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, cosmology and space science.

PPARC is government funded and provides research grants and studentships to scientists in British universities, gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Organisation for Nuclear Research, CERN, and the European Space Agency. It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI National Facility.

PPARC's Public Understanding of Science and Technology Awards Scheme provides funding to both small local projects and national initiatives aimed at improving public understanding of its areas of science.

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