The D/H Abundance Ratio in Local Interstellar Gas

Does the D/H ratio vary within ~ 100 pc of the Sun? If it does, what is the range of these variations? Are the D/H variations correlated to any physical property of the absorbing cloud? To answer these questions we have begun a project to obtain high-precision D/H abundance ratios in the Local Interstellar Medium within ~ 100 pc (Sahu et al. 1999, ApJ 523, L159). For this purpose, we primarily use HST-Space Telescope Imaging Spectrograph (STIS) data towards nearby hot, white dwarfs (WDs). The advantages of using STIS for this study are: (1) The high velocity resolution of STIS in the UV ( ~ 2 km s^-1 in the E140H and E230H modes) allows the velocity structure in the sightline to be resolved and reliable component-to-component variations can be studied (2) WDs provide a strong, smooth UV continuum against which other diagnostic interstellar absorption lines can be measured. The wider wavelength coverage ( ~ 200 Angstroms per setting) of STIS in the high-resolution E140H mode allows important interstellar lines such as N i, Si ii, O i and S ii to be measured in addition to D i and H i, at no extra cost in HST time (3) The superior echelle scatter and background corrections possible with the 2-dimensional STIS-MAMA detectors allow more accurate D/H determinations from WD spectra. In this poster, we present a status report of our project along with some recent results.