D/H ALONG THE LINE OF SIGHT OF REJ 1738+665

Abstract

We present a new D/H measurement along the sightline to REJ 1738+665, one of the hottest DA stars (T eff ~ 76,000 K, log g ~ 7.85), probing a line of sight in the direction (l = 9689, b = 3195) and extending over a path-length of ~243 pc. Hot DA stars are useful sources for mapping interstellar absorption because of the lack of metal lines, which can lead to systematic errors, making REJ 1738+665 an attractive target for measuring D/H at distances and column densities intermediate between those in the Local Bubble (LB) and the more distant targets studied by the Far Ultraviolet Spectroscopic Explorer (FUSE). The FUSE spectrum towards REJ 1738+665 is rich in interstellar features (H I, H2, D I, C II, N I, N II, O I, Si II, P II, Ar I, and Fe II) and offers another opportunity to probe D/H and D/O outside the LB. We determine the column densities of the species mentioned above. N(H I) is determined using archival Hubble Space Telescope data in conjunction with model atmospheres and synthetic spectra computed in non-local thermodynamic equilibrium in order to better define the stellar continuum in the vicinity of Lyα. Our analysis yields D I/H I = (1.78 ± 0.28 0.25) × 10–5 and D I/O I = (5.25 ± 0.93 0.82) × 10–2 along this sight line (errors stated at the 1σ level). The D/H ratio is consistent with the LB value, while the D/O ratio is only marginally consistent with the LB value (1.6σ above). We find O I/H I = (3.39 ± 0.65 0.57) × 10–4, consistent with that measured in the local interstellar medium (ISM) and N I/H I = (5.01 ± 0.86 0.77) × 10–5, somewhat lower than typical values measured in the ISM..