Fast flipping star is a magnetic mystery
Alien solar cycle Astronomers have discovered a star that flips its magnetic poles ten times more often than the Sun.
Dr Rim Fares of the University of St. Andrews in Scotland and colleagues, made the surprising discovery, while studying the star Tau Boötis, located 51 light years away.
They found the star, which is about one and a third times more massive than the Sun, reverses its magnetic polarity every year.
Their discovery, which is the first ever study of the magnetic cycle of a star other than our Sun, is presented today at the National Astronomy Meeting in St. Andrews, Scotland.
"To know there's a star with such a fast magnetic cycle … helps us understand how the dynamo generating the magnetic field evolves from the way the Sun's acts to that seen in other stars," says Fares.
"The magnetic field also affects the habitability and environment of orbiting planets, because it influences the strength of the stellar wind which can cause evaporation of planetary atmosphere."
A star's magnetic field can be compared to a giant bar magnet, with north and south poles.
Every 11 years, during solar maximum, the peak of sunspot activity, the Sun's poles reverse polarity.
It takes two flips to restore the magnetic field to its original orientation, giving the Sun a 22-year magnetic cycle.
Tau Bootis follows the same cycle, but much faster.
Fares and colleagues made the discovery while studying the magnetic fields of ten stars being circled by hot Jupiters, massive gas planets orbiting extremely close to their host star.
They wanted to compare the magnetic fields of these stars, to those of stars without hot Jupiters, allowing them to understand how the magnetic environment of stars affects planets, and whether planets also influence the magnetic behaviour of stars.
Tau Boötis was selected because it's orbited every 3.3 days by a planet six times the mass of Jupiter.
"This planet is gravitationally locked, so the same side always faces the star," says Fares.
"The star also has the same 3.3 day rotational period as its planet's orbit."
Although that's much faster than the Sun, which takes 28 days, it's only slightly faster than stars similar to Tau Boötis.
"Its rotation is linked somehow to the orbital period of the planet," says Fares.
"The planet might somehow be influencing the star's rotation through tidal interactions. But only the outer layers of the star, which is possible, not the whole star."
Fares and colleagues first observed Tau Boötis magnetic field flip in 2007 and decided to keep monitoring it.
They identified four further reversals, confirming the star has a rapid magnetic cycle of just two years.
But it is unclear what's causing rapid cycle, says Fares, who has two hypotheses.
"It could be that the planet is synchronising the outer convective layers of the star, which means that it might influence the dynamo," says Fares.
"We know that the solar magnetic field is generated by a dynamo, and we think it's the same for these stars.
"The second hypothesis might involve the strong differential rotation [of different parts of the star], but other stars also have strong differential rotation, but don't show a magnetic cycle, at least not detected yet."
The astronomers also found two other stars that had magnetic cycles, but their cycles appear to be different to Tau Boötis, with one flipping much slower, and the other showing no synchronisation to its planet.
"We found that, aside from Tau Bootis, the magnetic field properties of these stars, were just like stars without planets," says Fares.