Coronal activity of stars more active than our Sun in 3D magneto-hydrodynamic modeling

Numerical experiments solving the 3D MHD equations of the corona of a star

Cool stars like our Sun are surrounded by a million K hot corona. This is one of the most interesting problems in stellar-astrophysics, because it is an open question how the cool stellar surface, typically at a couple of thousand K, can sustain the hot outer atmosphere. One of the key observations in the framework of this “coronal heating problem” is how the X-ray luminosity scales with stellar rotation: faster rotating stars showing higher magnetic field strengths at their surface have much brighter coronae (seen in X-rays), easily more than 1000 times brighter than the corona of our Sun.

Numerical models of active regions of the Sun proved successful in reproducing some of the key aspects of coronal structures. So the time is ripe to apply such model to stars more active than our Sun and understand this relation between the surface magnetic field and the heat input and X-ray luminosity of the corona arising from this. While these relations are very well studied from an observational point of view, our understanding of the cause for this relation is still quite poor.

For this numerical experiments should be run that solve the 3D MHD equations of (part of) the corona of a star. Just as in our solar models the heat input into the corona will be through Ohmic dissipation of currents that are induced by the magnetic field at the surface being driven around by convective motions. However, here we will change the structure and strength of the magnetic field and the convective motions. Preliminary models showed promising results for this.

To mimic the conditions on the different types of stars (from F to M) in the model basically the lower boundary at the stellar surface has to be changed. This includes studies of various levels of magnetic field, density, temperature, and gravitation at the surface level and investigations how that changes the energy input, structural appearance, and radiative output of a stellar corona.

The 3D MHD code is ready and set-up to tackle this problem. Depending on the qualification and interest of the PhD student modifications of the code can be applied to speed-up the computational time that would allow to run more models in the same time.

More information on the Solar and stellar coronae group and the group's work can be found at the Solar and stellar coronae group homepage. For questions please contact Hardi Peter.