2015MNRAS.449.2386H

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 21cm, 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 M_☉/yr. 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 asymptotic giant branch (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 AGB. 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.