A High-precision Survey of the D/H Ratio in the Nearby Interstellar Medium

Abstract

We present high signal-to-noise ratio measurements of the H i Lyα absorption line toward 16 Galactic targets that are at distances between approximately 190 and 2200 pc, all beyond the wall of the Local Bubble. We describe the models used to remove stellar emission and absorption features and the methods used to account for all known sources of error in order to compute high-precision values of the H i column density with robust determinations of the uncertainties. When combined with H2 column densities from other sources, we find total H column densities ranging from 1020.01 to 1021.25 cm−2. Using deuterium column densities from Far Ultraviolet Spectroscopic Explorer observations we determine the D/H ratio along the sight lines. We confirm and strengthen the conclusion that D/H is spatially variable over these H i column density and target distance regimes, which predominantly probe the interstellar medium outside the Local Bubble. We discuss how these results affect models of Galactic chemical evolution. We also present an analysis of metal lines along the five sight lines for which we have high-resolution spectra and, along with results reported in the literature, discuss the corresponding column densities in the context of a generalized depletion analysis. We find that D/H is only weakly correlated with metal depletion and conclude that the spatial D/H variability is not solely due to dust depletion. A bifurcation of D/Htot as a function of depletion at high depletion levels provides modest support that deuterium-rich gas is infalling onto the Galactic plane.