Shape of a slowly rotating star measured by asteroseismology

Gizon, L, Sekii, T, Takata, M, Kurtz, Donald Wayne orcid iconORCID: 0000-0002-1015-3268, Shibahashi, H, Bazot, M, Benomar, O, Birch, AC and Sreenivasan, KR (2016) Shape of a slowly rotating star measured by asteroseismology. Science Advances, 2 (11). e1601777-e1601777.

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Stars are not perfectly spherically symmetric. They are deformed by rotation and magnetic fields. Until now, the study of stellar shapes has only been possible with optical interferometry for a few of the fastest-rotating nearby stars. We report an asteroseismic measurement, with much better precision than interferometry, of the asphericity of an A-type star with a rotation period of 100 days. Using the fact that different modes of oscillation probe dif- ferent stellar latitudes, we infer a tiny but significant flattening of the star’s shape of DR/R = (1.8 ± 0.6) × 10−6. For a stellar radius R that is 2.24 times the solar radius, the difference in radius between the equator and the poles is DR = 3 ± 1 km. Because the observed DR/R is only one-third of the expected rotational oblateness, we conjecture the presence of a weak magnetic field on a star that does not have an extended convective envelope. This calls to question the origin of the magnetic field.

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