Diffuse Circumstellar Bands

Abstract

A review of searches for diffuse circumstellar bands is presented. There have been several recent articles reporting the detection of diffuse bands in the circumstellar environment. The most important instances are reviewed and discussed, with the conclusion that there remain enough ambiguities in the interpretation of the data to leave the matter uncertain. The largest problem area is in distinguishing the circumstellar material from foreground interstellar material. Because of this uncertainty, the reported detections cannot put hard constraints on the various proposed diffuse band carriers. However, the non-dectection of bands in targets with circumstellar material can and does constrain the carrier theories. The detection of PAHs in targets without diffuse bands implies that neutral PAHs cannot be the carriers. Observations of bands in dark clouds are also reviewed briefly, showing that the bands are weaker in these high density environments. Together, the circumstellar and dark clouds observations present a challenge that must be met by any successful carrier theory.