Shape of a slowly rotating star measured by asteroseismology

TL;DR

This paper demonstrates asteroseismology as a highly precise method to measure the slight asphericity of a slowly rotating star, revealing a tiny flattening that challenges existing expectations about stellar shape and magnetic fields.

Contribution

It introduces a novel asteroseismic approach to measure stellar shape deformation with unprecedented precision, providing new insights into stellar magnetic fields and rotation effects.

Findings

Measured stellar flattening of ΔR/R = (1.8 ± 0.6) × 10^{-6}

Detected a radius difference of 3 ± 1 km between equator and poles

Observed flattening is one-third of expected rotational oblateness

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.