Nuclear Physics Division

Age and rotation of stars

All stars like the Sun are born in rotation. As they age, their rotation slows down due to magnetic winds in a process called “magnetic braking”. Research published in 2016 with participation from DAp, provided the first clues that stars at a similar life stage to the Sun were spinning faster than magnetic braking theories had previously predicted. The results of this study were based on measuring dark spots on the surface of stars and by tracking them as they move with the star's rotation. Although the method has been shown to be robust in measuring rotation in younger stars, stars older than the Sun have fewer spots, making it difficult to determine their rotation. To circumvent these difficulties and increase detections in cases where the signal is weak, methods using the most modern artificial intelligence techniques have been developed (Breton et al. 2021). These new techniques provided a complete view of the rotation of the young and cold stars in the sample observed by the NASA Kepler satellite. However, it was necessary to change the methodology to find the rotation of older stars.

It is for this reason that, in this new study published in Nature Astronomy, researchers used a different approach to confirm that older stars are spinning faster than expected. They used asteroseismology to calculate the star's rotation. This relatively new field of study, and successfully applied to the Sun, allows scientists to measure the oscillations caused by sound waves trapped inside the star. By measuring the different characteristics of these waves, we can probe the stars, and determine their size or age. In addition, as the star rotates, these modes break down into different frequencies, some higher and others lower (i.e. frequencies higher or lower than the quiescent frequency). This is a similar effect to the change in sound (frequency) experienced when an ambulance or car approaches or moves away from you. By measuring this frequency difference, it is possible to calculate the star's rotation rate, both young and old.

Although we have suspected since 2016 that older stars are spinning faster than predicted by magnetic braking theories, these new asteroseismic data obtained by the Kepler satellite are the most convincing to date to demonstrate that this "weakened magnetic braking" is actually at work in low mass stars similar to the sun.

Understanding the complex interplay between the wind, the magnetic field and the rotation of stars is one of the cornerstones of understanding the dynamics and evolution of stars. To obtain a complete and coherent vision of these mechanisms, the combination of observations, theory and numerical simulations is necessary and under development at CEA/Irfu/DAp/LDE3, in particular as part of the preparation of the mission. space ESA M3 PLATO.

Contact : Rafael A. García (S. Mathis et A. Strugarek)
 

 Publication:

•  Hall et al (2021). “New asteroseismic rotation rates of Kepler dwarfs show strong agreement with weakened magnetic braking on the late-age main sequence”. Nature Astronomy. DOI: 10.1038/s41550-021-01335-x

Note:

• Breton et al (2021). “ROOSTER: a machine-learning analysis tool for Kepler stellar rotation periods” Astronomy & Astrophysics 647, A125.

See  : - News from  CEA-Press  (23 avril 2021, in French)
         - Press reease from Birmingham University   (22 avril 2021)

See also :
- Vibration of a star and galactic collision (13 January 2020)
- Synergy between asteroseismology and planetary transit (19 July 2019)