The D/H ratio in interstellar gas varies on scales of a few hundred pc in the local Milky Way , with D/H values ranging from \sim 7 ppm to \sim 22 ppm .
The reduction in D/H relative to the primordial value of D/H \approx 26 ppm is usually attributed to “ astration ” – conversion of D into other elements by nuclear fusion in stars .
However , it is shown here that astration has difficulty accounting for the observations because the expected associated variations in O/H are not seen .

The lower D/H values are instead likely due to “ depletion ” of the D onto dust grains .
Polycyclic aromatic hydrocarbons ( PAHs ) are a possible repository for the missing D , and it appears possible for gas-grain reactions to achieve extreme deuteration of carbonaceous grain material .
Grain destruction will release D from the grains ; the gas phase abundance of D should therefore be positively correlated with the gas phase abundances of other elements that exhibit strong depletions , such as Mg , Si , Ti , and Fe , which also will be returned to the gas by grain destruction .