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We assess what information HST observations of stellar Ly-alpha lines canprovide on the heliosheath, the region of the heliosphere between thetermination shock and heliopause. To search for evidence of heliosheathabsorption, we conduct a systematic inspection of stellar Ly-alpha linesreconstructed after correcting for ISM absorption (andheliospheric/astrospheric absorption, if present). Most of the stellar linesare well centered on the stellar radial velocity, as expected, but the threelines of sight with the most downwind orientations relative to the ISM flow(Chi1 Ori, HD 28205, and HD 28568) have significantly blueshifted Ly-alphalines. Since it is in downwind directions where heliosheath absorption shouldbe strongest, the blueshifts are almost certainly caused by previouslyundetected heliosheath absorption. We make an initial comparison between theheliosheath absorption and the predictions of a pair of heliospheric models. Amodel with a complex multi-component treatment of plasma within the heliospherepredicts less absorption than a model with a simple single-fluid treatment,which leads to better agreement with the data. Finally, we find thatnonplanetary energetic neutral atom (ENA) fluxes measured by the ASPERA-3instrument on board Mars Express, which have been interpreted as being from theheliosheath, are probably too high to be consistent with the relative lack ofheliosheath absorption seen by HST. This would argue for a local interplanetarysource for these ENAs instead of a heliosheath source.Comment: 27 pages, 7 figures, AASTEX v5.0, accepted by Ap

Lyα Absorption from Heliosheath Neutrals