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Plus ça change...

posted Jan 5, 2017, 9:28 AM by Jeremy Drake
Our nearest stellar neighbour, the aptly monickered Proxima (actually dubbed "Proxima Centurus" by its discoverer, Robert Innes, of the Union Observatory in Johannesburg), shot to stardom recently after having been found to harbour a habitable zone planet with a mass only 30% larger than that of the Earth.  Before this jaunt down the red dwarf carpet, Proxima was merely a conveniently nearby M6 dwarf - a faint, diminutive star with only an eighth of the mass of the Sun and a seventh of its radius. Proxima is cool, in the literal sense - just 3000 K at its surface compared with the Sun's 5800 K.  So cool, in fact, that its internal structure is quite different to that of the Sun.  

The Sun has an outer convection zone taking up the top 30% or so of its radius.  Inside that, its structure is stable and "purely radiative".  It has been thought that the interface between the radiative and convective zones on the Sun is key for its dynamo that generates magnetic fields visible at the surface in the form of sunpots and energetic UV to X-ray emission. Proxima does not have a central radiative zone - it is convective all the way to the centre. So, logically, its dynamo should operate quite differently.  

An earlier posting presented new X-ray evidence that the dynamos of fully convective stars, like Proxima, are instead surprisingly solar-like.  Further evidence has now emerged showing common dynamo action in another way: cyclic behaviour. The solar dynamo has a well-known cycle that results in the surface magnetic field waxing and waning and reversing polarity every 11 years.  SAO scientist Brad Wargelin has lead a team analysing long-term optical, UV and X-ray observations of Proxima that have probed its long-term magnetic behaviour. The visible light data reveal a slow modulation in brightness with a period of seven years. This brightening and dimming is the result of changes in the number of starspots on the stellar surface stemming from a magnetic cycle - in Proxima's case a 7 year one instead of 11.  X-ray emission measured by a collection of different satellites over the years also shows a sympathetic secular variation in phase with the starspots.  The implication of these new observations is that the magnetic fields of all stars with outer convection zones, including the Sun, are generated in the same way within convection zone and do not depend on the presence of a central stable radiative zone.  This work was published in the 2017 January 21 edition of Monthly Notices of the Royal Astronomical Society.