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 930–1150Å with \lambda / \Delta \lambda \sim 80 , 000 .
The interstellar \ion D1 features are resolved and cleanly separated from interstellar \ion H1 in the Ly \delta and Ly \epsilon profiles of both sight lines , and also in the Ly \gamma profile of \zeta Pup .
The \ion D1 profiles were modeled using a velocity template derived from several \ion N1 lines in the IMAPS spectra recorded at higher signal-to-noise .
To find the best \ion D1 column density , we minimized \chi ^ { 2 } for model \ion D1 profiles that included not only the N ( \ion D1 ) as a free parameter , but also the effects of several potential sources of systematic error which were allowed to vary as free parameters . \ion H1 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 ^ { +0.36 } _ { -0.31 } \times 10 ^ { \small - 5 } for \gamma ^ { 2 } Vel and 1.42 ^ { +0.25 } _ { -0.23 } \times 10 ^ { \small - 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 to 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 \pm 0.002 , which we regard as an upper limit since there is no correction for the destruction of deuterium in stars .