The D/H Ratio toward PG 0038+199

Abstract

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 H2 fraction. 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 log N(H ) = 20.41 ± 0.08, log N(D ) = 15.75 ± 0.08, and log N(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 pc (1 σ). [D + HD]/[H + 2H2] toward PG 0038+199 is 1.91 × 10-5 (2 σ). There is no evidence of component structure on the scale of Δv > 8 km s-1, 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.