Two New Low Galactic D/H Measurements from theFar Ultraviolet Spectroscopic Explorer

Abstract

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) × 10-5 toward JL 9 and D/H = (0.76 ± 0.36) × 10-5 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 log N(H ) 500 pc and log N(H ) > 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 interstellar medium mixing, then (D/H)LD = (D/H)LDg.