THE α CENTAURI LINE OF SIGHT: D/H RATIO, PHYSICAL PROPERTIES OF LOCAL INTERSTELLAR GAS, AND MEASUREMENT OF HEATED HYDROGEN (THE 'HYDROGEN WALL') NEAR THE HELIOPAUSE

摘要

We analyze high-resolution spectra of the nearby (1.34 pc) stars α Cen A (G2 V) and α Cen B (K1 V), which were obtained with the Goddard High Resolution Spectrograph on the Hubble Space Telescope. The observations consist of echelle spectra of the Mg II 2800 A and Fe II 2599 A resonance lines and the Lyman-α 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 I line profiles are T = 5400 ± 500 K and ξ = 1.20 ± 0.25 km s~(-1), respectively. However, single-component fits to the H I Lyman-α lines yield a Doppler parameter (B_(H I) = 11.80 km s~(-1)) that implies a significantly warmer temperature of 8350 K, and the velocity of the H I absorption (v = -15.8 ± 0.2 km s~(-1)) is redshifted by about 2.2 km s~(-1) with respect to the Fe II, Mg n, and D I lines. The one-component model of the interstellar gas suggests log N_(H I) = 18.03 ± 0.01 and D/H = (5.7 ± 0.2) x 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 I relative to the other interstellar lines.