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 and solves 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 the University’s 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 Dor 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 Dor.
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.
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Investigations of stellar surfaces are gathering pace with the help of a new generation of instruments. The highly-efficient spectropolarimeter nicknamed ESPaDOnS (French for swordfish, or Echelle SpectroPolArimetric Device for ObserviNg Stars) is currently under construction at Observatoire Midi-Pyrenees and will be mounted next year on the 3.6m Canada- France-Hawaii Telescope atop the Mauna Kea, an extinct volcano on the Big Island of Hawaii. Although AB Dor is too far south to be observed from Hawaii, ESPaDOnS will be able to detect the same phenomenon on many other similar stars, giving astronomers a clear view of how wide an impact this new discovery has on stellar physics.
This work is supported by the UK Particle Physics and Astronomy Research Council (PPARC) and by the French Centre National de la Recherche Scientifique (CNRS). The Anglo-Australian Observatory is funded by PPARC and by the Australian Government and is operated by the Anglo-Australian Telescope Board on behalf of the UK and Australian astronomical communities.
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/MB951 RV_abs.html
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