An HI Imaging Survey of Asymptotic Giant Branch Stars

Abstract

We present an imaging study of a sample of eight asymptotic giant branch (AGB) stars in the HI 21-cm line. Using observations from the Very Large Array, we have unambiguously detected HI emission associated with the extended circumstellar envelopes of six of the targets. The detected HI masses range from M_HI ~ 0.015-0.055 M_sun. The HI 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 HI 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 (V_space>56 km/s), 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" HI 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 HI to date.

Accepted to AJ. A version with full resolution figures is available at http://www.haystack.mit.edu/hay/staff/lmatthew/matthews_HI_survey.pdf

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• HI line observations of 290 evolved stars made with the Nancay Radio Telescope -- I. Data
• Evidence of a Mira-like tail and bow shock about the semi-regular variable V CVn from four decades of polarization measurements
• HI emission from the red giant Y CVn with the VLA and FAST
• Discovery of Raman-scattered He II $\lambda$6545 in the Planetary Nebulae NGC 6886 and NGC 6881
• Detached Shell Carbon Stars: Tracing Thermal Pulses on the Asymptotic Giant Branch