The Deuterium-to-Oxygen Ratio in the Interstellar Medium

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 5 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. 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 to ~2000 pc and log N(D I) from ~13 to ~16 cm-2. The D/O ratio is constant in the local interstellar medium out to distances of ~150 pc and N(D ) ≃ 1 × 1015 cm-2, i.e., within the Local Bubble. In this region of the interstellar space, we find D/O = (3.84 ± 0.16) × 10-2 (1 σ 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 few, 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 data set suggests a present-epoch deuterium abundance below 1 × 10-5, i.e., lower than the value usually assumed, around 1.5 × 10-5.