The classical definition of a star's "habitable zone" is the orbital distance at which a planet can be warmed sufficiently to sustain liquid water on its surface. In earlier postings on studies of the radiation environments of M dwarf stars, we highlighted an additional criterion: the magnetic activity of the planet-hosting star. Magnetism is responsible for what are arguably the most interesting aspects of the behaviour of stars: UV and X-ray emission, flares and prominences, coronal mass ejections, and rarified but hot and energetic winds. These phenomena are more than of academic interest: they drive planetary atmospheric loss, influence the chemistry, and over time can
Universitäts-Sternwarte München and European Southern Observatory PhD student Julian Alvarado-Gomez has been leading a study to understand the magnetic activity of planet-hosting stars. The first step is to measure the stellar surface magnetic field - a non-trivial task and an activity representing an entire subfield of stellar astronomy in itself. A technique called "Zeeman-Doppler Imaging" is employed - see the posting on a particular application of this to the young planet-hosting Sun-like star, HD 1237. The next step involves using the surface magnetic field map to drive a sophisticated numerical supercomputer model of the outer atmosphere and magnetosphere of the star.
The computer model solves all the equations involving heating and cooling, forces and motions of the tenuous outer atmosphere of the star and its magnetic field. The gas is mostly a fully-ionized hydrogen plasma comprising electrons and protons. Unlike neutral gas, plasma interacts with magnetic fields. The outer atmosphere is a battle between the heated plasma and the magnetic field that tries to contains it. If the plasma wins, it breaks open the magnetic field to freedom in the form of an outflowing stellar wind; if the magnetic field wins, it contains the plasma as a multi-million degree X-ray emitting corona. This type of modelling was applied to three planet-hosting stars for which surface magnetic field maps were derived, HD 1237, HD 147513, and HD 22049 (better known as Epsilon Eridani). The results, published in the 2016 March 14 edition of Astronomy & Astrophysics, were used to visualize the stars' coronae and magnetic field structures, and to verify that the methods could successfully match observations of their energetic emissions. The next step is to push the modeling further out, to the orbits of their planets and probe their plasma and magnetic field space environments.
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