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A star of the beat generation

posted Dec 2, 2017, 1:50 PM by Jeremy Drake
Stars like the Sun generate copious magnetic fields that induce all sorts of interesting behaviour at the stellar surface.  Sunspots betraying bundles of emerging magnetic field, and the diffuse white halo of light scattered off the magnetically-driven solar wind seen around the Sun during a total eclipse, are good examples of this. The magnetic field is generated by the combination of the Sun's rotation and the convection that characterises the outer 30% or so of its radius. The magnetic field is always changing and goes through a cycle in which the polarity of the fieldthe "north" and "south" of the magnetreverses and changes back again over a period of about 22 years.  Similar cyclic magnetic behaviour is seen in about 60% of Sun-like stars.

Iota Horologii is a star like the Sun but much younger, with an age of "only" about 600 million years. It came to prominence when found to host a planet with a mass of 2.5 Jupiters in an orbit similar to that of Earth.  ι Hor rotates four times faster than the Sun because it has not had time to lose as much angular momentum through its wind. This faster rotation drives a stronger magnetic dynamo. Earlier studies had found a much shorter magnetic cycle than the Sun's with a period of only 1.6 yearsin fact the shortest known to date. As part of a much wider investigation of this intriguing exoplanet host, SAO Postdoctoral Scholar Julian Alvarado-Goméz studied the light from ionized calcium atoms in the star's upper atmosphere, or "chromosphere."  The chromosphere is heated by energy dissipated from the surface magnetic field and so can be used to trace the magnetic cycle. In a paper published in the 2017 October 10 edition of the Monthly Notices of the Royal Astronomical Society, Alvarado-Goméz and colleagues find the cycle is actually two cycles, with periods of 1.4 and 2 years, superimposed and "beating" against each other to make an apparent 1.6 year average. Still to be done is to understand exactly how and why magnetic cycles on the Sun and stars arise at all.
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