Abstract
The Lyα line transfer in an extremely thick medium of neutral hydrogen is investigated by adopting an accelerating scheme in our Monte Carlo code to skip a large number of core or resonant scatterings. This scheme reduces computing time significantly, with no sacrifice in the accuracy of the results. We applied this numerical method to the Lyα transfer in a static, uniform, dustless, and plane-parallel medium. Two types of photon sources have been considered; the midplane source and the uniformly distributed sources. The emergent profiles show double peaks and absorption trough at the line center. We compared our results with the analytic solutions derived by previous researchers and confirmed that both solutions are in good agreement with each other. We investigated the directionality of the emergent Lyα photons and found that limb brightening is observed in slightly thick media, while limb darkening appears in extremely thick media. The behavior of the directionality is noted to follow that of the Thomson scattered radiation in electron clouds, because both Lyα wing scattering and Thomson scattering share the same Rayleigh scattering phase function. The mean number of wing scatterings just before escape is in exact agreement with the prediction of the diffusion approximation. The Lyα photons constituting the inner part of the emergent profiles follow the relationship derived from the diffusion approximation. We present a brief discussion on the application of our results to the formation of Lyα broad absorption troughs and P Cygni type Lyα profiles seen in the UV spectra of starburst galaxies.
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