• circumstellar matter;
  • stars: individual: α Ori;
  • stars: mass-loss;
  • supergiants;
  • radio lines: stars


We report the detection of the H i line at 21 cm in the direction of α Orionis (α Ori) with the Nançay Radiotelescope and with the Very Large Array.

The observations confirm the previous detection of H i emission centred on α Ori, but additionally reveal for the first time a quasi-stationary detached shell of neutral atomic hydrogen ∼4 arcmin in diameter (0.24 pc at a distance of 200 pc). The detached shell appears elongated in a direction opposite to the star’s space motion.

A simple model shows that this detached atomic gas shell can result from the collision of the stellar wind from α Ori with the local interstellar medium (ISM). It implies that α Ori has been losing matter at a rate of ∼1.2 × 10−6 M yr−1 for the past 8 × 104 yr.

In addition, we report the detection of atomic hydrogen associated with the far-infrared arc located 6 arcmin north-east of α Ori, which has been suggested to trace the bow shock resulting from the motion of the star through the ISM. We also report the detection by the Galaxy Evolution Explorer of a far-ultraviolet counterpart to this arc.