STRUCTURE OF THE OUTER REGIONS OF ACCRETION DISKS IN ACTIVE GALACTIC NUCLEI - THE INFLUENCE OF OPACITY

In order to complement another paper (Hurt et al. 1994, hereafter refe rred to as Paper 1) where is studied the structure of the outer region s of accretion discs, located at distances between 10(2) and 10(4) gra vitational radii, we discuss here the physical mechanisms governing th e thermal and chemical equilibrium of a mixture of atoms, ions and mol ecules at densities and temperatures relevant for the disc. We show th at LTE is reached for all important species. We discuss the role of op acity in the energy equation. We show that the Rosseland and the Planc k mean can differ by several orders of magnitudes. The influence of di fferent choices of opacities is illustrated using a reference disc mod el (called ''canonical model'' in Paper 1, and representing adequately a Seyfert 1 nucleus), and it is shown that a treatment including all opacity sources is required to get a correct solution in the whole ran ge of radii. We also show that the mean molecular weight should be com puted self-consistently.