D/H-ratio: Observations with ORFEUS II

During the second flight of the ORFEUS-SPAS satellite (Nov./Dec. 96) high resolution (λ/Δλ = 10,000) Echelle-spectra of BD + 28° 4211 in the wavelength regime 912 – 1400 A have been taken. Deuterium can be clearly identified in the ORFEUS II Echelle-spectra of this star. For the first time it was possible to take spectra of faint, not redshifted objects in the far ultraviolet with a sufficient spectral resolution to study the deuterium column density in the whole spectral range of the Lyman-series down to the Lyman-limit. We obtained a value of log(ND) = 14.7 (± 0.3) towards BD + 28° 4211. The hydrogen column density has been determined using ORFEUS Echelle- and IUE-spectra of Ly-α (log(NH) = 19.8 (± 0.2)). Thus a value of 8 × 10−6 can be obtained for the D/H-ratio on the line-of-sight towards BD + 28° 4211.