A H I IMAGING SURVEY OF ASYMPTOTIC GIANT BRANCH STARS

We present an imaging study of a sample of eight asymptotic giant branch stars in the H I 21 cm line. Using observations from the Very Large Array, we have unambiguously detected H I emission associated with the extended circumstellar envelopes of six of the targets. The detected H I masses range from MHI ≈ 0.015-0.055 Msun. The H I morphologies and kinematics are diverse, but in all cases appear to be significantly influenced by the interaction between the circumstellar envelope and the surrounding medium. Four stars (RX Lep, Y UMa, Y CVn, and V1942 Sgr) are surrounded by detached H I shells ranging from 0.36 to 0.76 pc across. We interpret these shells as resulting from material entrained in a stellar outflow being abruptly slowed at a termination shock where it meets the local medium. RX Lep and TX Psc, two stars with moderately high space velocities (Vspace > 56 km s–1), exhibit extended gaseous wakes (∼0.3 and 0.6 pc in the plane of the sky), trailing their motion through space. The other detected star, R Peg, displays a peculiar ''horseshoe-shaped'' H I morphology with emission extended on scales up to ∼1.7 pc; in this case, the circumstellar debris may have been distorted by transverse flows in the local interstellar medium. We briefly discuss our new results in the context of the entire sample of evolved stars that has been imaged in H I to date.