HI OBSERVATIONS OF THE ASYMPTOTIC GIANT BRANCH STAR X HERCULIS: DISCOVERY OF AN EXTENDED CIRCUMSTELLAR WAKE SUPERPOSED ON A COMPACT HIGH-VELOCITY CLOUD

We report Hi 21 cm line observations of the asymptotic giant branch (AGB) star X Her obtained with the Robert C. Byrd Green Bank Telescope (GBT) and the Very Large Array. We have unambiguously detected Hi emission associated with the circumstellar envelope of the star, with a mass totaling M-HI approximate to 2.1 x 10(-3) M-circle dot. The Hi distribution exhibits a head-tail morphology, similar to those previously observed around the AGB stars Mira and RS Cnc. The tail is elongated along the direction of the star's space motion, with a total extent of >= 6'.0 (0.24 pc) in the plane of the sky. We also detect a systematic radial velocity gradient of similar to 6.5 kms(-1) across the Hi envelope. These results are consistent with the Hi emission tracing a turbulent wake that arises from the motion of a mass-losing star through the interstellar medium (ISM). GBT mapping of a 2 degrees x 2 degrees region around X Her reveals that the star lies (in projection) near the periphery of a much larger Hi cloud that also exhibits signatures of interaction with the ISM. The properties of the cloud are consistent with those of compact high-velocity clouds. Using (CO)-C-12 J = 1-0 observations, we have placed an upper limit on its molecular gas content of N-H2 < 1.3 x 10(20) cm(-2). Although the distance to the cloud is poorly constrained, the probability of a chance coincidence in position, velocity, and apparent position angle of space motion between X Her and the cloud is extremely small, suggesting a possible physical association. However, the large Hi mass of the cloud (>= 2.4M(circle dot)) and the blueshift of its mean velocity relative to X Her are inconsistent with an origin tied directly to ejection from the star.