The kinematics in the large-scale environment of Betelgeuse from radio HI-line observations

Abstract

The observations of mass losing stellar sources in the H i line at 21 cm allow us to study in detail the kinematics in their large size circumstellar shells. We report on the results that have been obtained on Betelgeuse with the Nancay Radiotelescope (NRT) and with the Very Large Array (VLA). On the stellar position, we find a double-horn line profile characteristic of a freely expanding wind at a velocity of ~ 14 km s -1 . We find also that the stellar outflow is slowed down by the pressure from the ambient medium, and forms a quasi-stationary detached shell of ~ 4′ in diameter (0.24 pc at a distance of 200 pc). The H i line profile from this detached shell has a width of 3 km s -1 , and is centered at a velocity close to the star radial velocity ( V lsr = +3 km s -1 ). The bulk of the material detected in H i ( ~ 0.05 M ⊙ ) has been heated at a temperature ~ 6000 K, and is cooling down to ~ 200 K. Furthermore, due to the motion relative to the local interstellar medium, the detached shell is distorted and elongated in a direction close to the space motion. Finally, we have detected H i emission associated with the 6′ radius far-infrared arc seen by IRAS, and with a counterpart to this arc that we have recently discovered in the far-ultraviolet. This H i emission is found in a velocity range (from +6 to +10 km s -1 ) which matches that of the interstellar medium observed on the same line-of-sight.