EK Eridani: the tip of the iceberg of giants which have evolved from magnetic Ap stars

TL;DR

This study directly measures the magnetic field of the giant star EK Eri, revealing a strong, large-scale magnetic field likely inherited from a magnetic Ap star, which is unusual for its class.

Contribution

First direct measurement of EK Eri's magnetic field showing it is unusually strong for a slow-rotating giant, supporting its origin as a descendant of a magnetic Ap star.

Findings

Detected a magnetic field of about 100 G in EK Eri.

Inferred a mean surface magnetic field of approximately 270 G.

Large-scale magnetic field dominated by a poloidal component.

Abstract

We observe the slowly-rotating, active, single giant, EK Eri, to study and infer the nature of its magnetic field directly. We used the spectropolarimeter NARVAL at the Telescope Bernard Lyot, Pic du Midi Observatory, and the Least Square Deconvolution method to create high signal-to-noise ratio Stokes V profiles. We fitted the Stokes V profiles with a model of the large-scale magnetic field. We studied the classical activity indicators, the CaII H and K lines, the CaII infrared triplet, and H\alpha line. We detected the Stokes V signal of EK Eri securely and measured the longitudinal magnetic field Bl for seven individual dates spanning 60% of the rotational period. The measured longitudinal magnetic field of EK Eri reached about 100 G and was as strong as fields observed in RSCVn or FK Com type stars: this was found to be extraordinary when compared with the weak fields observed at the surfaces of slowly-rotating MS stars or any single red giant previously observed with NARVAL. From our modeling, we infer that the mean surface magnetic field is about 270 G, and that the large scale magnetic field is dominated by a poloidal component. This is compatible with expectations for the descendant of a strongly magnetic Ap star.