Because the ionization balances for H i , O i , and D i are locked together by charge exchange , the deuterium-to-oxygen ratio , D/O , is an important tracer for the value of the D/H ratio and for potential spatial variations in the ratio .
As the D i and O i column densities are of similar orders of magnitude for a given sight line , comparisons of the two values will generally be less subject to systematic errors than comparisons of D i and H i , which differ by about five orders of magnitude .
Moreover , D/O is additionally sensitive to astration , because as stars destroy deuterium , they should produce oxygen .
We report here the results of a survey of D/O in the interstellar medium performed with the Far Ultraviolet Spectroscopic Explorer ( FUSE ) .
We also compare these results with those for D/N .
Together with a few results from previous missions , the sample totals 24 lines of sight .
The distances range from a few pc to \sim 2000 pc and \log N ( D i ) from \sim 13 to \sim 16 ( cm ^ { -2 } ) .
The D/O ratio is constant in the local interstellar medium out to distances of \sim 150 pc and N ( D i ) \simeq 1 \times 10 ^ { 15 } cm ^ { -2 } , i.e . within the Local Bubble .
In this region of the interstellar space , we find D/O = ( 3.84 \pm 0.16 ) \times 10 ^ { -2 } ( 1 \sigma in the mean ) .
The homogeneity of the local D/O measurements shows that the spatial variations in the local D/H and O/H must be extremely small , if any .
A comparison of the Local Bubble mean value with the few D/O measurements available for low metallicity quasar sight lines shows that the D/O ratio decreases with cosmic evolution , as expected .
Beyond the Local Bubble we detected significant spatial variations in the value of D/O .
This likely implies a variation in D/H , as O/H is known to not vary significantly over the distances covered in this study .
Our dataset suggests a present-epoch deuterium abundance below 1 \times 10 ^ { -5 } , i.e . lower than the value usually assumed , around 1.5 \times 10 ^ { -5 } .