Stellar Switch: Sun not alone in making magnetic flip-flops

It’s a topsy-turvy world out there and astronomers have new evidence to prove it. Researchers have for the first time documented that a star other than the sun flips its magnetic poles. The magnetic reversal observed on the nearby star tau Bootis may shed light on the origin of the sun’s 11-year magnetic cycle, which can affect Earth’s climate. The finding also highlights the role that massive, close-in planets may play in regulating a star’s magnetic activity.

FLIP-FLOP. An artist’s rendering shows the magnetic field (as a bar magnet) of the star tau Bootis. Astronomers discovered that tau Bootis has flipped its north and south poles, perhaps with help from a large, close-in planet. It is the first time a flip has been observed in a star other than the sun. K. Teramura /Univ. of Hawaii

Every 11 years, the sun reverses the direction of its magnetic field, heralding the peak of the solar cycle. That’s when the number of sunspots—regions where bundled loops of magnetic fields concentrate—reaches its maximum, and the sun is more likely to hurl billion-ton clouds of charged particles into space. Those eruptions can harm spacecraft and damage power grids on Earth.

In seeking other examples of magnetic flips, Andrew C. Cameron of the University of St. Andrews in Scotland and his colleagues homed in on stars with closely orbiting planets. That includes tau Bootis, just 51 light-years from Earth. The star harbors a hulking planet, some 6.5 times heavier than Jupiter, which resides close enough to graze the star’s outer atmosphere.

Previous studies showed that mid-latitude regions on the star’s surface rotate in sync with the planet’s rapid 3.3-day orbit. That’s a sign that the planet’s gravity has spun up the star—and perhaps revved up the star’s magnetic engine.

The researchers used instruments on the Canada-France-Hawaii Telescope atop Hawaii’s Mauna Kea and the Bernard-Lyot Telescope at the Pic du Midi Observatory in France to record the polarization of light from the star and the magnetic splitting of spectral lines. The observations reveal that between the summers of 2006 and 2007, tau Bootis reversed its magnetic field, the team reports in an upcoming Monthly Notices of the Royal Astronomical Society.

Recording such a flip within just one year, the astronomers say, suggests that the star reverses its field much more frequently than does the sun—a conclusion also supported by a comparison with solar properties.

The rising and falling of parcels of charged gas within the sun’s convection zone, the outer one-third of its roiling interior, generates a current, which in turn produces a magnetic field. Because the outer parts of the sun rotate more quickly at the equator than at the poles, the sun’s overall magnetic field becomes distorted and twisted over time. The twisting gradually causes the field to reverse polarity.

On tau Bootis, notes Cameron, the convection zone is thinner and the relative difference in rotation between the equator and the pole is greater. Both traits can boost magnetic activity. So might the gravity of the nearby, massive planet.

This likelihood of frequent flips “makes it very attractive to monitor tau Bootis and possibly other similar stars with high differential rotation, which may improve our understanding of the generation and dynamics of magnetic fields in stars,” says Marina Romanova of Cornell University.

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