The Galactic deuterium gradient

The Galactic deuterium abundance gradient has been determined from observations of DCN in Galactic molecular clouds. This is the only way to observe D throughout the Galaxy because the molecular clouds are not limited to the 2 kpc region around the Sun observed with FUSE and from DI. We used an astrochemistry model and the DCN/HCN ratios to estimate the underlying D/H ratios in 16 molecular clouds including five in the Galactic Center. The resulting positive Galactic D gradient and reduced Galactic Center D/H ratio imply that there are no significant Galactic sources of D, there is continuous infall of low-metallicity gas into the Galaxy, and that deuterium is cosmological.