The Galactic Interstellar Deuterium Abundance Gradient

Don Lubowich

I propose to determine the Galactic interstellar deuterium abundance gradient from observations of deuterated molecules in galactic molecular clouds. The observations have been completed primarily using the Arizona Radio Telescope 12m telescope and include observations of DCN in four Galactic Center molecular clouds (including one in the magnetic arc and heated by cosmic-rays); observations of the 2 pc Galactic circumnuclear disk (completed with the Nobeyama mm-array); 10 interstellar molecular clouds (at 12 positions) located outside the Galactic Center; and in a molecular cloud at the outermost edge of the Galaxy (28 kpc from the Galactic Center).

If deuterium were produced via any stellar or Galactic nucleosynthesis process, then its abundance would be a maximum value in the Galactic Center, which is the most active region in the Galaxy. Conversely, if there are no Galactic source of D, then astration would reduce the D abundance toward the Galactic Center and there would be a positive gradient in the D/H ratio with increasing galactocentric distance. DCN was used as a tracer molecule because DCN is produced in hot and cold regions of molecular clouds. Using a 5300 chemical reaction model to analyze the deuterium fractionation the underlying D/H ratio will be estimated from deuterated molecular abundance ratios such as DCN/HCN. Thus the Galactic D/H gradient determined in this proposal will help determine it there are any additional sources of D other than the big-bang.