Title: Reading Between the Bumps: Unexplained Features in Optical Extinction Curves

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. Extinction features tend to be categorized based on width as they probe different carriers. 

These extinction features include narrow interstellar absorption features from Na, Ca and K and diffuse interstellar bands (DIBs) with widths on the scale of ~0.1-1.0 nm. On the intermediate scale, the well-known 2175A with a width of ~427A is thought to originate from carbonaceous grains. Massa et al 2020, reported three new intermediate-scale structures (ISS). Their analysis found that strength two of the three of these ISS features correlate with the 2175A bump strength, but none were correlated with the total-to-selective extinction ratio, R(V). \

Since then, three additional candidate ISS features were identified in the literature at 0.770, 0.540, 0.850 micron, all with widths greater than any known DIB. To help identify their carriers, I used 74 different lines-of-sight to study the three original ISS features as well as the candidate features. Furthermore, in this study I identified 10 more candidate ISS features observed in these sightlines. Lastly, I present an analysis of all these ISS features on how they relate to the extinction curve parameters, to each other and to the 2175A bump feature.