Tau Bootis A, a white-dwarf located 51 light-years away, has a 2-year stellar cycle as opposed to the Sun's 22-year counterpart. One possible reason is a giant planet orbiting it.

Once every 11 years, the Sun's magnetic field flips. Its magnetic north becomes its magnetic south, and vice versa. During the course of this flip, activity on the star's surface intensifies, producing especially violent solar flares, coronal mass ejections, more sunspots, etc.

Now, Scottish astronomers have found a Sun-like star whose magnetic-field flip occurs once every year. Compared to the Sun, that is a very, very short time, and raises interesting questions about how and why magnetic-field flips happen in stars.

Rim Fares, a research fellow at the School of Physics and Astronomy, University of St. Andrews, and her colleagues studied a group of 10 stars between 2006 and 2011, and detected a magnetic field in seven. One of them, Tau Boötis A, is 51 light-years away, and weighs 20 per cent more than the Sun. Astronomers estimate it is almost 3.8 times wide, too, and aged about 3 billion years.

Tau Boötis A

Back in 2008, Fares was part of a research group that confirmed that Tau Boötis A was the first star apart from the Sun that was found to display a global flip in magnetic field polarity, and also that this could be due to a similar phenomenon on the Sun, called differential rotation.

If a star exhibits differential rotation, the entire star doesn't rotate at the same rate like a solid body. Instead, material at its equator rotates faster than at its poles. Moreover, it is only the outer, convective layers that are moving. The radiative interior isn't.

On the Sun, the outer layers are plasma: they consist of a high-temperature soup of particles, especially electrons, which are free to move around. So, as the convective layers move, these electrons also move, generating an electric current. According to Ampere's law, the current generates a magnetic field; according to Faraday's law, these magnetic fields also generate electric fields, and so on. This is called the solar dynamo.

Of course, this is a simplified picture of what really happens. For instance, the plasma flow has to be:

1. Fully three-dimensional
2. Helical
3. Turbulent,

for the dynamo to be possible.

Two fields

Anyway, there are two such fields on the sun: azimuthal fields (pointing east-west or west-east) and meridional fields (pointing north-south or south-north). Over the course of time, the differential rotation acts on the meridional field and transforms it to an azimuthal one. The "meridional circulation" then acts on the azimuthal field, twists it and flips it back to meridional.

"This flip is observed for the Sun. For sun-like stars, we think it might be the same phenomena at action. For other types of stars, some have a quite stable magnetic field over many years - we still did not observe any flip in polarity in these stars," Fares said.

The most likely explanation for Tau Boötis A's remarkably short polarity-flip cycle is a massive planet orbiting the star. The planet weighs about 6 Jupiter masses, the star and the planet have the same rotation and orbital period (about 3 days), respectively, and they are 25 times closer than the Earth is to the Sun.

"[These reasons] suggest that the planet might have synchronised with the outer convective envelope of the star,” Fares said, adding that their gravitational interaction “might lead to extra shear at the base of the outer layers (between the outer layers and the interior), which can influence the dynamo of this star."

Only the second star

Other reasons the study suggests are the star's own differential rate, the surface shear corresponding to which was found to be 6 to 10 times larger than that of the Sun, in 2008. To test that, Fares plans to have her colleagues who work on dynamo simulations of stars to include in their computational code "the effect of synchronisation of the outer layers only."

Ultimately, Tau Boötis A is only the second star to display a proven regular magnetic-field flip. While the phenomenon is thought to be fully understood for the Sun, there is still a long way to go before we know how these exotic phenomena are conceived during stellar births. Even Fares's target comprised only 10 stars; billions more of highly differentiated varieties are out there.

Fares and her team are currently studying a group of solar twins, stars that are similar to the Sun: "Verifying if they have cycles is important because it might suggest that it is common to have cycles in sun-like stars."