2000A&A...355L...1L

A very low primordial deuterium abundance of D/H≃1.5x10^–5 has recently been proposed by Molaro et al. (1999A&A...349L..13M) in the Lyman limit system with logN_HI≃18.1cm^–2 at z_a=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 self-consistently both the physical parameters of the gas cloud and the projected velocity and density distributions along the line of sight. The absorption lines of HI, CII, CIV, SiIII, and SiIV were analyzed simultaneously. The result obtained shows a considerably lower neutral hydrogen column density logN_HI≃15.7cm^–2. 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 HI 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≃4x10^–5.