2005ApJ...625..210W

We determine the D/H ratio in the interstellar medium toward the DO white dwarf PG 0038+199 using spectra from the Far Ultraviolet Spectroscopic Explorer (FUSE), with ground-based support from Keck HIRES. We employ curve-of-growth, apparent optical depth, and profile-fitting techniques to measure the column densities and limits of many other species (H₂, Na I, C I, C II, C III, N I, N II, O I, Si II, P II, S III, Ar I, and Fe II), which allows us to determine related ratios such as D/O, D/N, and the H₂fraction. Our efforts are concentrated on measuring gas-phase D/H, which is key to understanding Galactic chemical evolution, and comparing it to predictions from big bang nucleosynthesis. We find column densities logN(HI)=20.41±0.08, logN(DI)=15.75±0.08, and logN(H₂)=19.33±0.04, yielding a molecular hydrogen fraction of 0.14±0.02 (2 σ errors), with an excitation temperature of 143±5 K. The high H I column density implies that PG 0038+199 lies outside of the Local Bubble; we estimate its distance to be 297⁺¹⁶⁴_–104pc (1 σ). [DI+HD]/[HI+2H₂] toward PG 0038+199 is 1.91^+0.52_–0.42x10^–5 (2 σ). There is no evidence of component structure on the scale of Δv>8 km/s, based on Na I, but there is marginal evidence for structure on smaller scales. The D/H value is high compared to the majority of recent D/H measurements but consistent with the values for two other measurements at similar distances. D/O is in agreement with other distant measurements. The scatter in D/H values beyond ∼100 pc remains a challenge for Galactic chemical evolution.