title: Shape of a slowly rotating star measured by asteroseismology.
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 radiusRthat 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/Ris 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.
authors.affiliation: Max-Planck-Institut für Sonnensystemforschung, 37077 Göttingen, Germany.; Institut für Astrophysik, Georg-August-Universität Göttingen, 37077 Göttingen, Germany.; National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan.; Center for Space Science, NYUAD Institute, New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi, UAE.
authors.forename: Laurent
authors.initials: L
authors.lastname: Gizon
authors.affiliation: National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Sekii
authors.affiliation: Department of Astronomy, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan.
authors.forename: Masao
authors.initials: M
authors.lastname: Takata
authors.affiliation: Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK.
authors.forename: Donald W
authors.initials: DW
authors.lastname: Kurtz
authors.affiliation: Department of Astronomy, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan.
authors.forename: Hiromoto
authors.identifier: 0000-0003-1733-953X
authors.initials: H
authors.lastname: Shibahashi
authors.affiliation: Center for Space Science, NYUAD Institute, New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi, UAE.
authors.forename: Michael
authors.initials: M
authors.lastname: Bazot
authors.affiliation: Center for Space Science, NYUAD Institute, New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi, UAE.; Department of Astronomy, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan.
authors.forename: Othman
authors.initials: O
authors.lastname: Benomar
authors.affiliation: Max-Planck-Institut für Sonnensystemforschung, 37077 Göttingen, Germany.
authors.forename: Aaron C
authors.initials: AC
authors.lastname: Birch
authors.affiliation: Center for Space Science, NYUAD Institute, New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi, UAE.; New York University, New York, NY 10012, USA.
authors.forename: Katepalli R
authors.initials: KR
authors.lastname: Sreenivasan
keywords: Kepler space mission; Stellar asphericity; asteroseismology; stellar magnetism; stellar oscillations; stellar rotation