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New aspects of absorption line formation in intervening turbulent clouds - III. The inverse problem in the study of H+D profiles

A new method, based on a Reverse Monte Carlo technique and aimed at the inverse problem in the analysis of interstellar (intergalactic) absorption lines is presented. We consider the process of line formation in media with a stochastic velocity field accounting for the effects of a finite correlation length (mesoturbulence). This approach generalizes the standard microturbulent approximation, which is commonly used to model the formation of absorption spectra in turbulent media. The method allows to estimate from an observed spectrum both, the physical parameters of the absorbing gas, and an appropriate structure of the velocity distribution parallel to the line of sight. The procedure is applied to a template H+D Ly-alpha profile which reproduces the Burles and Tytler [BT] spectrum of the quasar Q 1009+2956 with the DI Ly-alpha line seen at the redshift z = 2.504. The results obtained favor a low D/H ratio in this particular absorption system, although the inferred upper limit for the hydrogen isotopic ratio of about 4.6 10^{-5} is slightly higher than that of BT [log (D/H) = -4.60 +/- 0.08 +/- 0.06,i.e. D/H <= 3.47 10^{-5}]. This revision of the BT data in the framework of the mesoturbulent model leads to limits on D/H consistent with standard big bang nucleosynthesis [SBBN] predictions and observational constraints on both extra-galactic 4He mass fraction Y_p (Izotov et al. 1997) and 7Li abundance in the atmospheres of population II (halo) stars (Bonifacio and Molaro 1997). SBBN restricts the common interval of the baryon to photon ratio ηfor which there is concordance between the various abundances to 3.9 10^{-10} <= η<= 5.3 10^{-10}. It implies that 0.014 <= Ω_b h^2 <= 0.020, where h is the Hubble constant in units 100 km/s/Mpc.