Spatial Variability in the Ratio of Interstellar Atomic Deuterium to Hydrogen. II. Observations toward γ² Velorum and ζ Puppis by the Interstellar Medium Absorption Profile Spectrograph

Abstract

High-resolution far-ultraviolet spectra of the early-type stars γ² Vel and ζ Pup were obtained to measure the interstellar deuterium abundances in these directions. The observations were made with the Interstellar Medium Absorption Profile Spectrograph (IMAPS) during the ORFEUS-SPAS II mission in 1996. IMAPS spectra cover the wavelength range 930-1150 Å with λ/Δλ~80,000. The interstellar D I features are resolved and cleanly separated from interstellar H I in the Lyδ and Lyɛ profiles of both sight lines and also in the Lyγ profile of ζ Pup. The D I profiles were modeled using a velocity template derived from several N I lines in the IMAPS spectra recorded at higher signal-to-noise ratio. To find the best D I column density, we minimized χ² for model D I profiles that included not only the N(D I) as a free parameter, but also the effects of several potential sources of systematic error, which were allowed to vary as free parameters. H I column densities were measured by analyzing Lyα absorption profiles in a large number of IUE high-dispersion spectra for each of these stars and applying this same χ²-minimization technique. Ultimately we found that D/H=2.18^+0.36_-0.31×10^-5 for γ² Vel and 1.42^+0.25_-0.23×10^-5 for ζ Pup, values that contrast markedly with D/H derived in Paper I for δ Ori A (the stated errors are 90% confidence limits). Evidently, the atomic D/H ratio in the ISM, averaged over path lengths of 250-500 pc, exhibits significant spatial variability. Furthermore, the observed spatial variations in D/H do not appear to be anticorrelated with N/H, one measure of heavy-element abundances. We briefly discuss some hypotheses to explain the D/H spatial variability. Within the framework of standard big bang nucleosynthesis, the large value of D/H found toward γ² Vel is equivalent to a cosmic baryon density of Ω_Bh²=0.023+/-0.002, which we regard as an upper limit since there is no correction for the destruction of deuterium in stars. This paper is dedicated in memory of Judith L. Tokel, wife of the first author, who passed away on 2000 June 10. Her enthusiastic support and encouragement were essential to its successful completion.