Spatial Variability in the Ratio of Interstellar Atomic Deuterium to Hydrogen. II. Observations toward γ²Velorum and ζ Puppis by the Interstellar Medium Absorption Profile Spectrograph

Abstract

To measure interstellar atomic deuterium abundances, we used the Interstellar\nMedium Absorption Profile Spectrograph (IMAPS) to obtain spectra of gamma2 Vel\nand zeta Pup over the wavelength interval 930-1150 A at a resolving power of\n80,000. The interstellar D I features are resolved and cleanly separated from\ninterstellar H I in the Ly-delta and Ly-epsilon profiles of both sight lines,\nand also in the Ly-gamma profile of zeta Pup. The D I profiles were modeled\nusing a velocity template derived from several N I lines in the IMAPS spectra\nrecorded at higher S/N. To find the best D I column density, we minimized\nchi-squares for model D I profiles that included not only the N(D I) as a free\nparameter, but also the effects of several potential sources of systematic\nerror which could also be varied. For both stars, H I column densities were\nmeasured by analyzing Ly-alpha absorption profiles in a large number of IUE\nhigh dispersion spectra. Ultimately we found that D/H = 2.18(+0.36,-0.31)e-5\nfor gamma2 Vel and 1.42(+0.25,-0.23)e-5 for zeta Pup, values that contrast\nmarkedly with D/H derived in Paper I for delta Ori (the stated errors are 90%\nconfidence limits). Evidently, the atomic D/H ratio in the ISM, averaged over\npath lengths of 250 to 500 pc, exhibits significant spatial variability.\nFurthermore, variations in D/H do not appear to be anticorrelated with N/H.\nWithin the framework of standard Big Bang Nucleosynthesis, the large value of\nD/H found toward gamma2 Vel is equivalent to a cosmic baryon density of Omega_B\nh^2 = 0.023 (+-0.002), which we regard as an upper limit since there is no\ncorrection for the destruction of deuterium in stars.\n