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 find { D / H } = ( 1.00 \pm 0.37 ) \times 10 ^ { -5 } towards JL 9 , and { D / H } = ( 0.76 \pm 0.36 ) \times 10 ^ { -5 } towards LSS 1274 ( 2 \sigma 
uncertainties ) .
With distances of 590 \pm 160 pc and 580 \pm 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 \pm 0.04 ) \times 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 \pm 0.09 ) \times 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 } } .