Deuterium variations in the interstellar medium of the Milky Way

Deuterium, which is produced during primordial nucleosynthesis and then destroyed through astration, is a key element for observational cosmology. Whereas measurements in low-metallicity intergalactic clouds probe D/H at look-back times of similar to 13 - 14 Cyrs, the present epoch deuterium abundance (D/H)(PE) can be measured in the interstellar medium. D/H ratios for the interstellar medium have been determined through far ultraviolet absorption line measurements since the 70's. These measurements show dispersion, interpreted as the result of either unexpected spatial variations or underestimated systematic errors. Final resolution will have implications for our understanding of the interstellar medium, galactic chemical evolution, and Universe baryonic density. The FUSE mission has brought significant progress on these issues. It now appears that the D/H ratio likely has a single value in the Local Bubble, in the range (1.3 - 1.5) X 10(-5). This local ratio, however, should not be considered as a canonical value for D/H in the Milky Way, representative of the material at the present epoch. Indeed, beyond 100 pc, D/H shows a range of values that are in significant disagreement with the local value. Two interpretations are proposed for explaining this difference. One assumes deuterium is depleted onto dust grains and claims (D/H)(PE) = (2.3 + 0.4) x 10(-5). The second emphasizes the D/O measurements, which show less dispersion than D/H, and concludes (D/H)(PE) = (7 2) X 10(-6). Both would challenge models of deuterium evolution.