This web page serves to collect together material, primarily simulation results and preprints, that focuses on the phenomenon of the magnetic confinement of hot-star winds.
The Magnetically-Confined Wind Shock (MCWS) model of Babel & Montmerle (1997) proposes that a stellar wind upflow from opposite footpoints of closed magnetic loops collides to form strong, stationary shocks near the loop apex. This model may explain the hard X-ray spectrum from the O7V star θ1 Ori C. Magnetohydrodynamical (MHD) simulations support such a conclusion, revealing that the magnetic field of the star will be effective in channeling wind material toward the equatorial plane, where strong shocking occurs.
Below, we present results from the MHD simulations of θ1 Ori C. We show the time evolution of the plasma density, temperature and speed, over a ∼500 kilosecond run; on top of each, we plot the magnetic field lines.
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In the animation below, we illustrate our view of the rigid dipole field of an oblique magnetic rotator, assuming the geometry for θ1 Ori C of Donati et al. (2002): obliquity β=42°, inclination i=45° and period P=15.422 days. Phase φ=0 corresponds to X-ray and Hα emission maximum. The viewing angle varies from α=4° at phase 0.0 to α=87° at phase 0.5. IDL code courtesy Nikolai Piskunov.
Although MHD simulations are a very powerful technique for investigating magnetic wind confinement, there are some circumstances where their use becomes difficult. For stars which possess very strong magnetic fields, yet only modest wind outflows, the numerical timestep required to preserve the stability of an MHD simulation becomes very short, greatly increasing the computational cost of the simulation.
To model such stars, we have developed an alternative semi-analytical approach, based around the assumption that the magnetic field is completely rigid. With sufficiently rapid rotation, we find that wind material can steadily accumulate in a circumstellar magnetosphere, where - supported by centrifugal forces - it is constrained into rigid co-rotation by the magnetic field. Below, we present results from this Rigidly-Rotating Magnetosphere (RRM) model. Each animation shows a map of the optically-thick Hα emission produced by a dipole-field RRM, for a star rotating at 50% of its critical rate; we vary the obliquity β and observer inclination i.
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Applying the RRM approach to the strong-field (∼10 kG) B2p star σ Ori E, we find that the variable Hα emission and brightness of the star can be well reproduced by a model with a dipole obliquity β=55° and an inclination i=75°. Below, we show an animation of this model; clockwise from top left, the panels show a map of the Hα emission (as above), the star's light curve, the Hα line profile, and the disk-averaged longitudinal field strength. These synthetic diagnostics exhibit very good agreement with the observed behavior of the star.
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Further details on the MCWS and RRM models, and their respective application to θ1 Ori C and σ Ori E, can be found in the following papers: