Heliospheric, astrospheric, and interstellar Ly alpha absorption toward 36ophiuchi

We use high-resolution UV spectra taken by the Space Telescope Imaging Spec trograph instrument on board the Hubble Space Telescope to study the 5.5 pc line of sight to the K0 V star 36 Oph A. The one interstellar component de tected for this line of sight has a velocity inconsistent with the local in terstellar cloud (LIC) flow vector but consistent with the flow vector of t he so-called G cloud, a very nearby warm cloud in the Galactic center direc tion. From our data, we measure the following values for the interstellar t emperature, nonthermal velocity, H I column density, and D/H value: T = 590 0 +/- 500 K, xi = 2.2 +/- 0.2 km s(-1), log N-H = 17.85 +/- 0.15, and D/H = (1.5 +/- 0.5) x 10(-5). The temperature of the G cloud is somewhat lower t han that of the LIC, and Mg and Fe depletions are also lower, but the D/H v alue appears to be the same. Based on upper limits for the LIC absorption, we estimate the distance to the edge of the LIC to be d(edge) < 0.19 pc, wh ich the Sun will reach in t(edge) < 7400 yr. The H I Ly alpha absorption li ne has properties inconsistent with those of the other absorption lines, in dicating the presence of one or more absorption components not seen in the other lines. We present evidence that hot neutral hydrogen local to both th e Sun and the star is responsible for the excess Lya absorption. This hot H I is created by the interaction between the ISM and the winds of the Sun a nd 36 Oph A. The observed line of sight lies only 12 degrees from the upwin d direction of the LIC flow vector, where hydrodynamic models of the helios phere suggest that heliospheric H I. absorption should be particularly prom inent. The properties of the heliospheric absorption (T = 38,000 +/- 8000 K and log N-H = 14.6 +/- 0.3) are consistent with previous measurements of t his absorption for the alpha Cen line of sight 52 degrees from the upwind d irection.