On the possible interaction between magnetic field and HII region in AB Aur

Abstract

The Ae Herbig’s star, AB Aur, shows alternatively at different times, a Beals Type III and Type II P Cygni profile, in H _α and H _β lines. We investigate the possibility that such behaviour is produced by a dipolar magnetic field which forces the inner parts of the expanding envelope (stellar wind) to corotate with the star out to a few stellar radii.

An explanatory model is proposed for the corotation region (the so called magneto-sphere) including HII region. Since the magnetosphere has a non-spherical symmetry, if magnetic and rotation axes are misaligned, the magnetosphere axis undergoes a pre-cessional motion around the rotation axis, showing to the observer different corotation radii and consequently different radial velocity dispersion in the hydrogen lines. It is proposed that when the projected velocity of the corotating portion of the envelope is larger than the velocity of the stellar wind in the line-forming region, we observe an emission component in the blue side of the absorption feature (Beals Type III profile). Furthermore, the combined effects of the oblique rotator geometry and the mass loss produced by the centrifugal force give rise to the changing Type II P Cyg profile at H _α and H _β lines.