EFFECTS OF INTERNAL DUST ON THE NARROW-LINE REGION LYMAN AND BALMER DECREMENTS

We present detailed calculations on the effects of internal dust on th e Balmer and Lyman decrements for a spherically symmetric distribution of low covering factor clouds photoionized by a power law. In the fir st stage, we assume an open geometry with clouds represented as slabs and consider the effects of Lyalpha resonance and absorption by dust. We consider the important effects of perspective on the emergent fluxe s, which in our simplified scheme present either the photoionized face to the observer (''f'') or the back (''b'') face. We adopt canonical values for the gas excitation (U(f)) and for the ionizing energy distr ibution (alpha = -1.4, F(nu) is-proportional-to nu(+mu)). After review ing the effects of separately varying the gas density in the dust-free case, we adopt a single representative density of 5000 cm-3 in all ou r calculations and proceed to compute sequences of photoionization mod els in which the relative internal dust content by mass (mu) is progre ssively increased to values comparable to the local ISM (i.e., mu = 1) . Although we always adopt solar metallicity for the gas + dust phases , as we increase mu we deplete the gas phase abundances in accordance with current knowledge of the depletion indices following a procedure described in Appendix A. We find that for moderate amounts of internal dust mu = 0.2-0.3, radiation-bounded clouds result in Lyman and Balme r decrements in the range 32 37 and 3.0-3.1, respectively. To cover th e case in which the clouds are more massive than implied by the condit ion of being radiation-bounded, we also consider adding a neutral gasdust zone of column density N(H0)0 beyond the partially ionized zone. Our main result, valid for the calculations with standard NLR input pa rameters and for an open geometry in which multiple cloud covering is negligible, is that even with internal dust, while Halpha/Hbeta is red dened, Lyalpha/Hbeta turns out not very different from recombination c ase B. The Seyfert 2 observations show much lower Lyman decrements tha n predicted from the spherically symmetric model and a critical study is made of various explanations: dust- or line-scattering screens, a s emiopen geometry or a fully closed spherical geometry. We find that Ly alpha/Hbeta and Halpha/Hbeta obtained from calculations using a semiop en geometry can span the region covered by the Seyfert 2 observations. We also study the possibility that dust reddening of the continuum ma y account for the apparent deficit of ionizing photons seen in many Se yfert 2's. This reddening may be present in addition to, or in place o f, the anisotropic beaming/occultation of ionized radiation (cf. occul tation/reflection picture'') that is generally invoked to explain the deficit.