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B.E. Wood, J.L. Linsky (JILA, U. Colorado)
We present new HST/STIS observations of absorption lines seen in the UV spectrum of the nearby (d=5.5 pc) K0~V star 36~Oph~A. These data were taken to study both the local interstellar medium (LISM) along the line of sight, and to study H~I Lyman-\alpha absorption from hot neutral hydrogen in the outer heliosphere. This heliospheric absorption has previously been detected towards both \alpha~Cen and Sirius, which are 52\circ and 139\circ from the upwind direction, respectively (Linsky & Wood, 1996, ApJ, 463, 254; Izmodenov et al.\ 1999, A&A, 342, L13). Our target, 36~Oph~A, was chosen for its location only 12\circ from the upwind direction, where the velocity separation between the LISM and heliospheric absorption should be at its largest, making detection and measurement of the heliospheric component easier.
Analogous to what was seen in the \alpha~Cen analysis, the H~I Lyman-\alpha absorption is redshifted relative to the Mg~II, Fe~II, and D~I absorption lines, demonstrating that an absorption component exists on the red side of the highly optically thick H~I line which is not present in the other lines. As in the \alpha~Cen analysis, we interpret this extra component to be due to H~I in the outer heliosphere that is heated and decelerated by charge exchange processes. This detection provides additional information for mapping the distribution of heliospheric hydrogen and constraining heliospheric models. In addition to the heliospheric absorption, we also find evidence that absorption from analogous ``astrospheric'' material around the star may be present on the blue side of the line.
The author(s) of this abstract have provided an email address for comments about the abstract: woodb@marmot.colorado.edu