Interstellar Deuterium, Nitrogen, and Oxygen Abundances toward BD +28o4211: Results from the FUSE Mission

Abstract

High-resolution far-UV spectra of the O-type subdwarf BD +28°4211 were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) to measure the interstellar deuterium, nitrogen, and oxygen abundances in this direction. The interstellar D I transitions are analyzed down to Lyι at 920.7 A. The star was observed several times at different target offsets in the direction of spectral dispersion. The aligned and co-added spectra have high signal-to-noise ratios (S/N = 50-100). D I, N I, and O I transitions were analyzed with curve-of-growth and profile fitting techniques. A model of interstellar molecular hydrogen on the line of sight was derived from H2 lines in the FUSE spectra and used to help analyze some features where blending with H2 was significant. The H I column density was determined from high-resolution Hubble Space Telescope Space Telescope Imaging Spectrograph spectra of Lyα to be log N(H ) = 19.846 ± 0.035 (2 σ), which is higher than is typical for sight lines in the local interstellar medium (ISM) studied for D/H. We found that D/H = (1.39 ± 0.21) × 10-5 (2 σ) and O/H = (2.37 ± 0.55) × 10-4 (2 σ). O/H toward BD +28°4211 appears to be significantly below the mean O/H ratio for the ISM and the Local Bubble.