Spatial Variability in the Ratio of Interstellar Atomic Deuterium to Hydrogen. I. Observations toward δ Orionis by the Interstellar Medium Absorption Profile Spectrograph

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Abstract
Studies of the abundances of deuterium in different astrophysical sites are of fundamental importance to answering the question about how much deuterium was produced during big bang nucleosynthesis and what fraction of it was destroyed later. With this in mind, we used the Interstellar Medium Absorption Profile Spectrograph (IMAPS) on the ORFEUS-SPAS II mission to observe at a wavelength resolution of 4 km s-1 (FWHM) the Lyδ and Ly absorption features produced by interstellar atomic deuterium in the spectrum of δ Ori A. A χ2 analysis indicated that 0.96 < N(D I) < 1.45 × 1015 cm-2 at a 90% level of confidence, and the gas is at a temperature 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 for the additional uncertainties that could arise from various sources of systematic error. To derive a value for D/H, we measured the Lyα absorption features in 57 spectra of δ Ori in the IUE archive, with the objective 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 × 1020 cm-2, we found that N(D I)/N(H I) = 7.4+1.9-1.3 × 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 that for N(D I). Our result for D/H contrasts with the more general finding along other lines of sight that D/H ≈ 1.5 × 10-5. The underabundance of D toward δ 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.
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