First constraints on the magnetic field strength in extra-Galactic stars: FORS2 observations of Of?p stars in the Magellanic Clouds

¹ Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, UK
e-mail: sba@arm.ac.uk
² Institut d’Astrophysique et de Géophysique, Université de Liège, Quartier Agora (B5c), Allée du 6 Août 19c, 4000 Sart Tilman, Liège, Belgium
³ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁴ Las Campanas Observatory, Carnegie Observatories, Casilla 601, La Serena, Chile
⁵ Department of Physics, Royal Military College of Canada, PO Box 17000, Station Forces, ON, K7K 4B4, Canada
⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁷ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁸ Excellence Cluster Universe, 85748 Garching bei München, Germany
⁹ Departamento de Física, Universidad de La Serena, Av. Juan Cisternas 1200 Norte, La Serena, Chile

Received: 4 November 2016
Accepted: 23 February 2017

Abstract

Massive O-type stars play a dominant role in our Universe, but many of their properties remain poorly constrained. In the last decade magnetic fields have been detected in all Galactic members of the distinctive Of?p class, opening the door to a better knowledge of all O-type stars. With the aim of extending the study of magnetic massive stars to nearby galaxies, to better understand the role of metallicity in the formation of their magnetic fields and magnetospheres, and to broaden our knowledge of the role of magnetic fields in massive star evolution, we carried out spectropolarimetry of five extra-Galactic Of?p stars, and of a couple of dozen neighbouring stars. We were able to measure magnetic fields with typical error bars from 0.2 to 1.0 kG, depending on the apparent magnitude and on weather conditions. No magnetic field was firmly detected in any of our measurements, but we were able to estimate upper limits on the field values of our target stars. One of our targets, 2dFS 936, exhibited an unexpected strengthening of emission lines. We confirm the unusual behaviour of BI 57, which exhibits a 787 d period with two photometric peaks and one spectroscopic maximum. The observed strengthening of the emission lines of 2dFS 936, and the lack of detection of a strong magnetic field in a star with such strong emission lines is at odd with expectations. Together with the unusual periodic behaviour of BI 57, it represents a challenge for the current models of Of?p stars. The limited precision that we obtained in our field measurements (in most cases as a consequence of poor weather) has led to field-strength upper limits that are substantially larger than those typically measured in Galactic magnetic O stars. Further higher precision observations and monitoring are clearly required.