Detection of ultra-weak magnetic fields in Am stars: β Ursae Majoris and θ Leonis

¹ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
e-mail: aurore.blazere@obspm.fr
² Université de Toulouse, UPS-OMP, Institut de Recherche en Astrophysique et Planétologie, 31062 Toulouse, France
³ CNRS, Institut de Recherche en Astrophysique et Planétologie, 14 avenue Édouard Belin, 31400 Toulouse, France
⁴ IPAG, UJF-Grenoble 1/CNRS-INSU, UMR 5274, 38041 Grenoble, France
⁵ Department of Physics, Royal Military College of Canada, PO Box 17000 Station Forces, Kingston, ON K7K 0C6, Canada

Received: 14 October 2015
Accepted: 2 December 2015

Abstract

Context. An extremely weak circularly polarized signature was recently discovered in spectral lines of the chemically peculiar Am star Sirius A. A weak surface magnetic field was proposed to account for the observed polarized signal, but the shape of the phase-averaged signature, dominated by a prominent positive lobe, is not expected in the standard theory of the Zeeman effect.

Aims. We aim at verifying the presence of weak circularly polarized signatures in two other bright Am stars, β UMa and θ Leo, and investigating the physical origin of Sirius-like polarized signals further.

Methods. We present here a set of deep spectropolarimetric observations of β UMa and θ Leo, observed with the NARVAL spectropolarimeter. We analyzed all spectra with the least squares deconvolution multiline procedure. To improve the signal-to-noise ratio and detect extremely weak signatures in Stokes V profiles, we co-added all available spectra of each star (around 150 observations each time). Finally, we ran several tests to evaluate whether the detected signatures are consistent with the behavior expected from the Zeeman effect.

Results. The line profiles of the two stars display circularly polarized signatures similar in shape and amplitude to the observations previously gathered for Sirius A. Our series of tests brings further evidence of a magnetic origin of the recorded signal.

Conclusions. These new detections suggest that very weak magnetic fields may well be present in the photospheres of a significant fraction of intermediate-mass stars. The strongly asymmetric Zeeman signatures measured so far in Am stars (featuring a dominant single-sign lobe) are not expected in the standard theory of the Zeeman effect and may be linked to sharp vertical gradients in photospheric velocities and magnetic field strengths.