The discovery of exoplanets twenty years ago has spurred renewed interest in stellar magnetic activity. As highlighted in earlier posts, magnetic fields on stars like the Sun drive highly ionized supersonic winds, EUV and X-ray radiation, flares and coronal mass ejections that can affect the chemistry, evolution and survival of planetary atmospheres. Magnetic activity-related signatures can also muddy exoplanet detection by causing a jitter in radial velocity and light output.
Surface magnetic fields also cause a subtle alteration of the structure of atoms in the stellar atmosphere that can be detected in sensitive measurements of the stellar spectrum. By making measurements of a star at different rotational phases, it is possible to infer the surface distribution of magnetic fields - a technique known as "Zeeman-Doppler Imaging".
A detailed study of one such planet-hosting Sun-like star, HD 1237, revealed a surface magnetic field quite different to that of the Sun. HD 1237 is located about 57 light years away in the southern constellation of Hydrus. It is a few hundred degrees cooler than the Sun but is much younger - about 880 million years rather than 4.6 billion - and therefore expected to be more magnetically-active. The recovered magnetic field maps show a strong ring-like azimuthal field distribution that is not seen in the Sun. A complex radial field that dominates at mid-latitudes was also found. Similar magnetic field maps are recovered from data acquired five months apart, indicating that the magnetic field, at least on the larger scales that our measurements are sensitive to, can be fairly stable over long periods. This work was lead by Universitäts-Sternwarte München and European Southern Observatory PhD student Julian Alvarado-Gomez and was published in Astronomy & Astrophysics, Volume 582, 2015.
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