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The rotation of stars and their complex magnetism

posted May 7, 2013, 8:51 AM by Jeremy Drake   [ updated Jun 3, 2013, 11:57 AM ]
Sunspots are the manifestation of strong magnetic fields emerging at the solar surface.  The field is generated deeper inside the Sun as rotation and convective motions of the plasma combine to form a magnetic dynamo.  The Sun is a middle-aged star and quite slow rotator, with an equatorial rotation period (the higher latitude layers rotate more slowly) of 25 days or so.  When it was young - just a few tens of million years old for example - it would have been completing a rotation about once every day or less, just like the rapidly rotating young beta Pictoris moving group stars studied earlier.  It subsequenty slowed down over the intervening 4 billion years because of angular momentum loss through the solar wind.

Like any dynamo, one might expect that if we turn the handle faster we get more magnetic field.  This is what happens in stars, roughly speaking, but with some interesting twists. Young, fast rotators do generally produce much more magnetic field than slow ones. While measuring the magnetic field directly is challenging, we can see its effects indirectly using X-ray telecopes in orbit outside of the Earth's atmosphere.  The picture here shows the Sun as seen by the X-ray telescope on the Japanese Yohkoh satellite.  The bright structures are magnetic fields anchored in sunspots, filled with plasma that is heated, probably largely by dissipation of the magnetic energy, to temperatures of two million degrees.  The same happens for other stars like the Sun, except the fast rotating ones have plasma heated to 10 million degrees and emit 10,000 times more X-rays.

We can use the X-rays from stars to try to understand how their dynamos work.  We have taken X-ray data on stars of different types that have known rotation periods to see how their magnetic energy dissipation rates change with rotation and stellar mass.  One of the results is illustrated in the graph to the right.  Each dot represents a different star whose mass relative to that of the Sun is indicated by the y-axis and rotation period by the x-axis. We have mapped out different dynamo regimes, showing how these change with mass.  Some of the most rapid rotators, with periods of a fraction of a day, appear to show "supersaturated" X-ray emission, lower than that of stars rotating with a period of a day or so.  Between periods of a day to a few days, stars have mostly "saturated" X-ray emission at a constant high level that does not depend on the exact rotation period.  Slower rotators, like the Sun, are in the "unsaturated" regime, in which faster rotation speed (shorter period) still leads to more X-rays and more magnetic field. Stars a little bit more massive than the Sun pass directly from a supersaturated dynamo to an unsaturated one. The origin of these regimes is still uncertain, and hotly debated.  This work was lead by SAO postdoc Nick Wright, who is, at the time of writing, a Royal Astronomical Society Fellow at the University of Hertfordshire.  It was published in the 2011 December 11 edition of the Astrophysical Journal.