On the deuterium abundance at z(a)=3.514 towards APM 08279+5255

A very low primordial deuterium abundance of D/H similar or equal to 1.5 x 10(-5) has recently been proposed by Molaro et al. in the Lyman limit syste m with log N-H (i) similar or equal to 18.1 Cm-2 at z(a) = 3.514 towards th e quasar APM 08279+5255. The D/H value was estimated through the standard V oigt fitting procedure utilizing a simple one-component model of the absorb ing region. We have investigated this system using our new Monte Carlo inve rsion procedure which allows us to recover self-consistently both the physi cal parameters of the gas cloud and the projected velocity and density dist ributions along the line of sight. The absorption lines of H I, C II, C IV, Si III, and Si IV were analyzed simultaneously. The result obtained shows a considerably lower neutral hydrogen column density log N-H (i) similar or equal to 15.7 cm(-2). Hence, the measurement of the deuterium abundance cm in this system is rather uncertain. We find that the asymmetric blue wing of the hydrogen Ly alpha 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 concordan ce with D/H similar or equal to 4 x 10(-5).