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The Optics of Superflares

posted Mar 4, 2020, 8:20 AM by Jeremy Drake   [ updated Mar 4, 2020, 8:21 AM ]
Stellar flares are explosive events in the outer atmospheres of stars powered by the sudden release of stored magnetic energy generated in the interior. The analogy of suddenly releasing a stretched elastic band is often drawn: the elastic is magnetic field that is stretched and stressed by the roiling convection of plasma beneath the stellar surface, or photosphere.  As the magnetic field snaps back into a lower energy configuration it accelerates electrons and protons that impact the photosphere, heating and "evaporating" plasma into the corona where it reaches temperatures of millions of K and radiates in X-rays. 

The Kepler spacecraft can detect the impact of flare-accelerated particles on the stellar photosphere because of a local increase in the photospheric temperature at the flare footpoints - from 6000 K to 10,000 K for a flare on a Sun-like star for example.  X-ray satellites, like ESA's XMM-Newton, can detect the flare in X-rays.  Both satellites observed the young Pleiades cluster at the same time back in 2015 February with the aim of detecting flares and understanding the way energy is distributed between the hot X-ray emitting gas and the optical emission of the flare footpoints. 

The Pleiades is a cluster of stars 136 parsecs away formed about 100 million years ago - very young stars in the context of the 4.6 billion-year-old Sun. As detailed in earlier postings, young stars rotate rapidly - up to once every day or so - engendering strong magnetic fields and flaring, with flare energies up to 10,000 times more energetic than seen on the Sun. Solar flares typically release 50 or more times more energy in visible light than in X-rays, and this large difference is key for understanding the physics and energy requirements of flaring.  What happens then in flares many times more energetic, such as on the young Pleiades Sun-like stars? 

A paper lead by Palermo Observatory scientist Mario Guarcello and published in Astronomy and Astrophysics found that the Kepler visible flare signature typically corresponded to energies 2-3 times higher than seen in the X-rays with XMM-Newton, a much smaller difference than in lower energy solar flares. This suggests that more energetic flares are less efficient in evaporating plasma into the corona, although much work still remains to understand the details of why.
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