The Spinning Magnet of a Sun-like Star
|The magnetic field of the sun-like star tau Bootis has flipped its north and south poles, the first time this has been observed in a star other than our sun. The shortened cycle of this event may be due to interactions with its nearby massive planet. Credit: Karen Teramura (UH IfA).
An international group of astronomers that includes the University of Hawaii's Evgenya Shkolnik reported today that they have discovered that the sun-like star tau Bootis flipped its magnetic field from north to south sometime during the last year.
It has been known for many years that the Sun's magnetic field changes its direction every 11 years, but this is the first time that such a change has been observed in another star. The team of astronomers, who made use of Canada-France-Hawaii Telescope atop Mauna Kea, are now closely monitoring tau Bootis to see how long it will be before the magnetic field reverses again.
Magnetic field reversals on the sun are closely linked to the varying number of sunspots seen on the sun's surface. The last "solar minimum," the time when number of sunspots was the lowest and the magnetic flip occurred, was in 2007. The first sunspot of the new cycle appeared just last month.
The magnetic cycle of the Sun impacts the Earth's climate and is believed to have caused the little ice age in the seventeen century. The Earth's magnetic field also flips, although much less frequently and more erratically.
The international team led by Jean-Francois Donati and Claire Moutou of France caught tau Bootis in the process of flipping its magnetic field while they were mapping the magnetic fields of stars.
What makes tau Bootis even more interesting is that it harbors a giant planet orbiting very close to the surface of the star. The planet is actually so close (only one twentieth the distance between the sun and Earth) and so massive (about 6.5 times the size of Jupiter) that it succeeded in forcing the surface of the star to co-rotate with the planet's orbital motion through tidal torques. This is the same effect that causes the moon to co-rotate around Earth so that we see only one side of the moon.
Since the astronomers managed to catch tau Bootis in this state of magnetic flipping during just two years of observations, it is likely that this event is much more frequent on tau Bootis than it is on the sun. It is possible that the giant planet that has already managed to speed up the surface of tau Bootis is also spinning up the magnetic engine of its host star. The astronomers will keep their telescopes focused on tau Bootis in coming years to make sure they catch the star's next magnetic turnover. Their goal is a better understanding of how magnetic engines work in stars, including our sun.
Slightly hotter and 20 percent more massive than the sun, tau Bootis is fairly bright and visible with the naked eye and located only 51 light-years away from us. It is currently rises about midnight and is visible for most of the night near the bright star Arcturus in the constellation Bootis in the northeast part of the sky.
This work is published this week in the British journal Monthly Notices of the Royal Astronomical Society.
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