Credit: David Aguilar, CfA
The first observations of the complete magnetic cycle of a star other than the Sun are proving a puzzle to astronomers.
Tau Boötis, known as Tau Boo (τ Boo), is a yellowish star that is a little brighter than our Sun. It is located 51 light years away in the constellation of Boötes.
It is host to a giant exoplanet about six times the mass of Jupiter, which orbits Tau Boo every 3.3 days.
In 2007, the magnetic field of Tau Boo was seen to flip: the first time this was observed to happen in a star other than the Sun.
Since then the team has observed four reversals in polarity and is now able to confirm that the star has a rapid magnetic cycle of no more than two years – compared to 22 years for the Sun.
This cycle will subject the orbiting hot-Jupiter to very fast changes in its surrounding environment. Dr Rim Fares will present findings at the National Astronomy Meeting in St Andrews on Thursday 4 July.
"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 behaviour 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."
Credit: Karen Teramura, University of Hawaii Institute for Astronomy
The reasons for Tau Boo's fast cycle are still unclear. As well as having the only proper cycle yet observed, Tau Boo is also unique in being the only star where magnetic reversals have been seen that is orbited by detected planets.
Dr Fares and her colleagues made the discovery whilst undertaking a mini-survey of 10 stars orbited by hot-Jupiters, massive Jupiter-like planets that orbit very close to their star and experience scorching temperatures.
Observations of the stars' magnetic fields were compared to observations of stars without hot-Jupiters.
The team aimed 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 behaviour of the star.
"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.
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