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

Abstract

Abstract. We apply the "solar paradigm" for stellar magnetic activity to the post-main-sequence evolution of stars in the mass range 1 M ≤ M ≤ 3 M . 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".

Modeling the Rise of Fibril Magnetic Fields in Fully Convective Stars

“…To facilitate some level of comparison between these initial condition assumptions, we also perform some simulations where flux tubes initially in MEQ rise through a quiescent convection zone. Similar to TFT models in stars with small radiative cores (e.g., Granzer et al 2000;Holzwarth & Schüssler 2001), we find that most of these flux tubes slip poleward, driven by a magnetic tension force that is dominant compared to buoyancy. However, strongly super-equipartition flux tubes can develop low-order ( = m 1 2 -) unstable modes if the magnetic field strength and radius of curvature arelarge enough.…”

“…Traditional TFT models tend to assume that the dynamo mechanism generates toroidal flux tubes at the interface between the radiative interior and the convection zone, often assumed to be in mechanical force equilibrium (MEQ) and neutral buoyancy (e.g., Caligari et al 1995;Granzer et al 2000;Holzwarth & Schüssler 2001;Weber et al 2011). However, recent simulations of global-scale dynamo action in spherical shells (e.g., Nelson et al 2014) have demonstrated that, at least in some parameter regimes, buoyant magnetic loops can be built by a distributed dynamo without a tachocline region.…”

“…The condition of TEQ deviates from the state of neutral buoyancy (r r = e ) often used in TFT simulations for stars with convectively stable radiative interiors (e.g., Caligari et al 1998;Granzer et al 2000;Holzwarth & Schüssler 2001;Weber et al 2011). Within the long-favored solar interface dynamo paradigm, it is assumed that toroidal magnetic fields are amplified and stored in the stably stratified convective overshoot/tachocline region.…”

Structure of the outer layers of cool standard stars

“…The transition in topology is ascribed to a change in the origin of the field: as the stellar rotation drops below a critical value, the spin-catalyzed dynamo gives way to a field generation mechanism requiring fluid turbulence as found in the convection zone. It has also been suggested that the magnetic flux tubes that rise up from underneath the convection zone to the stellar surface where they form large scale coronal loops, become trapped in the convective envelope as the convection zone deepens to the right of the DLs (Holzwarth & Schüssler 2001). However, Ayres et al (1997Ayres et al ( , 2003 have found evidence in "non-coronal" giants that coronal loops can still rise to the stellar surface: the loops extend beyond the cold molecular layer just above the stellar photosphere, but are at least partially submerged in the chromosphere/COmosphere, where the coronal X-rays are attenuated by overlying material.…”

Precessing planetary magnetospheres in SiO stars?

“…If the convection zone extends to at least 80% of the stellar radius (Holzwarth & Schüssler 2001), magnetic flux loops are trapped there, because magnetic stresses at the tips of the loops act against convective buoyancy. This also explains the weakness of the coronal X-ray luminosity, and why the magnetic fluctuations are rare.…”