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

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 have carried out spectropolarimetry of five extra-Galactic Of?p stars, as well as a couple of dozen neighbouring stars. We have been 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 has been firmly detected in any of our measurements, but we have been able to estimate upper limits to 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.

Accepted by A&A

References in corpus (13)

• Pre-Supernova Evolution of Massive Single and Binary Stars
• The empirical metallicity dependence of the mass-loss rate of O- and early B-type stars
• The MiMeS Survey of Magnetism in Massive Stars: Introduction and overview
• The MiMeS survey of magnetism in massive stars: Magnetic analysis of the O-type stars
• Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
• B fields in OB stars (BOB): low-resolution FORS2 spectropolarimetry of the first sample of 50 massive stars
• The metallicity dependence of envelope inflation in massive stars
• A Modern Search for Wolf-Rayet Stars in the Magellanic Clouds: First Results
• The FORS1 catalogue of stellar magnetic field measurements
• On the consistency of magnetic field measurements of Ap stars: lessons learned from the FORS1 archive
• Magnetometry of a sample of massive stars in Carina
• Photometric identification of the periods of the first candidate extragalactic magnetic massive stars
• Spectral Variations of Of?p Oblique Magnetic Rotator Candidates in the Magellanic Clouds

Cited by in corpus (8)

• The Magnetic Early B-type Stars III: A main sequence magnetic, rotational, and magnetospheric biography
• A Modern Search for Wolf-Rayet Stars in the Magellanic Clouds. IV. A Final Census
• Searching for the weakest detectable magnetic fields in white dwarfs. Highly-sensitive measurements from first VLT and WHT surveys
• The effects of surface fossil magnetic fields on massive star evolution: IV. Grids of models at Solar, LMC, and SMC metallicities
• Testing the fossil field hypothesis: could strongly magnetised OB stars produce all known magnetars?
• Magnetism in High-Mass Stars
• A search for strong magnetic fields in massive and very massive stars in the Magellanic Clouds
• Detection of extragalactic magnetic massive stars