Two New Low Galactic D/H Measurements from FUSE

We analyze interstellar absorption observed towards two subdwarf O stars, JL 9 and LSS 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 H 2 ), 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 towards JL 9, and D / H = (0 . 76 ± 0 . 36) × 10 − 5 towards LSS 1274 (2 σ uncertainties). With distances of 590 ± 160 pc 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 log N (H I) < 19 . 2 suggest that the gas-phase D/H value within the Local Bubble is (D / H) LBg = (1 . 56 ± 0 . 04) × 10 − 5 . However, the four longest Galactic lines of sight with measured D/H, which have d > 500 pc and log N (H I) > 20 . 5, suggest a significantly lower value for the true local-disk gas-phase D/H value, (D / H) LDg = (0 . 85 ± 0 . 09) × 10 − 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) LDg for the true local-disk D/H, (D / H) LD . However, if (D / H) LDg is different from (D / H) LBg simply because of variable astration and incomplete ISM mixing, then (D / H) LD = (D / H) LDg .