We have used the Robert C. Byrd Green Bank Telescope to perform the most sensitive search to date for neutral atomic hydrogen (HI) in the circumstellar envelope (CSE) of the carbon star IRC+10216. Our observations have uncovered a low surface brightn
ess HI shell of diameter ~1300 (~0.8 pc), centered on IRC+10216. The HI shell has an angular extent comparable to the far ultraviolet-emitting astrosphere of IRC+10216 previously detected with the GALEX satellite, and its kinematics are consistent with circumstellar matter that has been decelerated by the local interstellar medium. The shell appears to completely surround the star, but the highest HI column densities are measured along the leading edge of the shell, near the location of a previously identified bow shock. We estimate a total mass of atomic hydrogen associated with IRC+10216 CSE of M_HI~3x10e-3 M_sun. This is only a small fraction of the expected total mass of the CSE (<1%) and is consistent with the bulk of the stellar wind originating in molecular rather than atomic form, as expected for a cool star with an effective temperature T_eff<~2200 K. HI mapping of a 2 deg x 2 deg region surrounding IRC+10216 has also allowed us to characterize the line-of-sight interstellar emission in the region and has uncovered a link between diffuse FUV emission southwest of IRC+10216 and the Local Leo Cold Cloud.
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 o
f 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.
We report HI 21-cm line observations of the AGB star X Her obtained with the Green Bank Telescope (GBT) and the Very Large Array (VLA). We have detected HI emission totaling M_HI=2.1e-03 M_sun associated with the circumstellar envelope of the star. T
he HI distribution exhibits a head-tail morphology, similar to those previously observed around Mira and RS Cnc. The tail extends ~6.0 (0.24 pc) in the plane of the sky, along the direction of the stars space motion. We also detect a velocity gradient of ~6.5 km/s across the envelope, consistent with the HI tracing a turbulent wake that arises from the motion of a mass-losing star through the ISM. GBT mapping of a 2x2deg 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 ISM interaction. The properties of the cloud are consistent with those of compact high-velocity clouds. Using CO observations, we have placed an upper limit on its molecular gas content of N_H2<1.3e20 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 (~>2.4~M_sun) and the blueshift of its mean velocity relative to X Her are inconsistent with an origin tied directly to stellar ejection. (abridged)
We report the detection of an HI counterpart to the extended, far-ultraviolet-emitting tail associated with the asymptotic giant branch star Mira (o Ceti). Using the Nancay Radio Telescope (NRT), we have detected emission as far as 88 north of the st
ar, confirming that the tail contains a significant atomic component (M_HI ~ 4x10e-3 M_sun). The NRT spectra reveal a deceleration of the tail gas caused by interaction with the local interstellar medium. We estimate an age for the tail of ~1.2x10e5 years, suggesting that the mass-loss history of Mira has been more prolonged than previous observational estimates. Using the Very Large Array (VLA) we have also imaged the HI tail out to ~12 (0.4 pc) from the star. The detected emission shows a ``head-tail morphology, but with complex substructure. Regions with detected HI emission correlate with far-ultraviolet-luminous regions on large scales, but the two tracers are not closely correlated on smaller scales (<1). We propose that detectable tails of HI are likely to be a common feature of red giants undergoing mass-loss.
