Discovery of a detached HI gas shell surrounding alpha Orionis

TL;DR

This paper reports the discovery of a detached HI gas shell around alpha Orionis, indicating stellar wind interaction with the interstellar medium and providing insights into the star's mass-loss history.

Contribution

First detection of a quasi-stationary detached HI shell around alpha Ori, revealing its interaction with the ISM and star's mass-loss over 80,000 years.

Findings

Detached HI shell ~0.24 pc in diameter detected

Star's mass-loss rate estimated at ~1.2x10^-6 solar masses/year

Far-UV counterpart and bow shock associated with alpha Ori identified

Abstract

We report the detection of the HI line at 21 cm in the direction of alpha Ori with the Nancay Radiotelescope and with the Very Large Array. The observations confirm the previous detection of HI emission centered on alpha 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 alpha Ori with the local interstellar medium (ISM). It implies that alpha Ori has been losing matter at a rate of ~ 1.2x10^-6 solar masses per year for the past 8x10^4 years. In addition, we report the detection of atomic hydrogen associated with the far-infrared arc located 6 arcmin. north-east of alpha Ori, that has been suggested to trace the bow shock resulting from the motion of the star through the ISM. We report also the detection by the Galaxy Evolution Explorer (GALEX) of a far-UV counterpart to this arc.