The Magnetic Field of Binary Stars

The complex joint magnetosphere of the binary system V4046 Sgr. (Image credit: V. Holzwarth, KIS; S. G. Gregory, Univ. St Andrews)

Close binary stars with late-type, solar-like components are among the most active stellar objects known. The combination of vigorous convective motions with rapid rotation promotes efficient dynamo operation, which entails a plethora of magnetic activity considerably stronger than on the Sun. Examples include extensive dark spots in the photosphere, enhanced chromospheric emission, and high-energetic flaring events in the corona. The magnetic field has a decisive influence on the structural, dynamical and thermal properties of the atmosphere of these stars. However, since even in the case of the Sun direct observations of coronal magnetic fields are difficult to accomplish, extrapolation methods are frequently used to infer them from observed magnetic field distributions in the photosphere.

We have extended the potential field source surface extrapolation technique, a basic method which was originally developed for the case of the Sun, to binary star systems to investigate the structure and properties of their joint magnetosphere. As an example of the applicability of the new technique, the picture shows the complex, inter-connected magnetosphere (movie) of the binary system V4046 Sgr, for which magnetic surface maps for both stellar components have been reconstructed based on Zeeman-Doppler imaging observations.


  • Holzwarth, V, Gregory, S.G., Potential magnetic field extrapolation in binary star systems in 18th Cambridge Workshop on Cool Stars, Stellar Systems, edited by G. van Belle & H. Harris (submitted)
  • Donati et al. 2011, MNRAS 417,1747