Research ArticleASTRONOMY

Shape of a slowly rotating star measured by asteroseismology

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Science Advances  16 Nov 2016:
Vol. 2, no. 11, e1601777
DOI: 10.1126/sciadv.1601777

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Abstract

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 ± 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 ΔR = 3 ± 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.

Keywords
  • Stellar asphericity
  • asteroseismology
  • stellar oscillations
  • stellar rotation
  • stellar magnetism
  • Kepler space mission

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