Internal rotational dynamics of a star influences star evolution through poorly understood transport and mixing mechanisms. Magnetic fields can transport angular momentum and chemical elements. However, it is still being determined where the magnetism in the radiation layers of stars came from.

The star’s core contract under specific conditions. When this happens, they begin to spin faster than the outer layers of the star. Amazingly, the cores of these stars spin slower than predicted, according to asteroseismology, which studies star oscillations.

Why is this so?

Three French scientists from CNRS, INRIA and ENS-PSL have studied this question. They used numerical simulations to show how a stellar dynamo can form in purely radiative layers (meaning they have no convection). They identified a subcritical transition from laminar flow to turbulence caused by the generation of a magnetic dynamo.

Their numerical simulations, which model the plasma flow in the deep layers of a star, have shown that an internal magnetic field can slow down the core. More specifically, plasma flow can amplify a magnetic field to the point where it generates turbulent solid motion. Such turbulence can further amplify the magnetic field until the star’s core begins to spin downward.

The results of the research team’s simulations are consistent with numerous asteroseismological observations of stars. In addition, the simulations show that the magnetic field would be obscured by the star’s outer layers, which explains why this kind of magnetic field has not been detected using modern methods until now.

Magazine reference:

  1. Ludovic Petitdemange et al. Spin-down by dynamo action in simulated radiant stellar layers. Science. DOI: 10.1126/science.abk2169