Interstellar deuterium, nitrogen, and oxygen abundances toward GD 246, WD 2331-475, HZ 21, and Lanning 23:: Results from the FUSE mission

The interstellar abundances of D I, N I, and O I in the local ISM are studied using high-resolution spectra of four hot white dwarfs. The spectra of GD 246, WD 2331 - 475, HZ 21, and Lan 23 were obtained with the Far Ultraviolet Spectroscopic Explorer ( FUSE) in the wavelength range 905-1187 Angstrom. The line of sight to GD 246 probes the Local Interstellar Cloud and at least one other H I cloud inside the Local Bubble, which contains most of the gas seen along this line of sight. The column densities of HI, C II*, S II, and Si II are measured using archival Hubble Space Telescope STIS echelle-mode observations. The H I column density is determined by fitting the strong damping wings of interstellar Lyalpha and using a model atmosphere to account for the stellar continuum. The sight line-averaged ratios for GD 246 are DIi/H I = 1.51(-0.33)(+0.39) x 10(-5), O I/H I = 3.63(-0.67)(+0.77) x 10(-4), and D I/O I = 4.17(-1.00)(+1.20) x 10(-2) (uncertainties are 2 sigma). This line of sight provides the fourth reliable FUSE measurement of the Local Bubble D/H ratio. For the WD 2331-475 line of sight, we find sight line-averaged ratios D I/O I = 5.13(-1.69)(+2.20) x 10(-2) and D I/N I = 4.57(-1.45)(+1.88) x 10(-1). Toward HZ 21 the sight line-averaged ratios are D I/O I = 4.57(-1.63)(+2.22) x 10(-2) and D I/N I = 4.27(-1.44)(+1.96) x 10(-1). In the higher column density sight line to Lan 23, the sight line-averaged ratios are D I/O I = 3.24(-2.06)(+3.27) x 10(-2) and D I/N I = 3.16(-1.23)(+1.56) x 10(-1). Molecular hydrogen, corresponding to rotational levels J less than or equal to 3, is clearly seen along this line of sight. No reliable H I measurements are available for WD 2331-475, HZ 21, or Lan 23. We combine the different abundance ratios computed here with previous published values to produce revised FUSE abundance ratios for DI/ H I, O I/ H I, N I/ H I, D I/ N I, D I/ O I, and O I/ N I.