Spatial variability in the ratio of interstellar atomic deuterium to hydrogen. I. Observations toward delta Orionis by the interstellar medium absorption profile spectrograph

Studies of the abundances of deuterium in different astrophysical sites are of fundamental importance to answering the question about how much deuteri um was produced during big bang nucleosynthesis and what fraction of it was destroyed later. With this in mind, we used the Interstellar Medium Absorp tion Profile Spectrograph (IMAPS) on the ORFEUS-SPAS II mission to observe at a wavelength resolution of 4 km s(-1) (FWHM) the Ly delta and Ly epsilon absorption features produced by interstellar atomic deuterium in the spect rum of delta Ori A. A chi(2) analysis indicated that 0.96 < N(D I) < 1.45 x 10(15) cm(-2) at a 90% level of confidence, and the gas is at a temperatur e of about 6000 K. In deriving these results, we created a template for the velocity profile defined by seven different N I transitions recorded at a high signal-to-noise ratio. Extra free parameters in the analysis allowed f or the additional uncertainties that could arise from various sources of sy stematic error. To derive a value for D/H, we measured the Ly alpha absorpt ion features in 57 spectra of delta Ori in the IUE archive, with the object ive of arriving at an H I column density more accurate than those reported by other investigators. From our measurement of N(H I) = 1.56 x 10(20) cm(- 2), we found that N(D I)/N(H I) = 7.4(-1.3)(+1.9) x 10(-6) (90% confidence) . Systematic errors in the derivation of N(H I) probably dominate over the very small formal error, but their relative value should be smaller than th at for N(D I). Our result for D/H contrasts with the more general finding a long other lines of sight that D/H approximate to 1.5 x 10(-5). The underab undance of D toward delta Ori A is not accompanied by an overabundance of N or O relative to H, as one might expect if the gas were subjected to more stellar processing than usual.