The D/H Ratio in Interstellar Gas toward G191-B2B

We reinvestigate the question of spatial variation of the local D/Habundance, using both archival GHRS spectra, and new echelle spectra ofG191-B2B obtained with the Space Telescope Imaging Spectrograph (STIS) aboardHST. Our analysis uses stratified line-blanketed non-LTE model atmospherecalculations to determine the shape of the intrinsic WD Lyman-alpha profile andestimate the WD photospheric contamination of the interstellar lines. Althoughthree velocity components were reported previously towards G191-B2B, we detectonly two velocity components. The first component is at V(hel) ~ 8.6 km/s andthe second at V(hel) ~ 19.3 km/s, which we identify with the Local InterstellarCloud (LIC). From the STIS data we derive D/H = 1.60(+0.39,-0.27)X10^-5 for theLIC component, and D/H > 1.26X10^-5 for the 8.6 km/s component (uncertaintiesdenote 2-sigma or 95% confidence limits). The STIS data provide no evidence forlocal or component-to-component variation in the D/H ratio. Despite using twovelocity components for the profile fitting and using a more physicallyrealistic WD Lyman-alpha profile for G191-B2B, our re-analysis of the GHRS dataindicates a component-to-component variation as well as a variation of the D/Hratio in the LISM, neither of which are supported by the newer STIS data. Webelieve the most probable cause for this difference is the characterization ofthe background due to scattered light in the GHRS and STIS spectrographs. Thetwo-dimensional MAMA detectors of STIS measure both the spatial and wavelengthdependences of scattered light, allowing more accurate scattered lightcorrections than was possible with GHRS.Comment: Accepted for publication in Astrophysical Journal Letters. 10 pages + 3 figures. (Abstract is abridged.