Spatial variability in the ratio of interstellar atomic deuterium to hydrogen.: II.: Observations toward γ2 Velorum and ξ Puppis by the interstellar medium absorption profile spectrograph

High-resolution far-ultraviolet spectra of the early-type stars gamma (2) Vel and zeta 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 Angstrom with lambda/Delta lambda similar to 80,000. The interstellar D I AZ features are resolved and cleanly separated from interstellar H I in the Ly delta and Ly epsilon profiles of both sight lines and also in the Ly gamma profile of zeta 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 chi (2) 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 Ly alpha absorption profiles in a large number of IUE high-dispersion spectra for each of these stars and applying this same chi (2)-minimization technique. Ultimately we found that D/H = 2.18(-0.3)(1+0.36) x 10(-5) for gamma (2) Vel and 1.42(-0.23)(+0.25) x 10(-5) for zeta Pup, values that contrast markedly with D/H derived in Paper I for delta 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 gamma (2) Vel is equivalent to a cosmic baryon density of Omega (B) h(2) = 0.023 +/- 0.002 which we regard as an upper limit since there is no correction for the destruction of deuterium in stars.