We present HI line profiles for various models of circumstellar shells around red giants. In the calculations we take into account the effect of the background at 21 cm, and show that in some circumstances it may have an important effect on the shape
and intensity of the observed line profiles. We show that self-absorption should also be considered depending on the mass loss rate and the temperature reached by circumstellar gas. HI emission from circumstellar shells has been mostly reported from stars with mass loss rates around 10$^{-7}$ solar masses per year. We discuss the possible reasons for the non detection of many sources with larger mass loss rates that are hallmarks of the end of the AGB phase. Although radiative transfer effects may weaken the line emission, they cannot alone account for this effect. Therefore, it seems likely that molecular hydrogen, rather than atomic hydrogen, dominates the composition of matter expelled by stars at the end of their evolution on the Asymptotic Giant Branch. However sensitive HI observations can still yield important information on the kinematics and physical properties of the circumstellar material at large distances from central stars with heavy mass loss, despite the low abundance of atomic hydrogen.
We present a phenomenological study of CO(1-0) and CO(2-1) emission from the circumstellar envelope (CSE) of the Asymptotic Giant Branch (AGB) star RS,Cnc. It reveals departures from central symmetry that turn out to be efficient tools for the explor
ation of some of the CSE properties. We use a wind model including a bipolar flow with a typical wind velocity of $sim$8 km,s$^{-1}$ decreasing to $sim$2 km,s$^{-1}$ near the equator to describe Doppler velocity spectral maps obtained by merging data collected at the IRAM Plateau de Bure Interferometer and Pico Veleta single dish radio telescope. Parameters describing the wind morphology and kinematics are obtained, together with the radial dependence of the gas temperature in the domain of the circumstellar envelope probed by the CO observations. Significant north-south central asymmetries are revealed by the analysis, which we quantify using a simple phenomenological description. The origin of such asymmetries is unclear.
