Abstract. An accurate value of the deuterium/hydrogen (D/H) ratio in the local interstellar medium (LISM) and a better understanding of the D/H variations with position in the Galactic disk can provide essential information on the primordial D/H ratio in the Galaxy at the time of the protosolar nebula, and the amount of astration and mixing in the Galaxy over time. Recent measurements have been obtained with UV spectrographs on FUSE, HST, and IMAPS using hot white dwarfs, OB stars, and late-type stars as background light sources against which to measure absorption by D and H in the interstellar medium along the lines of sight. Recent analyses of FUSE observations of seven white dwarfs and subdwarfs provide a weighted mean value of D/H = (1.52 ± 0.08) × 10 −5 (15.2 ± 0.8 ppm), consistent with the value of (1.50 ± 0.10) × 10 −5 (15.0 ± 1.0 ppm) obtained from analysis of lines of sight toward nearby late-type stars. Both numbers refer to the ISM within about 100 pc of the Sun, which samples warm clouds located within the Local Bubble. Outside of the Local Bubble at distances of 200 to 500 pc, analyses of far-UV spectra obtained with IMAPS indicate a much wider range of D/H ratios between 0.8 to 2.2 ppm, providing information on inhomogeneous astration in the Galactic disk.
Why are Accurate Measurements of the D/H Ratio Important?Measurements of D/H, the number ratio of deuterium in all forms to hydrogen in all forms, are important for at least two reasons. First, an accurate measurement of the primordial ratio, (D/H) prim , counts the number of baryons in the universe to determine the ratio Ω B of the baryon density to the closure density, and tests our assumptions concerning nucleosynthesis during the first 100-1,000 seconds of the universe (e.g., Burles et al., 2001). Deuterium is our best probe of primordial nucleosynthesis because theory predicts that D was formed only in the very early universe, D is the easiest isotope to be destroyed by nuclear reactions in stars (astration), and Ω B is a very sensitive single-valued function of (D/H) prim . The best approximation to (D/H) prim would be an accurate measurement of D/H in gas where there has been little chemical fractionation or star formation, as indicated by very low metal abundances, but such measurements remain difficult.Second, measurements of D/H in different locations in our Galaxy will provide an accurate test of the assumptions underlying Galactic chemical evolution models. A major problem in astrophysics is understanding how galaxies evolve and, in particular, how the chemical element abundances evolve. In broad overview, we know that stars form out of gas clouds and over time they destroy D, create metals, and return some of this deuterium-poor and metal-rich material to the ISM by winds and supernova explosions. The detailed rates for these processes depend on the Space Science Reviews 95/96: 1-12, 2003.