The Galactic Deuterium Distribution

Don Lubowich

The deuterium abundance in Galactic molecular clouds located throughout the Galaxy (0.01 to 10 kpc) was determined from the J = 1-0 lines of DCN and HC15N in five Galactic-center molecular clouds and ten molecular clouds outside the Galactic Center (GC), using the 12-m telescope of the Arizona Radio Observatory. Because molecular clouds have embeded massive stars, supernovae and supernova remnants, and high rates of star formatiom, they are ideal sources in which to seach for deuterium nucleosyntheis. DCN was not detected in the circumnuclear disk cloud (2 pc from the GC) and the cloud at the outer edge of the Galactic disk (20 kpc). The underlying D/H ratio was deduced form an astrochemistry model containing 5260 chemical reactions. The D/H ratios ranges from 1.3 ppm at 35 pc to 20 ppm at 9.8 kpc from the Galactic Center. The data shows that average GC D/H = 2.3 ± 0.5 ppm which is ~106 times less than the D/H ratio predicted from astration models without an additional source of D and 5.3 times less than the average D/H of 11.0 ± 2.2 ppm in local molecular clouds (8 * 10 kpc from the GC). The low GC D/H ratios imply that there is continuous infall of low-metallicity gas enriched in deuterium into the GC; the GC has not been active nucleus for an extended period; and if any D nucleosynthesis does occur it is not correlated with massive stars, star formation, cosmic rays, stellar flares or molecular clouds. )regions with massive stars and star.

A non-parameteric ranked correlation analysis shows that the null hypothesiss that there is no correlation between the D/H ratio and galactocentric distance is ruled out at the 99.8 confidence level. This positive gradient in the Galactic D/H ratio (a linear regression gives D/H (ppm) = 1.15 R (kpc) + 1.49) is strong evidence that deuterium is primarily cosmological with no significant Galactic sources of deuterium and that the D/H ratio is determined by infall, astration, and mixing.

One possible explanation for the distribution and variations of the D/H ratios is that the GC, bulge, and disk have significantly different Galactic evolution histories with differing infall and astration rates. These results and future measurements of the D/H ratios may be used to estimate the infall and astration rats throughout the Galaxy and to allow construct better deuterium chemical evolution models. The methods developed in this research will allow the D/H ratio to be determined in other galaxies.