Cardelli continues his study of the nature of interstellar dust through the analysis of UV extinction. As part of this work, he has found that variations in the width of the 2175Å extinction bump appear to be related to specific environments, with regions of bright nebulosity exhibiting narrow bumps while cold dark clouds appear to be represented by fairly broad bumps. One argument is that broad bumps are due to surface coatings on graphite grains which are subsequently removed by radiation yielding narrow bumps in regions of bright nebulosity. This behavior has been further quantified by new results that show that the width of the bump is strongly linearly correlated with the fractional abundance of the molecule CH as well as H.
Cardelli continues his study of the characteristics of UV extinction in extremely low density interstellar gas using distant supergiants as probes. A major result thus far is that the 2175 Å bump in low density gas appears to be fairly narrow, much like in regions of bright nebulosity. However, the bumps are probably narrow because the processes for broadening the bump (surface coatings) are probably very inefficient at low density. In addition, this is likely aided by processes that erode surface coatings which are likely to dominate in low density gas. Work on ISM dust is supported in part by NASA grants NAG5-832 and NAGW-4607.