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Astron. Astrophys. 322, 633-645 (1997)

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7. Conclusion

In this paper, constraints on the possible clustering state and dielectric function of the interstellar UV feature carrier were derived using various models.

Compact clusters of graphite spheres were shown to satisfy qualitatively the observational constraint that the UV bump peak position is very stable while its width exhibits a wide range of values. The main drawbacks of this model were that the peak position was wrong, and that it did not allow for variations in chemical composition. The latter is essential to account for the fact that variations in peak position and width are not correlated, except for the widest bumps (observed in dense interstellar regions where clustering of the grains is expected).

To remedy this, a single-Lorentz oscillator model in conjunction with clustering was considered. It was shown to satisfy the observational constraints, including the (correct) correlated shift in peak position observed for very wide bumps.

But since the UV bump is usually assigned to some carbonaceous material, then the carrier of this feature must also produce a FUV feature, the tail of which would be the FUV curvature observed along most lines of sight. A relatively simple two-Lorentz oscillator model was shown to reproduce the extinction along various lines of sight for a wide range of FUV curvatures. However, in this model, clustering shifted the peak position to smaller wavenumbers for the widest bumps, contrary to what is observed. This model allowed a derivation of some optical constants of bump grains along specific lines of sight, although the solution was not unique. Thus, more measurements of the FUV part of the spectrum of carbonaceous interstellar dust analogues would be most desirable to further constrain the model.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998