2020ApJ...891...67M

New Hubble Space Telescope/STIS optical spectra were obtained for a sample of early-type stars with existing International Ultraviolet Explorer UV spectra. These data were used to construct optical extinction curves whose general properties are discussed elsewhere. In this paper, we identify extinction features in the curves that are wider than diffuse interstellar bands (DIBs) but narrower than the well known broadband variability. This intermediate scale structure, or ISS, contains distinct features whose peaks can contribute a few percent to 20% of the total extinction. Most of the ISS variation can be captured by three principal components. We model the ISS with three Drude profiles and show that their strengths and widths vary from one sight line to another, but their central positions are stable, near 4370, 4870, and 6300 Å. The very broad structure (VBS) in optical curves appears to be a minimum between the 4870 and 6300 Å absorption peaks. We find relations among the fit parameters and provide a physical interpretation of them in terms of a simplistic grain model. Finally, we note that the strengths of the 4370 and 4870 Å features are correlated to the strength of the 2175 Å UV bump, but that the 6300 Å feature is not, and that none of the ISS features are related to R(V). However, we verify that the broadband curvature of the continuous optical extinction is strongly related to R(V).