A Detailed Analysis of Intermediate-scale Structure in Optical Extinction Curves: Expanded Census and Two-family Classification

Abstract

The features of interstellar extinction curves serve as powerful diagnostics for interstellar dust, revealing information about its composition, size distribution, and the physical and chemical processes that shape it. D. Massa et al. reported three faint but wide extinction features, termed intermediate-scale structures (ISS) at 4370, 4870, 6300 Å. Since then, three additional ISS features have been reported in the literature at 7700, 5400, 8500 Å, all with widths greater than any known diffuse interstellar band. We present new optical and UV Hubble Space Telescope/STIS spectra for a sample of 24 early-type OB stars. We used these data combined with 50 literature targets for a systematic, homogeneous analysis of ISS features, with the aim of investigating their observational behavior to help constrain their carriers. This analysis revealed nine more candidate ISS features. We also find that ISS features can be arranged into two main families, which we call $\alpha$ and $\beta$, according to correlations between the feature strengths. Finally, we find strong correlations between the 2175 Å bump strength and ISS features at 4353, 4847, 6443, 7710 Å, suggesting their possible carbonaceous origin.

22 pages, 12 figures

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