Evidence of Correlated Titanium and Deuterium Depletion in the Galactic Interstellar Medium

Abstract

Current measurements indicate that the deuterium abundance in diffuseinterstellar gas varies spatially by a factor of ?4 among sight linesextending beyond the Local Bubble. One plausible explanation forthe scatter is the variable depletion of D onto dust grains. To testthis scenario, we have obtained high signal-to-noise, high- resolutionprofiles of the refractory ion Ti II along seven Galactic sight lineswith D/H ranging from 0.65 to 2.1 � 10-5. These measurements, acquiredwith the recently upgraded Keck/HIRES spectrometer, indicate a correlationbetween Ti/H and D/H at the better than 95% confidence level Therefore,our observations support the interpretation that D/H scatter is associatedwith differential depletion. We note, however, that Ti/H values takenfrom the literature do not uniformly show the correlation. Finally,we identify significant component-to-component variations in thedepletion levels among individual sight lines and discuss complicationsarising from this behavior.