Magnetic Fields in Rotating Stars

Some stars contain magnetic fields which we can observe in their spectra via the Zeeman effect. These fields can be very strong: around 1 tesla, which is around 20,000 times stronger than the Earth's magnetic field. These fields are formed as the star forms; whatever magnetic field is left over from the cloud from which the star formed organizes itself into an equilibrium configuration in which it then remains for the whole of the star's lifetime. Very recently, however, it has been discovered that the other 90% of the stellar population displays very much weaker magnetic fields, about 0.0001 tesla, and that no stars have fields between about 0.0003 and 0.03 tesla. This is something of a puzzle: why should there be this bimodal distribution of magnetic field strengths? Moreover, what is the nature of these very weak fields? There is so far only one hypothesis: that the magnetic fields in these stars somehow failed to find their way into an equilibrium because the fast rotation of the star slowed down this process. There is a terrestrial analogy: the air in the atmosphere "wants" to flow from high pressure to low pressure, but is forced by the Earth's rotation to flow instead in circles, giving rise to cyclones and anticyclones. In this project, the student will perform simulations of the magnetic field evolving towards an equilibrium and will investigate the effect of the star's rotation, with the aim of testing this hypothesis. Further reading: http://arxiv.org/abs/1201.5646. 

For more information, please contact: