Deuterium toward Two Milky Way Disk Stars: Probing Extended Sight Lines with the Far Ultraviolet Spectroscopic Explorer

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 (B1 Ib) and HD 191877 (B0 V), 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 ± 200 and 2200 ± 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 Lyα. 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, as well as H I measured from Lyα absorption in the STIS spectrum of HD 195965. Toward HD 195965 we find D/H = (0.85) × 10-5 (2 σ), O/H = (6.61) × 10-4, and N/H = (7.94) × 10-5. Toward HD 191877 we find D/H = (0.78) × 10-5 (2 σ) and N/H = (6.76) × 10-5. The O I column density toward HD 191877 is very uncertain. Our preferred value gives O/H = (3.09) × 10-4, but we cannot rule out O/H values as low as O/H = 1.86 × 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 ± 0.15) × 10-5 (2 σ in the mean) found with FUSE for the local interstellar medium (~37-179 pc from the Sun). These observations lend support to earlier detections of variation in D/H over distances greater than a few hundred parsecs. 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 ~1000 pc. These observations demonstrate that although D/H studies through Lyman absorption may become impractical at d > 2500 pc and log N(H ) > 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.