Image: A. CAMERON, M. JARDINE AND K. WOOD/University of St. Andrews |
Astronomers have observed for the first time a star doing the stellar version of the twist, according to a new report. Andrew Collier Cameron of Scotland's University of Saint Andrews, along with a colleague, found that the difference between the rotation rates of the equator and poles of a fast-spinning star called AB Doradus (AB Dor) changed over time: after the poles sped up, the equator slowed down. Scientists had predicted that magnetic fields would cause such twisting, and gaining a better understanding of this process may give them insights to our own sun's behavior. These results will be published in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.
Cameron likens a star's magnetic field to a boiling pot full of spaghetti, in which the boiling water takes the place of ionized gas molecules in the star and the pieces of spaghetti are magnetic field lines. If there are just a few strands of pasta in the pot, the water pushes them around, but dump in a whole package and the noodles push back on the water, creating a feedback loop. Astronomers couldn't observe this feedback effect in the sun, he says, because the magnetic field is too weak to make its presence felt on the star's surface. So the two researchers turned to AB Dor, which spins 50 times faster than the sun and should therefore have a stronger magnetic field. They measured the rotation of the star from 1988 to 1996 by tracking the motions of its starspots. "The nice thing about this [result]," Cameron notes, "is really seeing at the surface of the star¿where everybody can get at it¿a phenomenon that's difficult to observe on the sun because it's buried so deeply."
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The finding also supports a theory that explains deviations in the orbits of binary stars as a consequence of magnetically induced shimmying and hence flattening of one member of the pair, which alters its gravitational pull. Cameron says he is preparing to make more observations of AB Dor to see if the twisting pattern is repeating itself or, as he puts it, "wandering all over itself in a chaotic way" from the feedback.
