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Shape of a slowly rotating star measured by asteroseismology

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 different stellar latitudes, we infer a tiny but significant flattening of the star's shape of $ΔR/R = (1.8 \pm 0.6) \times 10^{-6}$. For a stellar radius $R$ that is $2.24$ times the solar radius, the difference in radius between the equator and the poles is $ΔR = 3 \pm 1$ km. Because the observed $ΔR/R$ 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.