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The wound-up magnetosphere of AB Doradus

posted Apr 15, 2013, 8:32 AM by Jeremy Drake   [ updated Jun 3, 2013, 12:00 PM ]
The Sun's outer atmosphere, or "corona" comprises plasma at temperatures of 2 million degrees Kelvin contained within magnetic fields in loop-like configurations.  This X-ray emitting gas is now routinely imaged by satellites such as NASA's Solar Dynamics Observatory (SDO) and the Japanese Hinode satellite.  This image shows the solar corona at 171 Angstroms glowing in the light of million degree highly ionized iron emission imaged by SDO on March 30 2011. The loop structures are the extensions of the magnetic fields in sunspots we can see on the solar disk in visible light.

Other stars like the Sun - so-called late-type stars with outer convection zones - show similar properties, but can be much more extreme.  The K0 dwarf AB Doradus is only 75 million years old and rotating extremely rapidly with a period of half a day - 50 times faster than the Sun.
 This rapid rotation engenders vigorous dynamo activity that sustains a violent and energetic corona that emits X-rays with several thousand times the intensity of the solar corona.  We have modelled the corona of AB Dor using sophisticated supercomputer magnetohydrodynamic models that are usually applied to model the solar corona and wind.  The results are the first detailed models of such a rapidly-rotating magnetized corona and wind.  Such models are important for understanding how stars spin down with age. The Sun was also once a fast rotator, when it was only a few tens of millions of years old, but has slowed over its 4.6 billion years or so due to angular momentum loss from its magnetized wind. The model of AB Dor shows that its coronal loops, shown as the yellow magnetic field lines in this image, are bent over by its fast rotation and the drag of the wind coming from the surface - much like the solar corona would have looked when it was much younger.  The model and its implications for the spin-down of AB Dor was published in the 2010 September 20 edition of the Astrophysical Journal.