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Old red dwarfs teach us new tricks

posted Jul 28, 2016, 1:19 AM by Jeremy Drake
Study of the so-called "magnetic activity" of stars like the Sun dates back to early Chinese astronomers who made naked eye observations of sunspots at least as early as 28 BC. But it took almost 2000 years and George Ellery Hale's brilliant 1908 discovery that sunspots were regions of strong magnetic field to really kick off the study of stellar magnetism. The most fundamental part of the problem is the origin of the magnetic fields themselves.  

Stars are giant balls of ionized gas - a plasma of mostly free electrons and protons. Such electrically charged particles are forced to spiral around magnetic fields, and magnetic fields essentially become attached or "frozen in" to the plasma.  When the pressure of the plasma is stronger than the magnetic field pressure, plasma motions drag the magnetic field with it. Stellar interiors are very dynamic, with regions of strong convective flows.  Stars also do not spin uniformly with the same rotation period - the Sun's equator rotates faster than its pole by 50% or so. There is broad understanding that this differential rotation, combined with convective motions, generates and amplifies the magnetic field by winding and folding it - much like stretching and folding an elastic band - but the details of the process and exactly where in the Sun it happens has remained a subject of intense debate.

Since the 1980's, the main driver of the solar dynamo has been thought to be the differential rotation at the tachocline - a region of strong rotational shear between the outer convection zone and inner purely radiative and non-convecting zone of the Sun. As magnetic field strength reaches a certain threshold, the fields and their trapped plasma become buoyant and rise to the surface to form sunspots like those first studied by the Chinese more than two millennia ago. These areas of surface magnetism are also associated with X-ray emission that originates when energy stored and channelled in the magnetic fields is dissipated at the surface, heating the very rarified outer atmosphere to form the solar "corona".  We can see coronae on other stars like the Sun using X-ray telescopes, and X-rays turn out to be a powerful way of probing their surface magnetism. 

When current Earnest Rutherford Fellow Nick Wright of Keele University was a postdoc at Smithsonian, he and I examined the way stellar rotation influenced X-ray output from stars.  Faster rotating stars generate more magnetic field and are brighter in X-rays.  We were missing some key types of star from the sample though: cool, slowly rotating old M dwarfs - stars with less than half the mass of the Sun.  We subsequently observed two of them using NASA's Chandra X-ray Observatory and found X-ray data for two more from older observations.  To our surprise, the trend with stellar rotation for these M dwarfs was the same as for Sun-like stars.  We were surprised because these stars do not have the central radiative zone that the Sun does, but are convective all the way to the centre. They have no tachocline like that in the Sun, but their magnetic behaviour with rotation is the same. The results indicate that the tachocline is not a significant factor in stellar magnetic dynamos: magnetic field must be generated elsewhere in the convection zone, likely by the differential rotation that also exists there.  This study was published in the 2016 July 28 issue of Nature and also featured in a Chandra press release.