Study shows sun-like stars turn differently

Like our sun, distant stars are rotating spheres of hot gas. However, stars do not rotate like solid spheres: regions at different latitudes rotate at different speeds. A group of researchers from the University of New York and the Max Planck Institute for Solar System Research (MPS) in Germany has now measured rotational profiles from a sample of 39, stars similar to the sun. They have identified 13 stars that rotate in a similar way to our Sun: their equators rotate faster than their mid-latitudes. This rotation pattern is however much more pronounced than in the Sun: the star's equators rotate twice as fast as their mid-latitudes. This difference in rotational speed is much greater than that suggested by theories.

What do we know about distant stars apart from their brightness and their colors? Is our Sun a typical star? Or does it show some properties that make it special or even unique? A property that is not fully understood is rotation. In its outer layers, the Sun has a rotation pattern that scientists call "latitudinal differential rotation". This means that different latitudes rotate at different rates. While at the equator of the Sun, a complete rotation takes about 25 days, the higher latitudes rotate more slowly. Near the poles of the Sun, a complete rotation takes about 31 days.

In their new work, the scientists studied the rotation of 40 stars resembling the Sun in relation to the mass. Among these, the 13 stars for which the differential rotation could be measured with confidence all show a differential rotation similar to that of the Sun: the equators rotate faster than the higher latitudes. In some cases, however, the difference in rotation speed between the equator and the mid-latitudes is much greater than that of the sun.

Classically, stellar rotation is determined by following the curves of stars in different photometric curves. This method is however limited because we do not know the latitudes of the points of stars. "Using observations from NASA's Kepler mission, we can now probe the interior of stars with asteroseismology and determine their rotational profiles at different latitudes and depths," says Laurent Gizon, director of MPS.

The stars are too far away to be resolved in astronomical images. They are like points. However, scientists can indirectly obtain spatial information on stellar interiors using stellar oscillations. The stars undergo global acoustic oscillations which are excited by convective movements in their outer layers. Different modes of oscillations probe different regions of a star. Thus, oscillation frequencies tell us about different regions. In this study, scientists used stellar oscillations to measure rotation at different latitudes in the outer convection zone. "Oscillation modes that propagate in the direction of rotation move faster than modes that propagate in the opposite direction, so their frequencies are slightly different," says Gizon.

"Our best measurements all show solar rotation stars," says Gizon. The most surprising aspect of this research is that the differential rotation in latitude can be much stronger in some stars than in the sun. Scientists did not expect such values, which are not predicted by numerical models.

This work is important because it shows that asteroseismology has fantastic potential to help us understand the inner workings of stars. "The information on stellar differential rotation is essential for understanding the processes that generate magnetic activity," Gizon explains. The combination of information on internal rotation and activity, as well as modeling, will most likely reveal the root causes of magnetic activity in the stars. However, many Sun-like stars need to be studied for this to happen. In 2026, the European Space Agency will launch the PLATO mission (an exoplanet mission such as Kepler) to characterize tens of thousands of bright, sun-like stars using precision astero-seismology. Numerous statistics will be essential to study the physics of stars and their evolution.

Published by O. Benomar, et al., "Asteroseismic Detection of Latitude Differential Rotation in 13 Sun-like Stars," Science 21 Sept. 2018: Vol. 361, No. 6408, pp. 1231-1234 DOI: 10.1126 / science.aao6571