An Investigation of the Magnetic Properties of the Classical Be Star  omega Ori by the MiMeS Collaboration

C. Neiner1, J.H. Grunhut2,3, V. Petit4, A. ud-Doula5, G.A. Wade3, J. Landstreet6, B. de Batz1, F. Cochard7, J. Gutiérrez-Soto8,9, and A.-L. Huat10

1LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris Diderot; 5 place Jules Janssen, 92190 Meudon, France
2Department of Physics, Engineering Physics & Astronomy, Queen's University, Kingston, Ontario, Canada, K7L 3N6
3Department of Physics, Royal Military College of Canada, P.O. Box 17000, Station Forces, Kingston, Ontario, Canada, K7K 7B4
3LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris Diderot; 5 place Jules Janssen, 92190 Meudon, France
4Dept. of Geology &amp Astronomy, West Chester University, West Chester, PA, 19383, USA
5Penn State Worthington Scranton, 120 Ridge View Drive, Dunmore, PA USA, 18512
6Physics &amp Astronomy Department, The University of Western Ontario, London, Ontario, Canada N6A 3K7
7Shelyak Instruments, Les Roussets, 38420 Revel, France
8Valencian International University (VIU), Prolongación C/ José Pradas Gallen s/n, 12006 Castellón de la Plana, Spain
9Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
10GEPI, Observatoire de Paris, CNRS UMR 8111, Université Paris Diderot; 5 place Jules Janssen, 92190 Meudon, France

One-hundred and twenty-five new high-precision spectropolarimetric observations have been obtained with ESPaDOnS at CFHT and Narval at TBL to investigate the magnetic properties of the classical Be star omga Ori. No Stokes V signatures are detected in our polarimetric data. Measurements of the longitudinal magnetic field, with a median error bar of 30 G, and direct modelling of the mean Least-Squares Deconvolved Stokes V profiles, yield no evidence for a dipole magnetic field with polar surface strength greater than ~80 G. We are therefore unable to confirm the presence of the magnetic field reported by Neiner et al. (2003). However, our spectroscopic data reveal the presence of periodic emission variability in H and He lines analogous to that reported by Neiner et al. 2003, considered as evidence of magnetically confined circumstellar plasma clouds. We revisit this hypothesis in light of the new magnetic analysis. Calculation of the magnetospheric Kepler radius RK and confinement parameter eta* indicate that a surface dipole magnetic field with a polar strength larger than 63?G is sufficient to form of a centrifugally supported magnetosphere around omega Ori. Our data are not sufficiently sensitive to detect fields of this magnitude; we are therefore unable to confirm or falsify the magnetic cloud hypothesis. Based on our results, we examine 3 possible scenarios that could potentially explain the behaviour of omega Ori: (1) that no significant magnetic field is (or was) present in omega Ori, and that the observed phenomena have their origin in another mechanism or mechanisms than co-rotating clouds. We are, however, unable to identify one; (2) that ? Ori hosts an intermittent magnetic field produced by dynamo processes; However, no such process has been found so far to work in massive stars and especially to produce a dipolar field; and (3) that omega Ori hosts a stable, organised (fossil) magnetic field that is responsible for the observed phenomena, but with a strength that is below our current detection threshold. Of these three scenarios, we consider the second one (dynamo process) as highly unlikely, whereas the other two should be falsifiable with intense monitoring.

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