Centrifugal breakout
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The steady accumulation of material in a co-rotating magnetosphere, as envisaged in the Rigidly Rotating Magnetosphere (RRM) and Rigid Field Hydrodynamics (RF-HD) models,
cannot continue indefinitely. Eventually, circumstellar densities approach levels where the outward centrifugal force begins to overwhelm the inward magnetic tension forces. Then, field lines sag outward and eventually snap, ejecting the previously-trapped magnetospheric plasma in a direction perpendicular to the rotation axis.
During one of these centrifugal breakout episodes, the reconnection of the stressed magnetic field lines releases the energy stored by the centrifugal force. The subsequent dissipation of this reconnection energy appears to be a strong candidate for the generation of the X-ray flares recently detected in σ Ori E (see Sanz-Forcada et al. 2004 and Groote & Schmitt 2004) and HD 38563S (see Yanagida et al. 2004).
MHD Simulations
Due to the assumption that the field lines always remain rigid, simulation of centrifugal breakout lies beyond the RRM and RF-HD model. However, MHD simulations are possible - albeit at much lower values of the magnetic confinement parameter than appropriate to, e.g., σ Ori E (
). As a preliminary test of the centrifugal breakout paradigm, Asif conducted 2D MHD simulations for a rapidly-rotating star with a dipole magnetic field having a moderate confinement parameter (
). The animations below show the time evolution of the plasma density, temperature and speed, over a 100 kilosecond run starting 400 kiloseconds after the initial condition.
| Density | Temperature | Speed |
|---|---|---|
 |  |  |
The stretching and snapping of magnetic field lines, associated with centrifugal breakout, can clearly be seen in these animations; note in particular the heating to temperatures in excess of 100 million Kelvin, arising from the energy released during magnetic reconnection. This heating appears sufficient to explain the hard component (> 2 keV) seen in the spectral energy distributions of the X-ray flares mentioned above (see ud-Doula et al. 2006).
