Radiative transfer effects in primordial hydrogen recombination

The calculation of a highly accurate cosmological recombination history has been the object of particular attention recently, as it constitutes the major theoretical uncertainty when predicting the angular power spectrum of Cosmic Microwave Background anisotropies. Lyman transitions, in particular the Lyman-alpha line, have long been recognized as one of the bottlenecks of recombination, due to their very low escape probabilities. The Sobolev approximation does not describe radiative transfer in the vicinity of Lyman lines to a sufficient degree of accuracy, and several corrections have already been computed in other works. In this paper, the impact of some previously ignored radiative transfer effects is calculated. First, the effect of Thomson scattering in the vicinity of the Lyman-alpha line is evaluated, using a full redistribution kernel incorporated into a radiative transfer code. The effect of feedback of distortions generated by the optically thick deuterium Lyman-alpha line blueward of the hydrogen line is investigated with an analytic approximation. It is shown that both effects are negligible during cosmological hydrogen recombination. Secondly, the importance of high-lying, non overlapping Lyman transitions is assessed. It is shown that escape from lines above Ly-gamma and frequency diffusion in Ly-beta and higher lines can be neglected without loss of accuracy. Thirdly, a formalism generalizing the Sobolev approximation is developed to account for the overlap of the high-lying Lyman lines, which is shown to lead to negligible changes to the recombination history. Finally, the possibility of a cosmological hydrogen recombination maser is investigated. It is shown that there is no such maser in the purely radiative treatment presented here.Comment: 23 pages, 4 figures, to be submitted to PR

Orion Nebula - High resolution Pa-12 mapping

A detailed map of the Paschen-12 (8750 A) line intensity across the central regions of the Orion Nebula is presented which was derived from observations with a tilting-filter Fabry-Perot interferometer mounted at the Cassegrain focus of a 182-cm telescope. The Pa-12 fluxes are compared with an H-alpha photograph and detailed H-alpha intensity tracings of the same regions in order to determine the variation of absorption in the different areas. Using recombination-line theory, it is found that the H-alpha absorption exceeds that in Pa-12 by 0.60 (plus or minus 0.20) magnitude over most of the visible nebula. The mottled appearance of the nebula is attributed to density variations of the gas inside the H II region and to nonuniformities in the dust distribution. Normalized column densities of the absorbing dust toward the Trapezium are plotted as a contour map, and interesting features in other areas are noted. It is concluded that the reddening law for starlight and nebular emission is the same

Near-infrared observations of the dust coma of Comet Kohoutek (1973f) with a tilting-filter Fabry-Perot photometer

By means of narrow-band Fabry-Perot filters, which exclude the interference from molecular line fluorescence, the brightness of Comet Kohoutek (1973f) has been measured at 8560 and 8748 \uc5. Data reduction on the basis of averaged Mie-scattering cross sections indicates that the dust production rate was different before and after perihelion at the same heliocentric distances. This asymmetry suggests that vaporization and dust entrainment were governed by fractionation of a multicomponent mixture of parent molecules in a comparatively porous cometary nucle

Orion Nebula - Fabry-Perot high resolution O I /8446 A/ mapping

The Fabry-Perot near-infrared spectrophotometer of the 182-cm Asiago telescope was used for absolute flux measurements of the neutral oxygen line at 8446 A in 22 positions across the surface of the Orion Nebula (M42). From the derived O I/H-beta ratios, starlight excitation appears to be the most efficient mechanism for explaining the unexpected strength of this line