OF LOCAL INTERSTELLAR GAS, AND MEASUREMENT OF HEATED HYDROGEN (THE "HYDROGEN WALL") NEAR THE HELIOPAUSE _

We analyze high-resolution spectra of the nearby (!.34 pc) stars _ Cen A (G2 V) and _ Cen B (K1 V), which were obtained with the Goddard High Resolution Spectrograph on the Huhhle Space Telescope. The observations consist of echelle spectra of the Mg II 2800 / _ and Fe I1 2599 A resonance lines and the Lyman-:t lines of hydrogen and deuterium. The interstellar gas has a velocity (v =- 18.0 + 0.2 km s 1) consistent with the local flow vector proposed for this line of sight by Lallement & Bertin (1992). The temperature and nonthermal velocity inferred from the Fe II, Mg II, and D 1 line profiles are T = 5400 __+500 K and ¢ = 1.20 + 0.25 km s 1, respectively. However, single-component fits to the H 1 Lyman- _ lines yield a Doppler parameter (b,t = 11.80 km s-1) that implies a significantly warmer tem-perature of 8350 K, and the velocity of the H 1 absorption (v =-15.8 _+0.2 km s 1} is redshifted by about 2.2 km s-1 with respect to the Fe lZ, Mg ii, and D I lines. The one-component model of the interstellar gas suggests log N.I = 18.03 _+ 0.01 and D/H = (5.7 _+0.2) × 10 6. These parameters lead to a good fit to the observed spectra, but this model does not explain the higher temperature and redshift of H l relative to the other interstellar lines. The most sensible way to resolve the discrepancy between H I and the other lines is to add a secon