Theoretical atmospheres of stars in the broad-line regions of active galaxies

Model atmospheres for main-sequence stars illuminated by incident power law flux characteristic of the broad-line regions of active galaxies are computed using the ATLAS 6 computer code. Modifications of the temperature-correction algorithm are given to allow improved convergence of atmospheres subjected to these extreme conditions. It is found that while the calculated Ly/sub α//H/sub β/ line ratio matches the observed values fairly well, H/sub α//H/sub β/ is predicted to be ≅0.5 which is low compared to observed values of 3 to 4. While this problem might be rectified by assuming a system of broad-line emitters that is composed of both a stellar and more standard photoionized cloud component, the number of stars required to reproduce the observed flux in H/sub β/ is much too high to be realistic for these regions. This large space density might be reduced by lowering the stellar surface gravity of the models, thus increasing the stellar radius. It was found that cool stars that could be typically found in these regions are better broad-line emitters, and that they go into emission at distances from the central source of continuum radiation that correspond to broad-line regions dimensions deduced form line variability observations