Very Large Array Observations of Hiin the Circumstellar Envelopes of Asymptotic Giant Branch Stars

(Abridged) We have used the VLA to search for neutral atomic hydrogen in thecircumstellar envelopes of five AGB stars. We have detected HI 21-cm emissioncoincident in both position and velocity with the semi-regular variable RS Cnc.The emission comprises a compact, slightly elongated feature centered on thestar with a mean diameter ~82'' (1.5e17 cm), plus an additional filamentextending ~6' to the NW. This filament suggests that a portion of the mass lossis highly asymmetric. We estimate MHI=1.5e-3 Msun and M_dot~1.7e-7 Msun/yr.Toward R Cas, we detect weak emission that peaks at the stellar systemicvelocity and overlaps with the location of its circumstellar dust shell andthus is probably related to the star. In the case of IRC+10216, we were unableto confirm the detection of HI in absorption against the cosmic backgroundpreviously reported by Le Bertre & Gerard. However, we detect arcs of emissionat projected distances of r~14'-18' (~2e18 cm) to the NW. A large separation ofthe emission from the star is plausible given its advanced evolutionary status,although it is unclear if the asymmetric distribution and complex velocitystructure are consistent with a circumstellar origin. For EP Aqr, we detectedHI emission comprising multiple clumps redward of the systemic velocity, but weare unable to determine unambiguously whether the emission arises from thecircumstellar envelope or from interstellar clouds along the line-of-sight.Regardless of the adopted distance for the clumps, their inferred HI masses areat least an order of magnitude smaller than their individual binding masses. Wedetected our fifth target, R Aqr (a symbiotic binary), in the 1.4 GHzcontinuum, but did not detect any HI emission from the system.