The Galactic Deuterium Abundance and Dust Depletion: Insights From an Expanded Ti/H Sample

The primordial abundance of deuterium (D/H) yields a measure of the density of baryons in the universe and is an important complement to determinations from cosmic microwave background (CMB) experiments. Indeed, the current small sample of high redshift D/H measurements from quasar absorption line studies are in excellent agreement with CMB-derived values. Conversely, absorption line measurements of the Galactic D/H ratio in almost 50 stellar sightlines show a puzzlingly large scatter outside the Local Bubble which is difficult to explain simply by astration from the primordial value. Here, we investigate the dust depletion scenario by studying the correlation between D/H and the abundance of titanium, one of the most refractory elements readily observed in the ISM. With a sample 3 times larger than previous work, we confirm a correlation between Ti/H and D/H at the 97% confidence level. However, the magnitude of this dependence is difficult to reconcile with a simple model of dust depletion for two reasons. First, contrary to what is expected from local depletion rates, the gradient of the highly refractory Ti is much shallower than that observed for Fe and Si. Second, we do not observe the established tight, steep correlation between [Ti/H] and the mean volume density of hydrogen. Therefore, whilst dust remains a plausible explanation for the local D/H variations, the abundances of at least some of the refractory elements do not provide unanimous support for this scenario. We also argue that the correlations of [Si/H], [Fe/H], and [Ti/H] with D/H are inconsistent with a simple infall model of low metallicity gas with approximately solar abundances as the dominant cause for D variations.