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 Å. 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 H I, 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 Lyα and using a model atmosphere to account for the stellar continuum. The sight line-averaged ratios for GD 246 are D I/H I = 1.51 × 10-5, O I/H I = 3.63 × 10-4, and D I/O I = 4.17 × 10-2 (uncertainties are 2 σ). 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 × 10-2 and D I/N I = 4.57 × 10-1. Toward HZ 21 the sight line-averaged ratios are D I/O I = 4.57 × 10-2 and D I/N I = 4.27 × 10-1. In the higher column density sight line to Lan 23, the sight line-averaged ratios are D I/O I = 3.24 × 10-2 and D I/N I = 3.16 × 10-1. Molecular hydrogen, corresponding to rotational levels J ≤ 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 D I/H I, O I/H I, N I/H I, D I/N I, D I/O I, and O I/N I.