Our Sun is turning - but not like the others

The rotation of the Sun is a complicated matter, and astronomers are quite a bit away from a full understanding of why this is. 13 other sunlike stars behave in almost the same way, according to recent Science paper with Jørgen Christensen-Dalsgaard as co-author

2018.09.19 | Ole J. Knudsen

Differential rotation in the Sun

A group of astrophysicists, counting Jørgen Christensen-Dalsgaard of the Department of Physics and Astronomy among them, recently had a paper published in the highly esteemed periodical Science. In this the rotational details of 13 sunlike stars observed with the NASA Kepler satellite are described.

To a first approximation the Sun rotates as if it consists of an inner ball of gas surrounded by a 'girdle'; a convection zone going 27% into the radius of the Sun. The inner ball rotates with a period of some 27 days, and the girdle has differential rotation with periods of 25 days at the equator and some 35 days at the poles. A difference of some 30%. This causes a special physical situation deep in the Sun where the two zones meet, and the interactions in the inner magnetic fields here may be the cause of the well known 11 year cycle of the sunspots. The astronomers are not yet able to explain why the rotation in the outer zone changes from fast at the equator to significantly slower at the poles.

Lead by O. Benomar, New York University Abu Dhabi, the research team has pulled data for 40 sunlike stars out of the enormous database produced over more than three years by the NASA Kepler satellite. Of these 40 stars 13 are close to the Sun in physical parameters, making it relevant to compare with our own best known star. The remainder have a much slower rotation, and for some of those the situation in the outer zone may be inversed: rotation at the pole is faster than at the equator.

                                    

                     Our Sun (left) shows small differential rotation compared to other Sun-like stars (right)

Tiny variations of brightness for 150 000 stars were measured by Kepler during the three years. These data have be used for finding lots of exoplanets orbiting the stars, and the same data can be used for studying the vibrations in the stars; a technique named asteroseismology. The inner conditions of the stars can be calculated in the same way that geologists are able to 'look' into the Earth by analyzing earthquakes.

Even though the 13 selected stars resemble our own Sun in many ways; diameters, age, brightness and chemical composition, the researchers find that their rotations differ greatly from that of the Sun. As a main rule the differences in rotational periods between the poles and the equators are up to 5 times bigger; 150% of what is known on the Sun. Such large differences amongst otherwise similar stars surprise the researchers. Finding a useful physical explanation for the differential periods is suddenly much more difficult than previously expected. The situation is even more unresolved, as some model calculations have shown that the Sun iteself ought to fit in with the ones with a slower rotation at the equator; obviously not in accordance with the actual observations. "Something is clearly wrong with our models of the Sun-like stars," professor Christensen-Dalsgaard comments, "and we have to look deeply and critically into our present computer models of these stars - which will be great fun!"

The paper in Science is available here, titeled "Asteroseismic detection of latitudinal differential rotation in 13 Sun-like stars", and the story is also covered on the Danish-language Videnskab.dk website.

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