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 $\Delta R/R=(1.8\pm 0.6)\times 10^{-6}$. For a stellar radius $R$ that is $2.24$ times the solar radius, the difference in radius between the equator and the poles is $\Delta R=3\pm 1$ km. Because the observed $\Delta R/R$ 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.

15 pages, 2 tables, 3 figures

References in corpus (4)

• Imaging the Surface of Altair
• Asteroseismic measurement of surface-to-core rotation in a main sequence A star, KIC 11145123
• Vega is a rapidly rotating star
• Fourier Analysis of Gapped Time Series: Improved Estimates of Solar and Stellar Oscillation Parameters

Cited by in corpus (10)

• The single-sided pulsator CO~Camelopardalis
• Latitudinal differential rotation in the solar analogues 16 Cyg A and B
• The two-dimensional internal rotation of KIC11145123
• Spectroscopic and asteroseismic analysis of the remarkable main-sequence A star KIC 11145123
• Transition of latitudinal differential rotation as a possible cause of weakened magnetic braking of solar-type stars
• Overstable Convective Modes in a Polytropic Stellar Atmosphere
• Bayesian rotation inversion of KIC 11145123
• Non-standard modeling of a possible blue straggler star, KIC 11145123
• Detecting active latitudes of Sun-like stars using asteroseismic a-coefficients
• Asteroseismic Signature of a Large Active Region