We have carried out an investigation of the abundance of deuterium along two extended sight lines through the interstellar medium ( ISM ) of the Galactic disk .
The data include Far Ultraviolet Spectroscopic Explorer ( FUSE ) observations of HD 195965 ( B1Ib ) and HD 191877 ( B0V ) , as well as Space Telescope Imaging Spectrograph ( STIS ) observations of HD 195965 .
The distances to HD 195965 and HD 191877 , derived from spectroscopic parallax , are 794 \pm 200 pc and 2200 \pm 550 pc , respectively , making these the longest Galactic disk sight lines in which deuterium has been investigated with FUSE .
The FUSE spectra contain all of the H i Lyman series transitions ( and the corresponding D transitions ) except Ly \alpha .
The higher Lyman lines clearly show the presence of deuterium .
We use a combination of curve of growth analyses and line profile fitting to determine the D i abundance toward each object .
We also present column densities for O i and N i toward both stars , and H i measured from Ly \alpha absorption in the STIS spectrum of HD 195965 .
Toward HD 195965 we find D/H= ( 0.85 \pm ^ { 0.34 } _ { 0.24 } ) \times 10 ^ { -5 } ( 2 \sigma ) , O/H= ( 6.61 \pm ^ { 1.03 } _ { 1.11 } ) \times 10 ^ { -4 } , and N/H= ( 7.94 \pm ^ { 1.69 } _ { 1.34 } ) \times 10 ^ { -5 } .
Toward HD 191877 we find D/H= ( 0.78 \pm ^ { 0.52 } _ { 0.25 } ) \times 10 ^ { -5 } ( 2 \sigma ) and N/H= ( 6.76 \pm ^ { 2.22 } _ { 1.97 } ) \times 10 ^ { -5 } .
The O i column density toward HD 191877 is very uncertain .
Our preferred value gives O/H= ( 3.09 \pm ^ { 1.98 } _ { 0.98 } ) \times 10 ^ { -4 } , but we can not rule out O/H values as low as O/H= 1.86 \times 10 ^ { -4 } , so the O/H value for this sight line should be taken with caution .
The D/H ratios along these sight lines are lower than the average value of ( 1.52 \pm 0.15 ) \times 10 ^ { -5 } ( 2 \sigma in the mean ) found with FUSE for the local interstellar medium ( \sim 37 to 179 pc from the Sun ) .
These observations lend support to earlier detections of variation in D/H over distances greater than a few hundred pc .
The O/H ratio toward HD 195965 is supersolar .
This star is part of an OB association , so there may be local enrichment by nearby massive stars .
The D/H and O/H values measured along these sight lines support the expectation that the ISM is not well mixed on distances of \sim 1000 pc .
These observations demonstrate that although D/H studies through Lyman absorption may become impractical at d > 2500 pc and log N ( H i ) > 21 , D/H studies in the distance range from 500 to 2500 pc may be very useful for investigating mixing and chemical evolution in the ISM .