Application of line transfer calculations to active nuclei and novae

We apply Monte Carlo calculations to realistic nebulae in order to study the effects of internal dust and level-2 population on the observed spectrum. We show that severe Lα trapping is obtained in all cases, and penetration of line photons to optical depths as large as 107 is common. We use photoionization calculations to mimic the photon generating function in quasars and other gaseous nebulae, and show that level-2 population is concentrated deep in the cloud, due to trappping and some collisional excitation of Lα. We calculate the Balmer optical depth and give simple formulae to deduce it in different situations. The optical depth tau/sub Halpha/ can reach a very large value, but an effective optical depth 8--15 times smaller must be used when calculating Balmer self-absorption. We use our results to study the collisional excitation of Balmer lines, the level-2 ionization by Lα, and thermalization. The last cannot amount to more than approx.30% destruction of Lα under typical active nuclei conditions (N/sub e/<1010 cm-3) and cannot therefore explain the anomalous observed Lα/Hβ. we study the destruction of Lα and some metallic lines like C IV lambda1549 in nebulae containing dust, and give their destruction as a function of both the dust and the line optical depths. Severe destruction of Lα can occur, but observed line ratios like He II lambda1640/Lα suggest that the amount of internal dust in QSOs is small; and if it is the only attenuation mechanism, then Lα/Hβ> or approx. = 15. External dust may be more important. Synthetic QSO spectra are shown for cases of internal dust with different dust/gas ratios. In an appendix we apply some results to novae, and show that they can nicely explain the observed Hα/Hβ and O I lambda8446 in V1500 Cygni. The calculations demonstrate the importance of using realistic ionization structure and dust properties in investigating line trapping and destruction