Buried magnetic flux tubes in giant stars near the "Coronal Dividing Line"

Max-Planck-Institut für Aeronomie, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany

Corresponding author: V. Holzwarth, holzwarth@linmpi.mpg.de

Received: 2 May 2001
Accepted: 2 August 2001

Abstract

We apply the "solar paradigm" for stellar magnetic activity to the post-main-sequence evolution of stars in the mass range . The model starts from a strong toroidal magnetic field generated by a dynamo working in the overshoot layer below the convection envelope. Once a critical field strength is exceeded, an undulatory (Parker-type) instability leads to rising flux loops. Upon emergence at the stellar surface, they form bipolar magnetic regions and large-scale coronal loops. By considering the stability, dynamics, and rise of magnetic flux tubes along evolutionary sequences of stellar models, we find that the flux loops become trapped in the stellar interior when the depth of convective envelope exceeds about 80% of the stellar radius. Trapping is caused by an increase of field line curvature at the loop summit, so that eventually the magnetic tension force dominates over the buoyancy force. The magnetic loops find a stable equilibrium configuration within the convection zone and do not emerge at the stellar surface. The transition from emerging to trapped flux tubes falls in the range of spectral types G7 to K0 for luminosity class III giants, which is close to the observed "coronal dividing line" in the Hertzsprung-Russell diagram. This result is remarkably stable within large ranges of stellar parameters (mass, rotation) and flux tube parameters (field strength, magnetic flux) and depends practically exclusively on the fractional radius of the stellar radiative core. We suggest that flux tube trapping is the cause for the strong decline of stellar X-ray emission across the "coronal dividing line" .