The Deuterium Abundance atz = 0.701 toward QSO 1718+4807

We present constraints on the deuterium-to-hydrogen ratio (D/H) in the metal-poor gas cloud at redshift z = 0.701 toward QSO 1718+4807. We use new Keck spectra in addition to Hubble Space Telescope (HST) and International Ultraviolet Explorer spectra. We use an improved redshift and a lower H I column density to model the absorption. The HST spectrum shows an asymmetric Lyα feature that is produced either by H I at a second velocity or by a high abundance of D. Three models with a single simple H + D component give 8 × 10-5 < D/H < 57 × 10-5 (95%), a much larger range than reported by Webb et al. A more sophisticated velocity distribution or a second component is necessary for lower D/H. With two components, which could be a part of one absorbing structure or separate clouds in a galaxy halo, we find D/H < 50 × 10-5. We do not know if this second component is present, but it is reasonable because 40%–100% of absorption systems with similar redshifts and H I column densities have more than one component.