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Variations in the D/H Ratio of Extended Sight Lines from Far Ultraviolet Spectroscopic Explorer Observations

We use new FUSE data to determine the column densities of interstellar D I, N I, O I, Fe II, and H2 along the HD 41161 and HD 53975 sight lines. Together with N(H I) from the literature, we derive D/H, N/H, and O/H ratios. These sight lines have log N(H ) > 21.00 and probe gas up to ~1300 pc, allowing us to determine the gas-phase D/H ratio in a hydrogen column density range, log N(H) > 20.70, where the only five measurements available in the literature yield a weighted average of D/H = (0.86 ± 0.08) × 10-5. We find D/H = (2.14) × 10-5 along the HD 41161 sight line. This ratio is ~3 σ higher than the weighted mean D/H ratio quoted above, while the D/H ratio for the HD 53975 line of sight, D/H = (1.02) × 10-5, agrees within the 1 σ uncertainties. Our D/H measurement along the HD 41161 sight line presents the first evidence of variations of D/H at high N(H). Our result seems to indicate either that the long sight lines that, according to the deuterium depletion model, are dominated by cold undisturbed gas where deuterium would be depleted onto carbonaceous grains occur at higher N(H) than previously thought, or that the clumping of low D/H values in the literature for the long sight lines has another explanation. Both of the O/H ratios derived here, (9.12) × 10-4 and (5.37) × 10-4 (for HD 41161 and HD 53975, respectively), are higher than what has been found by other authors. Finally, we derive (N/H) × 105 = 8.32 and 5.07, and (D/O) × 102 = 2.29 and 1.91, for HD 41161 and HD 53975, respectively. In addition, the relatively high signal-to-noise ratio of the HD 41161 data allows us to place constraints on the f-values of some neutral chlorine transitions present in the FUSE bandpass for which only theoretical values are available.