THE DIFFUSE INTERSTELLAR BANDS

The diffuse interstellar bands are absorption features observed in the spectra of stars seen through significant column densities of interstellar material. Of the 127 confirmed DIBs in the optical region between 0.4 and about 1.3 mu m, only 2 have (tentatively) been identified with a specific carrier. Because DIB strengths increase roughly in proportion to color excess, they were originally suspected of being produced on or in the interstellar grains, but current evidence favors some species of free polyatomic molecules, either neutral or ionized. DIBs are conspicuously broader than the atomic interstellar lines, having widths at half-depth ranging from 0.8 to about 30 Angstrom. These widths are presumably due to unresolved rotational structure, possibly compounded by lifetime broadening of the upper state. Recent proposals that C-60, some members of the PAH family, or polycarbon chains are responsible for the DIB spectrum are either not supported by observation or await better data.