining both the composition and the size distribution of interstellar dust.
We examine models consisting of two grain types -- amorphous silicate grains and carbonaceous grains -- each with an extended size distribution. The emission depends on the size distributions and on the physical properties of the grains -- cross sections for absorbing starlight photons, heat capacities, and cross sections for thermal emission of infrared photons. We assume that the carbonaceous grain population extends from grains with graphitic properties at radii a \gtrsim 0.01\mum, down to particles with PAH-like properties at very small sizes. The optical properties of the carbonaceous grains -- from the ultraviolet to the infrared -- are based on laboratory studies. The optical properties of the PAH-like particles include the prominent IR emission features at 3.3, 6.2, 7.7, 8.6, and 11.3\mum with the effects of ionization taken into account. The size distributions are constrained to be consistent with observed interstellar extinction curves and with observed depletions of C, Si, Fe, and other elements.
Since the temperature of a given grain is time-dependent, the temperature distribution function for each grain type is required. These temperature distribution functions have been calculated, including both quantized heating and cooling. The resulting emission spectra are compared to observations from IRAS, COBE/FIRAS, COBE/DIRBE, IRTS, and ISO. Adjusting the size distribution of the ultrasmall grain component, we are able to reproduce the observed emission from the diffuse interstellar medium with about 9% of the total C abundance contained in the very small (a < 20Å) grains. Furthermore, the maximum fraction of silicates which can be present in very small particles is limited by the fact that the 10\mum silicate emission feature is not detected in emission. Our model spectra thus allow us to place an upper limit on the abundance of a < 20Å\ silicate grains.
This research was supported in part by NASA grant NAG5-7030 and NSF grant AST-9619429.