Session 45 - Interstellar Medium I.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center

## [45.03] First Measurement of the Hydrogen Wall around the Heliosphere and the Interstellar Medium toward Alpha Centauri

J. L. Linsky, B. Wood (JILA/Univ. Colorado)

We analyze high-resolution GHRS spectra of the nearby (1.34 pc) stars \alpha Cen A (G2 V) and \alpha Cen B (K1 V). The observations consist of echelle spectra of the Mg II 2800 Å\ and Fe II 2599 Å\ resonance lines and the Lyman-\alpha lines of hydrogen and deuterium. The centroid velocity of the interstellar absorption of all lines except hydrogen is consistent with the local flow vector proposed for this line of sight by Lallement amp; Bertin (1992). The temperature and nonthermal velocity inferred from the Fe II, Mg II, and D I line profiles are T=5400\pm 500 K and \xi=1.20\pm 0.25 km s^-1 , respectively. However, single-component fits to the H I Lyman-\alpha lines yield a Doppler parameter (b_H I=11.80 km s^-1) that implies a significantly warmer temperature of 8350 K. Furthermore, the velocity of the H I absorption (v=-15.8\pm 0.2 km s^-1) is redshifted by about 2.2 km s^-1 with respect to the velocity of the Fe II, Mg II, and D I lines.

The most sensible way to resolve the discrepancy between H I and the other lines is to include a second absorption component when fitting the H I lines. Compared to the main absorption component, this second component is hotter (T\approx 30,000 K), 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 second component is located near the heliopause, consisting of the heated H I 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, velocity, and column density of H I in the hydrogen wall. We estimate that the parameters of hydrogen wall gas are \log N_H I^=14.74\pm 0.24, b_H I^(2)=21.9\pm 1.7 km s^-1 (corresponding to a temperature of T=29,000\pm 5,000 K), and v_H I^(2) > -16 km s^-1.

This work was sponsored by NASA Interagency Transfer S-56460-D to the National Institute of Standards and Technology.