The alpha Centauri Line of Sight: D/H Ratio, Physical Properties of Local Interstellar Gas, and Measurement of Heated Hydrogen (The 'Hydrogen Wall') Near the Heliopause

Abstract

The authors 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 GHRS on the HST. The observations consist of echelle spectra of the Mg II 2800 Å and Fe II 2599 Å resonance lines and the Lyman-α lines of hydrogen and deuterium. The interstellar gas has a velocity of v = -18.0±0.2 km s<SUP>-1</SUP>. The temperature and nonthermal velocity inferred from the Fe II, Mg II, and D I line profiles are T = 5400±500K and ξ = 1.20±0.25 km s<SUP>-1</SUP>, respectively. However, single component fits to the H I Lyman-α lines yield a Doppler parameter (b<SUB>HI</SUB> = 11.80 km s<SUP>-1</SUP>) that implies a significantly higher temperature of 8350K, and the velocity of the H I absorption is redshifted by about 2.2 km s<SUP>-1</SUP> with respect to the Fe II, Mg II, and D I lines. The one component model of the interstellar gas suggests log N<SUB>HI</SUB> = 18.03±0.01 and D/H = (5.7±0.2)×10<SUP>-6</SUP>. 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 I lines. This component is hotter, is redshifted relative to the primary component by 2 - 4 km s<SUP>-1</SUP>, and has a column density too low to be detected in the Fe II, Mg II, and D I lines. The authors propose that the gas responsible for this component is located near the heliopause, consisting of the heated H I gas from the interstellar medium that is compressed by the solar wind (the solar "hydrogen wall"). After considering the effects of a possible similar hydrogen wall around α Cen on the analysis, the authors derive for the solar hydrogen wall the parameters: log N<SUB>HI</SUB> = 14.74±0.24, b<SUB>HI</SUB> = 21.9±1.7 km s<SUP>-1</SUP> (corresponding to T = 29,000±5000K) and v<SUB>HI</SUB> &gt; -16 km s<SUP>-1</SUP>. Taking the solar hydrogen wall into account, the H I column density along the line of sight to α Cen is log N<SUB>HI</SUB> = 17.80±0.30, corresponding to n<SUB>HI</SUB> = 0.15 cm<SUP>-3</SUP> (± a factor of 2), and D/H is in the range (0.5 - 1.9)×10<SUP>-5</SUP>.