All in a spin about Sun-like stars

The rotating gas layers of stars behave in different ways depending on the age of the star and the circumstances of its birth. Studies have shown that the rotation rate of the Sun’s convection zone decreases towards higher latitudes: it spins 30 per cent slower near the poles than at the equator. This ‘latitudinal differential rotation’ may influence the solar magnetic field, possibly sustaining it through a dynamo-type mechanism. 

Now, Laurent Gizon at Max Planck Institute for Solar System Research, Germany, and New York University in Abu Dhabi, UAE, with co-workers in Germany, Japan, USA and Denmark, have used asteroseismology — the seismology of stars — to examine differential rotation in the convection zones of 40 Sun-like stars, using data gathered by NASA’s Kepler spacecraft.

“Asteroseismology has fantastic potential to help us understand the inner workings of stars,” says Gizon. “Traditionally, to infer the rotation of a star, you would analyse the star’s light intensity over time. Dark spots on the star’s surface rotate with the star, allowing us to work out its rotation period. Asteroseismology, on the other hand, monitors acoustic oscillations that are excited by convective motions in stars’ outer layers.”

Modes of oscillation that propagate in the direction of rotation move faster than modes that propagate in the opposite direction, and so their frequencies are slightly different, Gizon explains. This rotational frequency difference can be measured by analysing the star’s light curves. 

The team selected 40 stars with masses similar to the Sun, and visible oscillation patterns that suggested solar-like convection zones. They detected statistically significant differential rotation in 13 stars, and all of them were solar-like in that their equators rotated faster than their poles.

“We were very surprised that the latitudinal differential rotation in some stars is much stronger than in the Sun. In some cases, there was a 50 per cent difference in rotation speed between the equator and mid-latitudes,” says Gizon. 

Matthias Rempel, an expert in solar physics at the National Center for Atmospheric Research, USA, read the paper with great interest. "These measurements of differential rotation provide important constraints on our theories of turbulence in rotating stars and the overall evolution of magnetism and rotation with the age of a star."

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