2004ApJ...609..838W

We analyze interstellar absorption observed toward two subdwarf O stars, JL 9 and LS 1274, using spectra taken by the Far Ultraviolet Spectroscopic Explorer (FUSE). Column densities are measured for many atomic and molecular species (H I, D I, C I, N I, O I, P II, Ar I, Fe II, and H₂), but our main focus is on measuring the D/H ratios for these extended lines of sight, as D/H is an important diagnostic for both cosmology and Galactic chemical evolution. We find D/H=(1.00±0.37)x10^–5 toward JL 9 and D/H=(0.76±0.36)x10^–5 toward LS 1274 (2 σ uncertainties). With distances of 590±160 and 580±100 pc, respectively, these two lines of sight are currently among the longest Galactic lines of sight with measured D/H. With the addition of these measurements, we see a significant tendency for longer Galactic lines of sight to yield low D/H values, consistent with previous inferences about the deuterium abundance from D/O and D/N measurements. Short lines of sight with H I column densities of logN(HI)<19.2 suggest that the gas-phase D/H value within the Local Bubble is (D/H)_LBg=(1.56±0.04)x10^–5. However, the four longest Galactic lines of sight with measured D/H, which have d>500 pc and logN(HI)>20.5, suggest a significantly lower value for the true local disk gas-phase D/H value, (D/H)_LDg=(0.85±0.09)x10^–5. One interpretation of these results is that D is preferentially depleted onto dust grains relative to H and that longer lines of sight that extend beyond the Local Bubble sample more depleted material. In this scenario, the higher Local Bubble D/H ratio is actually a better estimate than (D/H)_LDgfor the true local disk D/H, (D/H)_LD. However, if (D/H)_LDgis different from (D/H)_LBgsimply because of variable astration and incomplete interstellar medium mixing, then (D/H)_LD=(D/H)_LDg.