21 - Stellar dynamics: Rotation, convection, and magnetic fields

Summary

Introduction

Stars are changing entities in a constant evolution during their lives. At non-secular time scales – from seconds to years – the effect of dynamical processes such as convection, rotation, and magnetic fields can modify the stellar oscillations. Convection excites acoustic modes in solar-like stars, while rotation and magnetic fields can perturb the oscillation frequencies, lifting the degeneracy in the azimuthal component m of the eigenfrequencies (see Chapter 9 for the case in which rotation is slow and first-order perturbative theory can be used). Moreover, the interaction between rotation, convection, and magnetic fields can produce magnetic dynamos, which sometimes yield to regular magnetic activity cycles.

In this chapter we review how stellar dynamics can be studied and explain what long-term seismic observations can bring to the understanding of this field. Thus, we show how we can study some properties of the convective time scales operating in a star like the Sun. We also compare the stratified information we can obtain on the internal (radial) differential rotation from main-sequence solar-like stars to the Sun, and to more evolved subgiants and giants. We complement this information on the internal rotation with the determination of the surface (latitudinal differential) rotation obtained directly from the lightcurves. Indeed, when stars are active there can be spots on their surfaces dimming the light emitted. When the star rotates, the emitted light will be modulated by the presence of these spots with a period corresponding to the rotation rate at the active latitudes (where the spots develop). We finally give a brief summary of stellar magnetic studies based on spectroscopic observations and then we discuss the use of seismology to better understand the stellar magnetism of solar-like stars and the existence of possible magnetic cycles. We conclude this chapter by discussing the seismology of fast rotating stars and, from a theoretical point of view, what are the current challenges to infer properties of the internal structure and dynamics of intermediate-and high-mass stars.