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

Abstract

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 the Canada–France–Hawaii Telescope. Eight observations of η Aql were acquired in 2017 covering its 7.2 d pulsation period, and single observations of Polaris, ζ Gem, δ 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 〈Bz〉. We detect magnetic signatures across all pulsation phases of η Aql (−0.89 ± 0.47 G$\, \lt \langle B_{z}\rangle \lt 1.27\pm 0.40$ G), as well as in the single observations of Polaris (0.59 ± 0.16 G), ζ Gem (0.41 ± 0.16 G), and δ 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.