Spatial variability in the ratio of interstellar atomic deuterium to hydrogen. II. Observations toward gamma(2) Velorum and xi Puppis by the interstellar medium absorption profile spectrograph

High-resolution far-ultraviolet spectra of the early-type stars gamma (2) V el and zeta Pup were obtained to measure the interstellar deuterium abundan ces in these directions. The observations were made with the Interstellar M edium Absorption Profile Spectrograph (IMAPS) during the ORFEUS-SPAS II mis sion in 1996. IMAPS spectra cover the wavelength range 930-1150 Angstrom wi th 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 an d 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 derive d 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, b ut also the effects of several potential sources of systematic error, which were allowed to vary as free parameters. H I column densities were measure d by analyzing Ly alpha absorption profiles in a large number of IUE high-d ispersion spectra for each of these stars and applying this same chi (2)-mi nimization technique. Ultimately we found that D/H = 2.18(-0.3)(1+0.36) x 1 0(-5) for gamma (2) Vel and 1.42(-0.23)(+0.25) x 10(-5) for zeta Pup, value s that contrast markedly with D/H derived in Paper I for delta Ori A (the s tated errors are 90% confidence limits). Evidently, the atomic D/H ratio in the ISM, averaged over path lengths of 250-500 pc, exhibits significant sp atial variability. Furthermore, the observed spatial variations in D/H do n ot appear to be anticorrelated with N/H, one measure of heavy-element abund ances. We briefly discuss some hypotheses to explain the D/H spatial variab ility. 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 d ensity of Omega (B) h(2) = 0.023 +/- 0.002 which we regard as an upper limi t since there is no correction for the destruction of deuterium in stars.