Finding magnetic north: an extraordinary magnetic field detection in Polaris and first results of a magnetic survey of classical Cepheids

TL;DR

This study reports the first detection of magnetic fields in classical Cepheids, revealing Zeeman signatures and complex magnetic topologies, which enhances understanding of stellar magnetism in these key astrophysical objects.

Contribution

It presents the first systematic detection of magnetic fields in classical Cepheids using high-resolution spectropolarimetry, demonstrating their magnetic properties and complexity.

Findings

Magnetic signatures detected in all observed phases of η Aql.

Polaris shows a high S/N magnetic detection with complex topology.

Magnetic fields of ~0.4 to 1.3 G observed in several Cepheids.

Abstract

Classical Cepheids are essential objects in the study of stellar evolution and cosmology; however, we know little about their magnetic properties. We report the detection of Stokes $V$ features interpreted as Zeeman signatures in four classical Cepheids using high-resolution spectropolarimetric observations obtained with ESPaDOnS at CFHT. Eight observations of $\eta$ Aql were acquired in 2017 covering its 7.2 d pulsation period, and single observations of Polaris, $\zeta$ Gem, $\delta$ Cep and RT Aur were obtained in 2020 as part of our ongoing systematic survey. We use mean circular polarization Stokes $V$ profiles generated using the Least-Squares Deconvolution procedure to diagnose Zeeman signatures and measure mean longitudinal field strengths $\langle B_{z}\rangle$. We detect magnetic signatures across all pulsation phases of $\eta$ Aql ($-0.89\pm0.47$ G$\,<\langle B_{z}\rangle<1.27\pm 0.40$ G), as well as in the single observations of Polaris ($0.59\pm0.16$ G), $\zeta$ Gem ($0.41\pm0.16$ G) and $\delta$ Cep ($0.43\pm0.19$ G). The Stokes $V$ profile of Polaris is detected at extremely high S/N and implies a complex magnetic field topology. It stands in stark contrast to all other detected Stokes $V$ profiles, which show unusual approximately unipolar positive circular polarization lobes analogous to those observed in some Am stars.