The Deuterium‐to‐Oxygen Ratio in the Interstellar Medium

Abstract

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 À2 . The D/O ratio is constant in the local interstellar medium out to distances of $150 pc and NðD iÞ ' 1 Â 10 15 cm À2 , i.e., within the Local Bubble. In this region of the interstellar space, we find D=O ¼ ð3:84 AE 0:16Þ Â 10 À2 (1 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 Â 10 À5 , i.e., lower than the value usually assumed, around 1:5 Â 10 À5 .

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