Detecting differential rotation of Sun-like stars

At its equator, our sun rotates with a period of 25 days, which is about 11% faster than at its midlatitudes (45°). What generates the differential rotation isn’t clear, but it’s thought to sustain the Sun’s magnetic field through a dynamo mechanism. To appreciate the relative magnitude of the Sun’s differential rotation, scientists want to compare it with that of other stars with Sun-like masses. Most rotation studies monitor starspots at different latitudes or use Doppler imaging to track the migration in latitude of magnetic features on the surface. But most stars, especially their surface features, are too distant to be clearly resolved in images.

An international collaboration led by Othman Benomar of New York University’s Abu Dhabi Center for Space Science adopted a more sensitive and precise method—asteroseismology, which probes features temporally rather than spatially. (See the article by Conny Aerts, Physics Today, May 2015, page 36 .) The technique measures the various resonant frequencies of acoustic waves and oscillations that are driven by motions in a star’s outer layers. Benomar and colleagues used NASA’s Kepler spacecraft to study the outer convection zones of some 40 stars for up to four years. Splitting the observed modes of oscillation into separate frequency components distinguishable by angular degree and azimuthal order provided data about the rotation rate and the shape of the stars. That information puts constraints on such physical processes as flows, tidal forces, and magnetic fields acting in the star. The decomposition has been used extensively in helioseismology but not previously in the analysis of other stars.

The Kepler measurements revealed that the equators of 13 of the stars in the sample rotated approximately twice as fast as their midlatitudes. That was a surprise. The latitudinal shear inferred from that rate is much larger than predictions from numerical simulations. (O. Benomar et al., Science 361, 1231, 2018, doi:10.1126/science.aao6571 .)