As the Galaxy evolves , the abundance of deuterium in the interstellar medium ( ISM ) decreases from its primordial value : deuterium is “ astrated ” .
The deuterium astration factor , f _ { D } , the ratio of the primordial D abundance ( the D to H ratio by number ) to the ISM D abundance , is determined by the competition between stellar destruction and infall , providing a constraint on models of the chemical evolution of the Galaxy .
Although conventional wisdom suggests that the local ISM ( i.e. , within \sim 1 - 2 kpc of the Sun ) should be well mixed and homogenized on timescales short compared to the chemical evolution timescale , the data reveal gas phase variations in the deuterium , iron , and other metal abundances as large as factors of \sim 4 - 5 or more , complicating the estimate of the “ true ” ISM D abundance and of the deuterium astration factor .
Here , assuming that the variations in the observationally inferred ISM D abundances result entirely from the depletion of D onto dust , rather than from unmixed accretion of nearly primordial material , a model-independent , Bayesian approach is used to determine the undepleted abundance of deuterium in the ISM ( or , a lower limit to it ) .
We find the best estimate for the undepleted , ISM deuterium abundance to be ( D/H ) _ { ISM } \geq ( 2.0 \pm 0.1 ) \times 10 ^ { -5 } .
This result is used to provide an estimate of ( or , an upper bound to ) the deuterium astration factor , f _ { D } \equiv ( D/H ) _ { P } / ( D/H ) _ { ISM } \leq 1.4 \pm 0.1 .