Local Interstellar Medium Properties and Deuterium Abundances for the Lines of Sight toward HR 1099, 31 Comae, β Ceti, and β Cassiopeiae^*

We analyze Goddard High-Resolution Spectrograph data to infer the properties of local interstellar gas and the deuterium/hydrogen (D/H) ratio for lines of sight toward four nearby late-type stars—HR 1099, 31 Comae, β Ceti, and β Cassiopeiae. The data consist of spectra of the hydrogen and deuterium Lyα lines, and echelle spectra of the Mg II h and k lines toward all stars except β Cas. Spectra of the RS CVn-type spectroscopic binary system HR 1099 were obtained near opposite quadratures to determine the intrinsic stellar emission line profile and the interstellar absorption separately. Multiple-velocity components were found toward HR 1099 and β Cet. The spectra of 31 Com and β Cet are particularly interesting because they sample lines of sight toward the north and south Galactic poles, respectively, for which H I and D I column densities were not previously available.

The north Galactic pole appears to be a region of low hydrogen density like the "interstellar tunnel" toward CMa. The temperature and turbulent velocities of the local interstellar medium (LISM) that we measure for the lines of sight toward HR 1099, 31 Com, β Cet, and β Cas are similar to previously measured values (T ≈ 7000 K and ξ = 1.0-1.6 km s^-1). The deuterium/hydrogen ratios found for these lines of sight are also consistent with previous measurements of other short lines of sight, which suggest D/H ≈ 1.6 × 10^-5. In contrast, the Mg abundance measured for the β Cet line of sight [implying a logarithmic depletion of D(Mg) = +0.30 ± 0.15] is about 5 times larger than the Mg abundance previously observed toward α Cen, and about 20 times larger than all other previous measurements for the LISM. These results demonstrate that metal abundances in the LISM vary greatly over distances of only a few parsecs.