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Star Tau Boo’s baffling magnetic flips

Last Updated on Monday, 01 July 2013 16:47
Published on Wednesday, 03 July 2013 23:01

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.

fares2Artist’s impression of the magnetic field of Tau Boötis. Credit: Karen Teramura, University of Hawaii Institute for Astronomy (click to enlarge)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.”

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. 

“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.”

Even so, stellar magnetic fields can be small and are hard to detect. Out of the sample of 10 stars, the team was not able to detect the fields of three stars. In the other seven stars, they detected magnetic fields and found (aside from Tau Boo) that they had properties similar to stars without planets.

“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 spins on its axis once every 3.3 days – the same period as its hot-Jupiter completes one orbit.  The vast planet is just ten times more massive than the total mass of the outer layers of the star.  One hypothesis for Tau Boo’s rapid cycle is that the planet is dragging on the star, making it rotate faster than usual, and this is affecting the generation of the magnetic field.

“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.





The team used the ESPaDOnS instrument on the Canada-France Telescope in Hawaii (CFHT) and the Narval instrument on the Bernard Lyot telescope in the French Pyrenees.

The study was carried out by an international team including Rim Fares (St Andrews), Claire Moutou (Marseille, France), Jean-Francois Donati (Toulouse, France), Claude Catala (Paris, France), Evgenya Shkolnik (Flagstaff, USA), Moira Jardine (St Andrews), Andrew Cameron (St Andrews), Magali Deleuil (Marseille, France).





Artist's impression of the giant exoplanet orbiting Tau Boötis, viewed through the star's magnetic arcs. (credit: David Aguilar, CfA)

Artist’s impression of the magnetic field of Tau Boötis.  Credit: Karen Teramura, University of Hawaii Institute for Astronomy  





Dr Rim Fares

Research Fellow

School of Physics and Astronomy

University of St Andrews

Scotland, UK

Tel: +44 (0) 1334 461672

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Dr Robert Massey

Royal Astronomical Society

Tel: +44 (0)20 7734 3307 / 4582 x214

Mob: +44 (0)794 124 8035

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Ms Anita Heward

Royal Astronomical Society

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Ms Emma Shea

Head of Development Communications

University of St Andrews

Tel: +44 (0)1334 462 167

Mob: +44 (0)785 090 0352

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Landline numbers in NAM 2013 press room (available from 9 a.m. to 5 p.m. from 1-4 July, 9 a.m. to 3 p.m. 5 July):   Tel: +44 (0)1334 462231, +44 (0)1334 46 2232





Bringing together more than 600 astronomers and space scientists, the RAS National Astronomy Meeting (NAM 2013) will take place from 1-5 July 2013 at the University of St Andrews, Scotland. The conference is held in conjunction with the UK Solar Physics (UKSP: and Magnetosphere Ionosphere Solar Terrestrial (MIST: meetings. NAM 2013 is principally sponsored by the RAS, STFC and the University of St Andrews and will form part of the ongoing programme to celebrate the University’s 600th anniversary.

Meeting arrangements and a full and up to date schedule of the scientific programme can be found on the official website at   The Royal Astronomical Society (RAS:, Twitter: @royalastrosoc), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3500 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.  


The Science and Technology Facilities Council (STFC:, Twitter: @stfc_matters) is keeping the UK at the forefront of international science and tackling some of the most significant challenges facing society such as meeting our future energy needs, monitoring and understanding climate change, and global security. The Council has a broad science portfolio and works with the academic and industrial communities to share its expertise in materials science, space and ground-based astronomy technologies, laser science, microelectronics, wafer scale manufacturing, particle and nuclear physics, alternative energy production, radio communications and radar. It enables UK researchers to access leading international science facilities for example in the area of astronomy, the European Southern Observatory.  


Founded in the 15th century, St Andrews is Scotland’s first university and the third oldest in the English speaking world. Teaching began in the community of St Andrews in 1410 and the University was formally constituted by the issue of Papal Bull in 1413. The University is now one of Europe’s most research intensive seats of learning – over a quarter of its turnover comes from research grants and contracts. It is one of the top rated universities in Europe for research, teaching quality and student satisfaction and is consistently ranked among the UK’s top five in leading independent league tables produced by The Times, The Guardian and the Sunday Times.   The University is currently celebrating its 600th anniversary and pursuing a £100 million fundraising campaign, launched by Patron and alumnus HRH Prince William Duke of Cambridge, including £4 million to fund the creation of an ‘Other Worlds’ Think Tank and Observatory. The new think tank and Observatory project will extend the University of St Andrews’ flagship work on extra-solar planets, and provide a creative environment for problem-focused research, education and continuing public engagement.   For further information go to: