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Astron. Astrophys. 322, 633-645 (1997) 2. Current theoretical models
Detailed theoretical comparisons between electromagnetic scattering
models involving small graphite grains and the interstellar UV feature
are complicated by the fact that graphite is a highly anisotropic
material. In particular, an approximation must be used when
computations involving Mie theory (valid only for isotropic spheres)
are carried out. The MRN model (Mathis et al. 1977) and its variants,
using a size distribution of spherical grains of silicate and
graphite, can explain the mean extinction curve, but fail to satisfy
the above observational constraints. Draine (1988) has studied the UV
feature produced by graphite particles using the discrete dipole
approximation (DDA). This method is ideally suited for handling the
anisotropic dielectric tensor of graphite. He found that only
particles of small elongation with equivalent radii in the range
100-200 Draine & Malhotra (1993) studied the variations in peak position, width, and strength of the bump for various models based on a size distribution of graphite grains to see whether they were compatible with the observational constraints. DDA calculations included spherical graphite grains with an ice coating, spheroidal graphite grains, and graphite spheres in contact with silicate spheres. All models using variations in shape or coating produced correlations between the peak position and width. Therefore, they concluded that the variations observed must be due to changes in the dielectric properties of the grains either through impurities or surface effects, rather than purely "geometric" effects. Mathis (1994) considered a model consisting of a graphite oblate
spheroidal core and a coating of material represented by an
appropriately chosen single Lorentz oscillator. The weakness of this
model lies in the fact that the shape of the grain had to be
unreasonably fine-tuned in order to reproduce the stability in peak
position of the UV feature. Having a coating to broaden the bump and
eliminate correlations between its width and its peak position can be
considered a second order effect, the overall shape of the grain being
the first order effect. Therefore, implicit in this model is the
unlikely assumption that all interstellar grains along every
possible line of sight have exactly the same shape. Any
deviation in shape shifts the peak position outside the observed
range. Furthermore, variations in shape introduce a correlation
between the strength, the width, and the peak position of the feature.
Since the width and peak position of the narrowest interstellar UV
features are narrower and shifted to smaller wavenumbers,
respectively, compared to those of Rayleigh graphite grains, Mathis
had to "tinker" with the dielectric function Henrard et al. (1993) considered a model in which the bump carrier was assumed to consist of small spherical onion shells of graphite. Again, an unreasonable fine-tuning in the number of shells was required to reproduce the peak position of the interstellar UV feature. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1997 Online publication: June 5, 1998 ![]() |