THE DIFFUSE INTERSTELLAR BANDS

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