Stellar magnetohydrodynamics

All stars rotate and many exhibit complex surface magnetic fields and activity. Coupled with the presence of turbulent convective envelopes or convective cores and stably stratified radiative zones, a large spectrum of nonlinear physical mechanisms and instabilities are at play in stars that deeply impact their evolution (Brun et al. 2015; Mathis 2013). It is essential to better characterise their relative amplitude and influence as a function of global parameters such as mass, age, rotation, and metallicity.

Understanding stellar magnetism with PLATO

The conditions to achieve cyclic magnetic activity or grand minima states (Augustson et al. 2015), stable inner magnetic field configurations (Duez & Mathis 2010), eruptive events such as flares or CME’s and various internal and surface rotation profiles are still poorly understood.

Confrontation between seismic data and surveys covering ranges of stellar masses and ages has shown that there is substantial variation in the nature of magnetic output (Petit et al. 2008; Marsden et al. 2014). This already provided constraints on theories of stellar convection and magnetism and results from numerical simulations (Alvan et al. 2015). A more detailed picture of magnetohydrodynamics of solar-like stars starts to emerge. Nevertheless, current multi-D simulations and theoretical modelling of stellar magnetohydrodynamics will benefit greatly from accurate constraints on convective powers and the extent of convective envelope or core, states of internal and surface rotation, properties of magnetic field, starspot coverage and period of cyclic activity, occurrence of flares and intense magnetic events. The asteroseismic techniques when applied to high quality data are able to assess how several key stellar parameters such as mass, age, rotation, metallicity and various stellar dynamical states are correlated and to improve our overall understanding of stars and their evolution. A larger survey than currently available is needed in order to constrain further multi-D numerical magnetohydrodynamics simulations of stars of various types. PLATO will provide such high quality survey. This will lead to a new generation of dynamical stellar evolution models, making more realistic their seismic modelling.

CoRoT provided evidence of a magnetic activity cycle (García et al. 2010), and initial constraints on stellar dynamo models under conditions different from those of the Sun (e.g. Mathur et al. 2013, 2014). This study was extended in terms of both the number of stars and measurement precision by Chaplin et al. (2011b) who used Kepler observations of a sample of Solar-type pulsators, and found a strong correlation between the strength of the activity and the level of inhibition of stochastically excited solar-like oscillations. Magnetic cycle has also an impact on the oscillation frequencies, whose values differ significantly depending on which phase of the cycle one observes. This bias must be taken into account when deriving the seismic properties of the host stars.

PLATO’s long time series will allow the study of magnetic activity cycles and spot decay time for various types of stars. The observation of starspot evolution, frequency, activity cycles, and surface distributions will allow us to better understand the physics behind activity phenomena, and to derive constraints for dynamo theories.

Surface rotation period and gyrochronology calibrated by seismology

For stars in the core programme that are too faint for detection of their oscillation frequency, the age will be determined by other means. One such technique is gyrochronology (e.g. Barnes 2007, 2011) that provides the age as a function of the surface period rotation of the star. Several such relations have been established empirically by fitting observational data for different sets of stars. Today, the precision on the age derived from such relations is clearly insufficient with respect to the PLATO specification (Lebreton & Goupil, 2014). Actually the relations between the rotation period and the age are not unique, they also depend on the effective temperature and the mass of the star. Theoretical investigations are necessary to understand better the physics behind these relations, and calibration with well seismically characterised stars will define the validity domain of these relations and how they can be used properly. Such studies have started with Kepler data (Angus et al. 2015; Van Saders et al. 2016) but will be fully developed with PLATO. Indeed, for a large set of bright stars ‒ therefore well spectroscopically characterised – PLATO will provide precise surface rotation and seismic age periods from the analysis of light curves. This will allow reliable statistical studies on the biases that keep gyrochronology today to provide precise and accurate stellar ages. Identifying these biases and understanding their physical origin will lead to improve the domain of validity of gyrochronology and its dating precision. These studies are very demanding in terms of the quality of observational data. This will be only possible with PLATO that will provide the appropriate large sample of fully well characterised stars.

Synergy star-planet

In direct link with the main goal of PLATO, that is to find earth-like planets around solar-like stars, a better knowledge of stellar nonlinear dynamics will improve our ability to understand their impact on the surrounding planets. Indeed, through either their magnetic activity, wind of particles and tidal effects, stars have a direct influence on planets and on their habitability (Strugarek et al. 2015; Mathis 2015). There is a complex feedback loop between rotation, magnetic field, and mass and angular momentum loss through stellar wind and tidal dissipation in stars, that needs to be  constrained. This will allow us to understand the orbital evolution of star-planet systems and the rotational evolution of their components on secular time scales, and to improve the realism of numerical simulations of such systems (Réville et al. 2015; Auclair-Desrotour et al. 2014). Stellar dynamical processes coupled with tidal dissipation in planets and MHD interactions with their magnetospheres will lead to a large diversity of configurations of star-planet systems that can be modelled. By providing constraints on the host star, surrounding planets and orbital architecture PLATO will improve greatly the study of star-planet interactions and the characterisation of key physical properties of stars, planets and planetary systems as a whole.

An important part of the stellar property characterisation for the exoplanet studies is therefore a precise knowledge of the surface activity of the host stars. One of the manifestations of stellar activity is spots at the surface of the star rotating with the star that generate temporal variability of the light curve. The high quality PLATO data will enable to measure micro variability and thereby provide highly valuable observational information on the surface rotation of the stars, on their magnetic activity on short and longer time scales (solar-like cycles) for a large set of stars. The information power of this type of investigation has been demonstrated with Kepler data (Bonomo & Lanza 2012; Lanza 2016; Moutou et al. 2016).

The occultation of starspots by transiting planets produces anomalies in the transit light curves that may lead to inaccurate estimates of transit duration, depth, and timing (Czesla et al. 2009; Oshagh et al. 2013). These inaccuracies can affect the precise derivation of the planet radius, and consequently affect estimates of the planet’s density. Thus, having an estimate of the size and position of starspots would help to overcome this issue. Furthermore, repeated starspot occultations can reveal the stellar rotation period (Silva-Valio 2008) and even differential rotation (Silva-Valio & Lanza 2011).

Thorough investigation of stellar evolution requires a large number of bright stars sampling all relevant stellar parameters (mass, age, rotation, chemical composition, environment).The PLATO mission will, for the first time, provide such necessary data in order to:

  • Improve understanding of internal stellar structure, including the identification of missing physics.
  • Better understand the pulsation content and its interaction with the physics of the star, in particular with respect to rotation and magnetism.

Improve our understanding of stellar evolution.




PLATO – Revealing habitable worlds around solar-like stars
Definition Study Report, ESA-SCI(2017)1, April 2017