Deuterium Abundance toward WD 1634–573: Results from the FUSE Mission

We use Far Ultraviolet Spectroscopic Explorer (FUSE) observations to study interstellar absorption along the line of sight to the white dwarf WD 1634-573 (d = 37.1 ± 2.6 pc). Combining our measurement of D I with a measurement of H I from Extreme Ultraviolet Explorer data, we find a D/H ratio toward WD 1634-573 of D/H = (1.6 ± 0.5) × 10-5. In contrast, multiplying our measurements of D /O = 0.035 ± 0.006 and D /N = 0.27 ± 0.05 with published mean Galactic interstellar medium (ISM) gas-phase O/H and N/H ratios yields D/HO = (1.2 ± 0.2) × 10-5 and D/HN = (2.0 ± 0.4) × 10-5, respectively. Note that all uncertainties quoted above are 2 σ. The inconsistency between D/HO and D/HN suggests that either the O I/H I or the N I/H I ratio toward WD 1634-573 must be different from the previously measured average ISM O/H and N/H values. The computation of D/HN from D I/N I is more suspect, since the relative N and H ionization states could conceivably vary within the local ISM, while the O and H ionization states will be more tightly coupled by charge exchange.