THE _ CENTAURI LINE OF SIGHT: D/H RATIO, PHYSICAL PROPERTIES 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 temperature 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 second absorption component to the H l lines. This second component is hotter (T _ 30,000 K}, is redshifted relative to the primary component by 2--4 km s 1, and has a column density too low to be detected in the Fe II, Mg II, and D I lines. We propose that the gas responsible for this component is located near the heliopause, consisting of the heated H l gas from the interstellar medium that is compressed by the solar wind. This so-called "hydrogen wall" is predicted by recent multifluid gasdynamical models of the interstellar gas and solar wind interaction. Our data provide the first measurements of the temperature and column density of H I in the hydrogen wall. After considering the effects that a corresponding hydrogen wall around _ Cen would have on our analysis, our best estimates for the parameters of the solar hydrogen wall are log N_21 = 14.74 + 0.24, b_21= 21.9_+ 1.7 km s z (corresponding to T = 29,000 _+ 5000 K), and v_2] > 16 km s 1. Unfortunately, the existence of this heated H I reduces our ability to compute the H I column density of the interstellar medium accurately because, with slight alterations to our assumed stellar Lyman-:_ profiles, we discovered that acceptable two-component fits also exist with log N. _ 17.6. We, therefore, quote large error bars for the H ! column density along the _ Cen line of sight, log N. = 17.80 _+0.30. For this range in N.I, nnl = 0.15 cm -3 (_+ a factor of 2) and D/H = (0.5-1.9) × 10 -_. This is the first direct measurement of the H I density in a local cloud and allows us to predict the distance from the Sun to the edge of the local cloud along various lines of sight. This range in D/H is consistent with the value D/H = 1.6 × 10-s previously derived for the Capella and Procyon lines of sight. We cannot tell whether the D/H ratio varies or is constant in the local interstellar medium, but we do find that the D I/Mg II ratio for the _ Cen line of sight is about 4 times smaller than for the Capella and Procyon lines of sight. Therefore, either D/H or the Mg depletion varies significantly over distance scales of only a few parsecs. Subject headings: ISM: abundances ISM: kinematics and dynamics radio lines' ISM stars: individual (_ Centauri) . ultraviolet" ISM