DEUTERIUM OBSERVATIONS IN THE GALAXY

An accurate measurement of the primordial value of D/H would provide o ne of the best tests of nucleosynthesis models for the early Universe and the baryon density. Such evaluations have been traditionally made using present estimations of the deuterium abundance in the interstell ar medium, extrapolated backwards in time with the use of galactic evo lution models. Direct estimations of the primordial deuterium abundanc e have been carried out only recently in QSOs absorbers at high redshi ft. We will summarize galactic observations of deuterium and suggest t hat, perhaps, a single D/H value for the interstellar medium is not re presentative. These evaluations mainly came from observations complete d in the far UV with first the Copernicus satellite over the Lyman lin es series followed then by H and D Lyman-alpha lines observations with both the IUE and the GHRS on the Hubble Space Telescope. We discuss d ifferent known systematics and show that the situation is not yet clea r. It is not possible today to claim that we know ''the'' D/H value in the interstellar medium, if any. Overall and in the context of additi onal D observations made in the solar system, we conclude that the act ual evolution of deuterium from Big-Bang nucleosynthesis to now is not yet understood. More observations, recently made with IMAPS (the Inte rstellar Medium Absorption Profile Spectrograph) and hopefully to be m ade with FUSE (the Far Ultraviolet Spectroscopic Explorer to be launch ed in the fall of 1998), at higher spectral resolution or in many diff erent galactic sites are certainly needed to help us reach a better gl obal view of the evolution of that key element, and thus better constr ain any evaluation of its primordial abundance.