Investigating the origin of cyclical wind variability in hot, massive stars - I. On the dipolar magnetic field hypothesis

A. David-Uraz$^{1,2}$, G. A. Wade$^1$, V. Petit$^3$, A. ud-Doula$^4$, J. O. Sundqvist$^{3,5}$, J. Grunhut$^6$, M. Shultz$^{1,2,7}$, C. Neiner$^8$, E. Alecian$^{9,8}$, H. F. Henrichs$^10$, J.-C. Bouret$^{11,12}$ and the MiMeS Collaboration

1 - Department of Physics, Royal Military College of Canada, PO Box 17000, Stn Forces, Kingston, Canada, K7K 4B4
2 - Department of Physics, Engineering Physics and Astronomy, Queen’s University, 99 University Avenue, Kingston, Canada, K7L 3N6
3 - Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
4 - Penn State Worthington Scranton, Dunmore, PA 18512, USA
5 - Institut fur Astronomie und Astrophysik der Universitat Munchen, Scheinerstr. 1, D-81679 Munchen, Germany
6 - European Organisation for Astronomical Research in the Southern Hemisphere, Karl-Schwarzschild-Str. 2, 85748,
Garching bei Munchen, Germany
7 - European Organisation for Astronomical Research in the Southern Hemisphere, Casilla 19001, Santiago 19, Chile
8 - LESIA, UMR 8109 du CNRS, Observatoire de Paris, UPMC, Universit´e Paris Diderot, 5 place Jules Janssen, F-92195
Meudon Cedex, France
9 - UJF-Grenoble 1/CNRS-INSU, Institut de Planetologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, Grenoble, F-38041, France
10 - Astronomical Institute Anton Pannekoek, University of Amsterdam, Science Park 904, NL-1098 XH Amsterdam, the Netherlands
11 - Laboratoire d’Astrophysique de Marseille, CNRS-Universite de Provence, Pole de l’Etoile Site de Chateau-Gombert, ´
38, rue Frederic Joliot-Curie 13388 Marseille cedex 13, France
12 - NASA/Goddard Space Flight Center, Greenbelt, MD 20771 USA

OB stars exhibit various types of spectral variability associated with wind structures, including the apparently ubiquitous discrete absorption components (DACs). These are proposed to be caused by either magnetic fields or non-radial pulsations (NRPs). In this paper, we evaluate the possible relation between large-scale, dipolar magnetic fields and the DAC phenomenon by investigating the magnetic properties of a sample of 13 OB stars exhibiting well-documented DAC behaviour.
Using high-precision spectropolarimetric data acquired in part in the context of the Magnetism in Massive Stars (MiMeS) project, we find no evidence for surface dipolar magnetic fields in any of these stars.

Using Bayesian inference, we compute upper limits on the strengths of the fields and use these limits to assess two potential mechanisms by which the field may influence wind outflow: magnetic wind confinement and local photospheric brightness enhancements. Within the limits we derive, both mechanisms fail to provide a systematic process capable of producing DACs in all of the stars of our sample. Therefore, this implies that dipolar fields are highly unlikely to be responsible for these structures in all massive stars, meaning that some other mechanism must come into play.

Reference: arXiv:1407.6417 (accepted in MNRAS)
Status: Manuscript has been accepted