| Abstract
| - We describe numerical simulations of hydrogen and deuterium Lyα lines formed in turbulent media with a stochastic velocity field. The line broadening is assumed to be the result of both macroscopic and thermal motions, and the line profile function Φ (Δλ) is a random function of coordinate along the line of sight. This is in contrast to the standard assumption of the microturbulence, where Φ(ΔΔ) is a given quantity and any bulk motions are treated as completely uncorrelated. The randomization of Φ(Δλ) stems from the chaotic character of the velocity field, which results from the Doppler shifts in the absorption coefficient. Numerical results are presented for models with NH1 = 1016, 1017 and 1018 cm−2; D/H = 2 × 105, 4 × 10−3, 10−4 and 3 × 10−4; rms turbulent velocities σ = 8, 12 and 16 km s−1; kinetic temperature Tkin = 5000K; and different velocity correlation lengths. A finite correlation length is shown to affect strongly the intensity ratio of D I to H I Lyα lines. In particular, uncertainties of the D/H ratio determination may be as large as a few orders of magnitude. It is important that the actual D/H ratios may appear to be higher or lower than the values obtained from the standard Voigt-fitting procedure applied to our simulated mesoturbulent H + D Lyα spectra. We suggest that deuterium absorption lines should be observable in the Lytα forest systems with the apparent hydrogen column density, which is obtained in the conventional microturbulent model fitting, NHI micro ∼ 1014−1015 cm−2. The actual HI densities in these systems may be 10-100 times higher if the clouds of the Lyα forest have correlated large-scale motions. An example is presented based on high-resolution observations of Q 1100-264 by Carswell et al.: two Lyα forest lines at z = 2.1029 and 2.1037 can be re-identified as DI and HI Lyα, respectively, at z = 2.1037. We conclude that the precise measurements of the D abundance are actually impossible without thorough investigation of the turbulent characteristics of the gas for each absorption system.
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