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Asteroseismology: Ancient stars spin faster than expected

​By studying the oscillation modes of 91 stars of all ages observed by NASA's Kepler space observatory, an international collaboration including CEA-Irfu has confirmed that "old" stars spin faster than previously accepted theories of magnetic braking predict.  This discovery will now make it possible to date stars more accurately!

Published on 23 April 2021

Stars are born as if launched in a rotational movement that will progressively slow down during their life, under the effect of magnetic winds.  This braking allows us to assign an age to stars from the measurement of their rotational speed (from the displacement of their dark spots).

A 2016 study with strong participation from Irfu provided the first evidence that stars at a similar stage to the Sun rotate faster than predicted by the "magnetic braking" theory, the "gyrochronology" method only proving robust for stars younger than the Sun.  The stars whose spots are low contrast or non-existent (for the oldest), remained difficult to study.

To circumvent these difficulties, methods using the most modern techniques of Artificial Intelligence were developed and provided a complete vision of the rotation of the young stars observed by Kepler.

For older stars, scientists had to use asteroseismology to calculate the rotation of the star. The spectral analysis of the oscillations caused by sound waves trapped in the star made it possible to find the rotation speed of the star, whether it is young or old.

The asteroseismological model proves to be more coherent and accurate than the magnetic braking model.  The researchers confirm that stars of the age of the Sun and beyond do not brake their rotational motion as efficiently as magnetic braking theory predicts.

Understanding the complex interplay between the wind, magnetic field and rotation of stars is a cornerstone of understanding stellar dynamics and evolution. To achieve this, Irfu scientists are now preparing the ESA space mission Plato (PLAnetary Transits and Oscillations of stars) by combining, as always, observations, theoretical models and numerical simulations.

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