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P. Ehrenfreund (Leiden Obs.)
Superimposed on the interstellar extinction curve are a large number of absorption lines, the diffuse interstellar bands (DIBs). Since the discovery of the first two DIBs in spectroscopic binaries in the 1920's the identification of the DIB carriers remains an important problem in astronomy. Due to their general correlation with dust extiction, the DIBs have been originally attributed to dust particles. Current observational data show that the number of 200 DIBs is still increasing, suggesting that more than 400 DIBs down to the confusion limit could be detected in the interstellar medium. The development of DIB research in recent years indicates that most DIB carriers could be large carbon-bearing molecules which reside ubiquitously in the interstellar gas. Polycyclic aromatic hydrocarbons, fullerenes and carbon chains are among the most promising carrier candidates. The first detection of substructures in the profile of several DIBs also indicates the molecular nature of some DIB carriers. Astronomical surveys performed over a large wavelength range suggest that all measured DIBs do originate from different carriers. However, some DIBs showing similar behaviour in different astronomical environments, may arise from structurally related carriers. An important tool to reveal the nature of the DIB carriers is to study the physical properties of their local environment, such as UV radiation and extinction parameters and to perform correlation studies of DIBs with other molecules and atoms residing in the diffuse interstellar medium. In a combined effort many laboratories, theoreticiens and observational astronomers try currently to solve this long-standing mystery and to identify the carriers of the DIBs.
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