On the nature of the Broad Line Region clouds

Abstract

The possibility that the broad line region (BLR) clouds in quasars and AGN may be irradiated stellar chromospheres is examined. This possibility is supported by the fact that stars are known to have peak densities in galactic nuclei and by data from symbiotic stars which show that cool stars irradiated by an ultraviolet source yield spectra with line ratios similar to those of AGN. Other evidence in favor of the theory includes the fact that stellar chromospheres do not require an external confining medium and the way in which line profiles can be accounted for by emission from parabolically orbiting objects of constant cross section. The main problem with this theory is that irradiated stars possess much more mass than traditional BLR models because only a small proportion of the stellar mass actually lies in the irradiated chromosphere. It is suggested that the irradiated stars must be dwarfs which have 'bloated' in response to the radiation field rather than normal giants.