A VAPID analysis of interstellar lithium in the ζ Oph sightline

ABSTRA C T We present observations of the Li I 6708 Adoublet in the z Oph sightline, obtained at a resolution of , 10 6 and a signal-to-noise ratio of , 1200, together with supplementary observations of K I l7699 and the Na I l3302 doublet. These observations marginally resolve the main ' 2 15 km s 21 ' 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 7 Li/ 6 Li 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.