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Ultra-weak magnetic fields in Am stars: β UMa and θ Leo

Published online by Cambridge University Press:  24 July 2015

A. Blazère ,
P. Petit ,
F. Lignières ,
M. Aurière ,
J. Ballot ,
T. Böhm ,
C. Folsom ,
A. López Ariste  and
G.A. Wade
A. Blazère
Affiliation:
LESIA, Observatoire de Paris, UMR 8109 du CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France email: aurore.blazere@obspm.fr Université de Toulouse & CNRS, Institut de Recherche en Astrophysique et Planétologie, Toulouse, France
P. Petit
Affiliation:
Université de Toulouse & CNRS, Institut de Recherche en Astrophysique et Planétologie, Toulouse, France
F. Lignières
Affiliation:
Université de Toulouse & CNRS, Institut de Recherche en Astrophysique et Planétologie, Toulouse, France
M. Aurière
Affiliation:
Université de Toulouse & CNRS, Institut de Recherche en Astrophysique et Planétologie, Toulouse, France
J. Ballot
Affiliation:
Université de Toulouse & CNRS, Institut de Recherche en Astrophysique et Planétologie, Toulouse, France
T. Böhm
Affiliation:
Université de Toulouse & CNRS, Institut de Recherche en Astrophysique et Planétologie, Toulouse, France
C. Folsom
Affiliation:
IPAG, UJF-Grenoble 1/CNRS-INSU, UMR 5274, 38041 Grenoble, France
A. López Ariste
Affiliation:
Université de Toulouse & CNRS, Institut de Recherche en Astrophysique et Planétologie, Toulouse, France
G.A. Wade
Affiliation:
Department of Physics, Royal Military College of Canada, PO Box 17000 Station Forces, Kingston, ON K7K 0C6, Canada
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Abstract

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An extremely weak circularly-polarized signature was recently detected in the spectral lines of the Am star Sirius A. With a prominent positive lobe, the shape of the phase-averaged Stokes V line profile is atypical of stellar Zeeman signatures, casting doubts on its magnetic origin. We report here on ultra-deep spectropolarimetric observations of two more bright Am stars: β Uma and θ Leo. Stokes V line signatures are detected in both objects, with a shape and amplitude similar to the one observed on Sirius A. We demonstrate that the amplitude of the Stokes V line profiles depend on various line parameters (Landé factor, wavelength, depth) as expected from a Zeeman signature, confirming that extremely weak magnetic fields are likely present in a large fraction of Am stars. We suggest that the strong asymmetry of the polarized signatures, systematically observed so far in Am stars and never reported in strongly magnetic Ap stars, bears unique information about the structure and dynamics of the thin surface convective shell of Am stars.

Keywords

Stars: magnetic fieldstars: chemically peculiar
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 10 , Symposium S305: Polarimetry: From the Sun to Stars and Stellar Environments , December 2014 , pp. 67 - 72
Copyright
Copyright © International Astronomical Union 2015 

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