Finding magnetic north: an extraordinary magnetic field detection in Polaris and first results of a magnetic survey of classical 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 $⟨B_z⟩$. We detect magnetic signatures across all pulsation phases of $\eta$ Aql ($-0.89\pm 0.47$ G$<⟨B_z⟩<1.27\pm 0.40$ G), as well as in the single observations of Polaris ($0.59\pm 0.16$ G), $\zeta$ Gem ($0.41\pm 0.16$ G) and $\delta$ Cep ($0.43\pm 0.19$ 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.

arXiv comment11 pages, 6 figures, accepted for publication in MNRAS

References in corpus (33)

• Magnetic fields of non-degenerate stars
• Least squares deconvolution of the stellar intensity and polarization spectra
• Direct Detection of the Close Companion of Polaris with the Hubble Space Telescope
• The MiMeS Survey of Magnetism in Massive Stars: Introduction and overview
• The MiMeS survey of magnetism in massive stars: Magnetic analysis of the O-type stars
• Revealing δ Cephei's Secret Companion and Intriguing Past
• PolarBase: a data base of high resolution spectropolarimetric stellar observations
• High resolution spectroscopy for Cepheids distance determination. I. Line asymmetry
• Detection of a weak surface magnetic field on Sirius A: are all tepid stars magnetic ?
• Cepheid Abundances: Multiphase Results and Spatial Gradients
• Multiplicity of Galactic Cepheids and RR Lyrae stars from Gaia DR2 - I. Binarity from proper motion anomaly
• Systematic detection of magnetic fields in massive, late-type supergiants
• Cepheid distances from the SpectroPhoto-Interferometry of Pulsating Stars (SPIPS) - Application to the prototypes delta Cep and eta Aql
• Large-scale magnetic field of the G8 dwarf xi Bootis A
• Detection of ultra-weak magnetic fields in Am stars: beta UMa and theta Leo
• Observational calibration of the projection factor of Cepheids. II. Application to nine Cepheids with HST/FGS parallax measurements
• Search for surface magnetic fields in Mira stars. First detection in chi Cyg
• Observations of Cepheids with the MOST satellite: Contrast between Pulsation Modes
• First detection of surface magnetic fields in Post-AGB stars : the cases of U Monocerotis and R Scuti
• Measuring surface magnetic fields of red supergiant stars
• Binary Properties from Cepheid Radial Velocities (CRaV)
• The Secret Lives of Cepheids: $\delta$ Cep -- the Prototype of a New Class of Pulsating X-ray Variable Stars
• Convective cells in Betelgeuse: imaging through spectropolarimetry
• The Secret Lives of Cepheids: Evolutionary Changes and Pulsation-Induced Shock Heating in the Prototype Classical Cepheid δ Cep
• Evolution of the magnetic field of Betelgeuse from 2009 - 2017
• The pulsating magnetosphere of the extremely slowly rotating magnetic $\beta$ Cep star ${\xi }^1$ CMa
• VEGA/CHARA interferometric observations of Cepheids. I. A resolved structure around the prototype classical Cepheid delta Cep in the visible spectral range
• Discovery of a complex linearly polarized spectrum of Betelgeuse dominated by depolarization of the continuum
• The period and amplitude changes of Polaris (alpha UMi) from 2003 to 2007 measured with SMEI
• Probing Polaris' Puzzling Radial Velocity Signals - Pulsational (In-)Stability, Orbital Motion, and Bisector Variations
• The Orbit of the Close Companion of Polaris: Hubble Space Telescope Imaging 2007 to 2014
• The pulsationally modulated radial crossover signature of the slowly rotating magnetic B-type star ${\xi }^1$ CMa
• X-rays in Cepheids: XMM-Newton Observations of $\eta$ Aql

Cited by in corpus (4)

• Non-thermal $X$-rays from pulsation-driven shocks in Cepheids
• First Detection of Radio Emission Associated with a Classical Cepheid
• Polarization position angle standard stars: a reassessment of $\theta$ and its variability for seventeen stars based on a decade of observations
• The VELOCE modulation zoo III. Detecting additional pulsation modes in optical spectra of classical Cepheids using semi-partial distance correlation periodograms