A team of Scottish and French astronomers have discovered a festive star that does the twist.

Discovered by astronomers from the University of St Andrews and the
Observatoire Midi-Pyrenees, Toulouse during Christmas observations in
Australia, the new phenomenon of twisting behaviour demonstrates a slow
change in the way the star spins on its axis.

This twisting behaviour has been predicted by some theories of the way in
which stars generate their magnetic fields, but until now it has never been
observed directly.

The phenomenon provides crucial new insights into the way the Sun generates
the magnetic fields that give rise to the 11-year sunspot cycle, deepening
our understanding of the dynamics of our parent star whilst solving a
long-standing mystery concerning the erratic orbits of some closely-orbiting
binary stars.

The star’s equator always spins faster than its poles – the stellar
equivalent of trade winds or ocean currents in Earth’s weather and ocean
systems. The new observations show that the equator speeds up and the pole
slows down for several years, then the cycle reverses as the equator slows
down and the pole speeds up.

The discovery stems from a painstaking analysis of observations made
annually since 1988 by Dr Andrew Collier Cameron of the University of St
Andrews and Dr Jean-Francois Donati of the Observatoire Midi-Pyrenees in
Toulouse. The team spent many a Christmas at the 3.9-metre Anglo-Australian
Telescope in New South Wales, Australia over an 8 year period to track
individual starspots at different latitudes on a young Sun-like star named
AB Doradus, located 50 light-years from the Earth in the southern
constellation of the Swordfish.

“The star is best placed for observation from the southern hemisphere in
December, which is why we often end up celebrating Christmas at the
telescope console”, said Dr Cameron, of St. Andrews University School of
Physics and Astronomy.

Observing the star for a few nights each year, the team mapped the changing
pattern of dark starspots on the star’s surface. Like sunspots, starspots
are produced where loops of strong magnetic field erupt from deep inside the
star, blocking the flow of energy from the star’s interior.

The new result provides the first clear observational evidence that magnetic
fields generated inside the star also act as “glue”, altering the
circulation of gas inside the star.

Since the time of Galileo, astronomers have watched sunspots drift across
the face of the Sun. From this motion, they have deduced that the Sun spins
on its axis once a month and that spots near the equator spin faster than
spots at the poles. As spots at different latitudes race around the Sun, it
takes a sunspot near the equator about 3 months to gain a complete lap on a
spot located near the Sun’s poles. AB Doradus rotates 50 times faster than
the Sun, spinning on its own axis in a mere 12.3 hours. Four years ago, Drs
Donati and Cameron discovered that AB Doradus showed a pattern of rotation
similar to the Sun, with its equator spinning slightly faster than its
poles.

This summer, they applied a sensitive new starspot tracking technique to
measure how long individual spots took to complete one circuit of the star.
Encouraged by a clear vindication of the earlier result, they set about
re-analysing data from previous years to build up a more complete picture of
how the spin rate of each spot depended on its distance from the star’s
equator. The surprise came when the team found that they could not reconcile
the pattern of spin rates of spots near the star’s equator and poles from
one year to the next.

The confirmation of a link between magnetic activity and twisting rotation
in stars could also solve a long-standing mystery involving close binary
stars whose orbits speed up and slow down for no apparent reason. As long
ago as 1992, Jim Applegate of Columbia University in New York had suggested
that, if the strength of the magnetic “glue” inside a star changed with the
star’s magnetic cycle, the star’s shape would change too. A star with a
fast-rotating equator would be more strongly flattened by its spin than a
star rotating like a solid body. This change in the shape of the star alters
slightly the gravitational pull on its companion, altering the duration of
the orbit by a few parts per million.

The periodic twisting seen in AB Doradus’ spin rate is the first direct
observation of the “Applegate mechanism” in action. The amount of twist is
sufficient to produce the observed orbital period changes in binaries
containing stars similar to AB Doradus.

The team is planning to extend the study for a few more years in order to
measure the duration of the complete cycle and investigate the relationship
between the twisting behaviour and the starspot cycle.

Weblinks:

http://star-www.st-and.ac.uk/~acc4

NOTE TO EDITORS

Images

An image showing a computer reconstruction illustrating the twisting
rotation of the star is available from the PPARC website www.pparc.ac.uk or
by contacting Mark Wells at the PPARC Press Office on 01793 442100 or
mark.wells@pparc.ac.uk

Picture and animation movie of ‘Twisting Star’ AB Doradus available from
Gayle Cook or Claire Grainger – contact details below.

Dr Andrew Collier Cameron and Dr Jean-Francois Donati are available for
interview today (Friday 7th December, 2001) between 08:00 and 12:00hrs GMT.
Please call them direct on +33 5 61 33 29 17.

Dr Andrew Collier Cameron is available for interview in St Andrews on Monday
10th December. Please call him direct on 01334 463147, or email
andrew.cameron@st-and.ac.uk

Gayle Cook – Press Office

University of St. Andrews

Tel: +44 (0)1334 467227

Mobile: 07900 050103

Email: gec3@st-andrews.ac.uk

Claire Grainger – Press Office

University of St. Andrews

Tel: +44 (0)1334 462530

Mobile: 07730 415015

Email: cg24@st-andrews.ac.uk

Gill Ormrod – PPARC Press Office

Particle Physics and Astronomy Research Council

Tel: +44 (0)1793 442012

Email: Gill.ormrod@pparc.ac.uk

This work is supported in the UK by the Particle Physics and Astronomy
Research Council (PPARC) which operates the Anglo-Australian Observatory in
partnership with the Australian astronomical community and in France by the
French Centre National de la Recherche Scientifique (CNRS). A paper on the
results of these findings will be published in the journal, ‘Monthly Notices
of the Royal Astronomical Society.’ The abstract of the paper, together with
links for downloading the full preprint, is available at
http://star-www.st-and.ac.uk/astronomy/preprints/MB951RV_abs.html

The Particle Physics and Astronomy Research Council (PPARC) is the UK’s
strategic science investment agency. It funds research, education and public
understanding in four broad areas of science – particle physics, astronomy,
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