On the deuterium abundance at z a = 3 . 514 towards APM 08279 + 5255 ⋆

A very low primordial deuterium abundance of D/H ≃ 1.5 × 10 has recently been proposed by Molaro et al. in the Lyman limit system with logNH i ≃ 18.1 cm at za = 3.514 towards the quasar APM 08279+5255. The D/H value was estimated through the standard Voigt fitting procedure utilizing a simple one-component model of the absorbing region. We have investigated this system using our new Monte Carlo inversion procedure which allows us to recover selfconsistently both the physical parameters of the gas cloud and the projected velocity and density distributions along the line of sight. The absorption lines ofH i, C ii, C iv, Si iii, and Si ivwere analyzed simultaneously. The result obtained shows a considerably lower neutral hydrogen column density logNH i ≃ 15.7 cm. Hence, the measurement of the deuterium abundance in this system is rather uncertain. We find that the asymmetric blue wing of the hydrogen Lyα absorption is readily explained by H i alone. Thus, up to now, deuterium was detected in only four QSO spectra (Q 1937-1009, Q 1009+2956, Q 0130-4021, and Q 1718+4807) and all of them are in concordance with D/H ≃ 4× 10.