The spinning magnet of Sun-like stars:
are close-in giant planets forcing the engine?
are close-in giant planets forcing the engine?
Stars like the Sun host a spinning engine - their magnetic fields. These magnetic fields are similar to those of bar magnets, except that they flip their north and south sides regularly, every 11 years in the case of the Sun. For the first time, an international team of astrophysicists1 has just caught one star other than the Sun - tau Bootis A - in the process of flipping its north and south magnetic poles. They however suspect that this event is much more frequent in tau Bootis A than in the Sun - is the magnetic engine in tau Bootis A spun up by the giant planet closely orbiting its host star? This discovery, published in the Monthly Notices of the Royal Astronomical Society, should help scientists understand better how magnetic engines work in stars like our Sun.
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Tau Bootis A, a naked eye star in the constellation of Bootes (© Philippe Durville) |
Stars like the Sun are not quiet and steady - they host a complex engine
producing a
magnetic field
very similar to that of a bar magnet.
The solar corona, best visible during
total eclipses,
visualises this
magnetic field just as iron filings would do for a bar magnet.
The engine producing
magnets of Sun-like stars
is called a
dynamo
and uses both
the spinning of stars and the boiling-like motions occuring
below their surfaces. The
Earth's magnet
is also generated by a dynamo engine.
An interesting and important property of the Sun's magnet is its ability to
flip its north and south sides regularly, every 11 years:
this is the
magnetic cycle
of the Sun, whose impact on the Earth climate is potentially strong
enough to have caused the
little ice age
in the seventeen century. The Earth's magnet is also flipping side, though
much less frequently and more eratically.
Sun-like stars also host magnetic fields and their magnets are
likely flipping side regularly as well;
however, observing such events was difficult until now, as they could only
be detected through indirect tracers, like for instance the very subtle
brightness changes occuring at very specific colors.
By looking directly at the magnetic fields of stars other than the Sun,
an international team of astrophysicists1, led by JF Donati
and C Moutou from CNRS (Toulouse and Marseille),
has recently caught one star, called tau Bootis A, in the process of
flipping its magnet, using twin instruments called
ESPaDOnS2 on the
Canada-France-Hawaii
Telescope3 (atop MaunaKea in Hawaii) and
NARVAL4 on the
Télescope Bernard
Lyot5 (atop
pic du Midi
en France). The
magnetic arches
they reconstruct for tau Bootis A are also fairly complex.
 |
Artist view of tau Bootis A with its rotating magnet and close-in giant planet (© art by Karen Teramura, University of Hawaii, Institute for Astronomy) |
If JF Donati, C Moutou and collaborators managed to catch tau Bootis A in this state of magnetic flipping, it is probably not just by chance. They suspect that this event is much more frequent in tau Bootis A than in the Sun - possibly every few years only as opposed to 11 years in the case of the Sun. Is the giant planet that already managed to speed up the surface of tau Bootis A, also spinning up the magnetic engine of its host star? Donati, Moutou and collaborators will undoubtedly keep their eyes tightly focussed on tau Bootis A in coming years to make sure not to miss any of the star's future regular magnetic turnover - with the hope of understanding better how magnetic engines work in stars like our Sun, and whether/how close-in giant planets manage to spin them up.
Press contacts in UK, USA/Hawaii & Canada:
in UK: Andrew Collier Cameron, University of StAndrews, Tel: +44-1334-463147, email: Andrew.Cameron[AT]st-and.ac.uk
in USA/Hawaii: Evgenya Shkolnik, University of Hawaii, Tel: 808-292-9088, email: shkolnik[AT]ifA.hawaii.edu
in Canada: David Bohlender, National Research Council Canada / Conseil national de recherches Canada, Tel: 250-363-0025, email: david.bohlender[AT]nrc-cnrc.gc.caLink to the UH press release