Diffuse Interstellar Bands in Individual Sightlines

Abstract

This paper reviews briefly the history of observations of diffuse interstellar bands (DIBs) — features covering the spectral range from the far UV to near IR — in individual sightlines, and describes their relations to other spectroscopic features originating in the same clouds. Four different kinds of inter- and circumstellar clouds are briefly characterized. DIB intensity ratios are not constant between ISM clouds, thus several carriers are responsible for the whole DIB spectrum. Several recent surveys have indicated the existence of hundreds of very weak interstellar features — the patterns of which are clearly related to the well-known, strong DIBs. These patterns may facilitate the identification of the DIB carriers, which most likely are big molecules or very small dust grains. In single cloud cases, the DIB intensity ratios clearly split into 3–4 groups, most probably characterizing different cloud environments. The ISMs of single OB stellar associations probably are quite homogeneous — the DIB spectra of all members of such aggregates are similar. The DIB intensity ratios apparently correlate with the shapes of vacuum UV extinction curves, as well as with absorption line strengths of simple interstellar molecules when individual clouds are considered. The patterns of the recently discovered weak DIBs as well as the strong DIB ratios suggest that more than one cloud and of different optical parameters exist along most of individual sightlines. The Be stars, characterized by the emission in the HeI D 3 line, are apparently immersed in circumstellar shells that do not contain the DIB carriers. The carriers are apparently absent or extremely underabundant also in high velocity clouds.KeywordsLine StrengthInterstellar CloudDiffuse BandInterstellar ExtinctionExtinction CurveThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.