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

We use Far Ultraviolet Spectrocopic Explorer (FUSE) observations to studyinterstellar absorption along the line of sight to the white dwarf WD1634-573(d=37.1+/-2.6 pc). Combining our measurement of D I with a measurement of H Ifrom Extreme Ultraviolet Explorer data, we find a D/H ratio toward WD1634-573of D/H=(1.6+/-0.5)e-5. In contrast, multiplying our measurements of D I/OI=0.035+/-0.006 and D I/N I=0.27+/-0.05 with published mean Galactic ISM gasphase O/H and N/H ratios yields D/H(O)=(1.2+/-0.2)e-5 andD/H(N)=(2.0+/-0.4)e-5, respectively. Note that all uncertainties quoted aboveare 2 sigma. The inconsistency between D/H(O) and D/H(N) suggests that eitherthe O I/H I and/or the N I/H I ratio toward WD1634-573 must be different fromthe previously measured average ISM O/H and N/H values. The computation ofD/H(N) from D I/N I is more suspect, since the relative N and H ionizationstates could conceivably vary within the LISM, while the O and H ionizationstates will be more tightly coupled by charge exchange.Comment: 23 pages, 5 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty; accepted by ApJ Supplemen