Deuterium in the Galactic Centre as a result of recent infall of low-metallicity gas

The Galactic Centre is the most active and heavily processed region of the Milky Way, so it can be used as a stringent test for the abundance of deute rium (a sensitive indicator of conditions in the first 1,000 seconds in the life of the Universe). As deuterium is destroyed in stellar interiors, che mical evolution models 1 predict that its Galactic Centre abundance relativ e to hydrogen is D/H = 5 x 10(-12), unless there is a continuous source of deuterium from relatively primordial (low-metallicity) gas. Here we report the detection of deuterium (in the molecule DCN) in a molecular cloud only 10 parsecs from the Galactic Centre. Our data, when combined with a model o f molecular abundances, indicate that D/H = (1.7 +/- 0.3) x 10(-6), five or ders of magnitude larger than the predictions of evolutionary models with n o continuous source of deuterium. The most probable explanation is recent i nfall of relatively unprocessed metal-poor gas into the Galactic Centre (at the rate inferred by Wakker(2)). Our measured D/H is nine times less than the local interstellar value, and the lowest D/H observed in the Galaxy. We conclude that the observed Galactic Centre deuterium is cosmological, with an abundance reduced by stellar processing and mixing, and that there is n o significant Galactic source of deuterium.