H I Observations of the Asymptotic Giant Branch Star X Herculis: Discovery of an Extended Circumstellar Wake Superposed on a Compact High-velocity Cloud

We report H I 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 H I emission associated with the circumstellar envelope of the star, with a mass totaling M _{H I} ≈ 2.1 × 10^-3 M _sun. The H I 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 >~ 6farcm0 (0.24 pc) in the plane of the sky. We also detect a systematic radial velocity gradient of ~6.5 km s^-1 across the H I envelope. These results are consistent with the H I 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° × 2° region around X Her reveals that the star lies (in projection) near the periphery of a much larger H I 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 ¹²CO J = 1-0 observations, we have placed an upper limit on its molecular gas content of N _H2 < 1.3 × 10²⁰ 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 H I mass of the cloud (gsim2.4 M _sun) and the blueshift of its mean velocity relative to X Her are inconsistent with an origin tied directly to ejection from the star.