Bartol Research Institute, University of Delaware, Newark

Redshifted Lya absorption toward a Cen has been interpreted by Linsky & Wood and Frisch et. al. as evidence for decelerated interstellar hydrogen piled up on the upstream side of the heliosphere. We utilize newly developed two-dimensional multifluid models of the solar wind interaction with the interstellar material to corroborate this interpretation by synthesizing the Lya absorption profile predicted for this "hydrogen wall." Both subsonic and supersonic inflow into the heliosphere are considered, corresponding to one-shock and two-shock global morphologies, respectively. We find that these two extremes give observably different redward absorption characteristics in the Lya profiles, and our preliminary conclusion is that the Lya profiles seen toward a Cen favor a barely subsonic model (Mach number 0.9). for such a model to hold, additional interstellar pressure terms, such as cosmic-ray or magnetic pressures, must contribute. To make this conclusion more certain, an extended-model parameter survey is required, coupled with Lya data along other lines of sight.

The Astrophysical J., 487, 259-270, 1997