Abundances of Deuterium, Nitrogen, and Oxygen in the Local Interstellar Medium: Overview of First Results from the FUSE Mission

Observations obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) have been used to determine the column densities of D I, N I, and O I along seven sight lines that probe the local interstellar medium (LISM) at distances from 37 to 179 pc. Five of the sight lines are within the Local Bubble, and two penetrate the surrounding H I wall. Reliable values of N(H I) were determined for five of the sight lines from Hubble Space Telescope (HST) data, International Ultraviolet Explorer (IUE) data, and published Extreme Ultraviolet Explorer (EUVE) measurements. The weighted mean of D I/H I for these five sight lines is (1.52 ± 0.08) × 10-5 (1 σ uncertainty in the mean). It is likely that the D I/H I ratio in the Local Bubble has a single value. The D I/O I ratio for the five sight lines within the Local Bubble is (3.76 ± 0.20) × 10-2. It is likely that O I column densities can serve as a proxy for H I in the Local Bubble. The weighted mean for O I/H I for the seven FUSE sight lines is (3.03 ± 0.21) × 10-4, comparable to the weighted mean (3.43 ± 0.15) × 10-4 reported for 13 sight lines probing larger distances and higher column densities. The FUSE weighted mean of N I/H I for five sight lines is half that reported by Meyer and colleagues for seven sight lines with larger distances and higher column densities. This result combined with the variability of O I/N I (six sight lines) indicates that at the low column densities found in the LISM, nitrogen ionization balance is important. Thus, unlike O I, N I cannot be used as a proxy for H I or as a metallicity indicator in the LISM.