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

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 the Canada-France-Hawaii Telescope. 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 < B-Z >. We detect magnetic signatures across all pulsation phases of eta Aql (-0.89 +/- 0.47 G < < B-Z > < 1.27 +/- 0.40 G), as well as in the single observations of Polaris (0.59 +/- 0.16 G), zeta Gem (0.41 +/- 0.16 G), and delta Cep (0.43 +/- 0.19 G). The Stokes V profile of Polaris is detected at extremely high signal-to-noise ratio 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.