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A Reverse Monte Carlo Study of H+D Lyman Alpha Absorption from QSO Spectra
Authors:Levshakov  S A  Kegel  W H  Takahara  F
Institution:(1) Institut für Theoretische Physik der Universität Frankfurt am Main, Postfach 11 19 32, 60054 Frankfurt/Main 11, Germany;(2) Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo, 192-03, Japan
Abstract:A new method based on a Reverse Monte Carlo RMC] technique and aimed at the inverse problem in the analysis of interstellar (intergalactic) absorption lines is presented. The line formation process in chaotic media with a finite correlation length (l > 0) of the stochastic velocity field (mesoturbulence) is considered. This generalizes the standard assumption of completely uncorrelated bulk motions (l nequiv 0) in the microturbulent approximation which is used for the data analysis up-to-now. It is shown that the RMC method allows to estimate from an observed spectrum the proper physical parameters of the absorbing gas and simultaneously an appropriate structure of the velocity field parallel to the line-of-sight.The application to the analysis of the H+D Lyagr profile is demonstrated using Burles and Tytler B&T] data for QSO 1009+2956 where the DI Lyagr line is seen at za = 2.504.The results obtained favor a low D/H ratio in this absorption system, although our upper limit for the hydrogen isotopic ratio of about 4.5 × 10-5 is slightly higher than that of B&T (D/H = 30 -0.5 +0.6 × 10-5). We also show that the D/H and N(HI) values are, in general, correlated, i.e. the derived D-abundance may be badly dependent on the assumed hydrogen column density. The corresponding confidence regions for an arbitrary and a fixed stochastic velocity field distribution are calculated.
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