2002ApJS..140...91W

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)x10^–5. In contrast, multiplying our measurements of DI/OI=0.035±0.006 and DI/NI=0.27±0.05 with published mean Galactic interstellar medium (ISM) gas-phase O/H and N/H ratios yields D/H_O=(1.2±0.2)x10^–5 and D/H_N=(2.0±0.4)x10^–5, respectively. Note that all uncertainties quoted above are 2 σ. The inconsistency between D/H_O and D/H_N 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/H_N 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.