A VAPID analysis of interstellar lithium in the ζ Oph sightline

We present observations of the Li I 6708 Angstrom doublet in the zeta Oph sightline, obtained at a resolution of similar to10(6) and a signal-to-noise ratio of similar to1200, together with supplementary observations of K ilambda 7699 and the Na ilambda 3302 doublet. These observations marginally resolve the main '-15 km s(-1)' system into its two principal clouds; we model the data using standard physical assumptions, but in a statistically rigorous manner, taking fully into account the hyperfine, doublet and isotopic structure for each species, in each cloud, using a new code, vapid. The average Li-7/Li-6 ratio determined in this sightline is 1.12+/-0.20 dex (68 per cent confidence interval), in excellent agreement with the solar-system (meteoritic) value. The ratios in the individual clouds are determined with less precision, but are also consistent with the solar-system value, as is the total lithium abundance (with little evidence for depletion). The thermal and 'turbulent' broadening in the two clouds is discussed on the basis of observed line widths.