Curious Flips Observed In Tau Boo's Magnetic Cycle
July 4, 2013

Complete Magnetic Cycle Of Tau Boo Observed

Lee Rannals for - Your Universe Online

One star sitting 51 light years away in the constellation of Bootes has a magnetic cycle that is puzzling astronomers.

Dr Rim Fares presented the first observations of the complete magnetic cycle of Tau Bootis, or Tau Boo, at the Royal Astronomical Society's National Astronomy Meeting in St Andrews this week. These are the first observations of the complete magnetic cycle of a star other than our Sun.

According to the findings, in 2007 the magnetic field of Tau Boo was seen to flip, which was the first time this was observed to happen in a star other than the Sun. The team has since observed four reversals in polarity and confirmed the star has a rapid magnetic cycle of no more than two years.

"The Sun's magnetic field is a bit like a giant bar magnet, with a north pole and south pole. Every 11 years, during solar maximum (the peak of sunspot activity), the Sun's poles swap over. It takes two flips to restore the magnetic field to its original orientation, so the Sun's magnetic cycle lasts 22 years," explained Dr Fares. "Tau Boo has the same magnetic behavior as the Sun, but its cycle is very fast compared to the solar one. We've seen changes at regular intervals of about a year that are clearly not chaotic, so we can now be sure that we are looking at the star's magnetic cycle lasting at most two years."

The scientists are unsure exactly why Tau Boo's magnetic cycles are fast, compared to the sun's 22 year cycle. Tau Boo is also unique in being the only star with orbiting planets where magnetic reversals have been seen.

Fares and her colleagues made the discovery while performing a mini-survey of 10 stars orbited by hot-Jupiters, which are exoplanets similar in size to Jupiter but with a very close orbit to their star. Observations of Tau Boo's magnetic fields were compared to observations of stars without hot-Jupiters. The team hoped to understand how the magnetic environment of stars affects the planets embedded within them and whether the planets themselves have an influence on the magnetic behavior of the star.

"A star's magnetic field can have powerful effects on a planet, causing aurora and evaporation (at least partially) of the atmosphere. To understand these phenomena, we should understand the different interactions at play and how they affect both the planet and the star," said Dr Fares. "Unfortunately, observing the planets is a really challenging task, so we decided to study the magnetic fields of the stars that host hot-Jupiters and see whether they were influenced by interactions with the planets."

The team was unable to detect the fields of three stars out of the 10 stars observed. However, in seven of the stars they detected magnetic fields and found they had properties similar to stars without planets, other than Tau Boo.

"For three stars in the sample, we had more than one epoch of observations, so we could actually follow how their magnetic fields changed with time and compare it to stars without hot-Jupiter. We found that, aside from Tau Boo, the magnetic field properties were just like those of stars without planets. For Tau Boo, tidal interactions between the star and the planet might be an important factor in accelerating the cycle, but we can't be sure of the cause," said Dr Fares.

Tau Boo's hot-Jupiter planet is ten times more massive than the total mass of the outer layers of the star. One reason for the star's rapid magnetic cycle is that the planet is dragging on the star, making it rotate faster than usual.

"There are still some big questions about what's causing Tau Boo's rapid magnetic cycle. From our survey, we can say that each planetary system is particular, that interactions affect stars and planets differently, and that they depend on the masses, distance and other properties of the system," explained Dr Fares.