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.

The magnetic early B-type stars – III. A main-sequence magnetic, rotational, and magnetospheric biography

“…This may reflect an incomplete characterization of the sample of magnetic O-type stars. Many such systems have been found to be spectroscopic binaries, leading to an increase in the inferred surface magnetic field strength (e.g., ζ Ori Aa and HD 148937; Blazère et al 2015;Wade et al 2019); B d is still a lower limit for the extremely slow rotator HD 108 (Shultz & Wade 2017); and while magnetic fields have still not been detected in the magellanic cloud Of?p stars (Walborn et al 2015;Bagnulo et al 2017), preliminary models of their magnetospheric variability suggest the mean B d of this population to be about 7 kG (Munoz et al 2018), consistent with that of the Galactic magnetic AB stars.…”

A Modern Search for Wolf-Rayet Stars in the Magellanic Clouds. IV. A Final Census*

“…The classification criteria come from Walborn et al (2010a) and describes Of-type stars whose C iii λ4650 emission is comparable in intensity to the N iii λλ4634, 42 emission. Of?p stars are also highly magnetic (Munoz et al 2018;Bagnulo et al 2017) and are believed to be oblique magnetic rotators (Walborn et al 2010a). A spectrum of the discovered Of?p star is shown in Figure 2.…”

A search for strong magnetic fields in massive and very massive stars in the Magellanic Clouds

“…and P = 7.9606 ± 0.0009d. This ephemeris is sufficiently precise to allow us to phase our new EWs with those of Bagnulo et al (2017). The resultant phase curves, illustrated in Fig.…”

“…In Paper I, we demonstrate that the EWs of Hβ and He ii λ4686 varied smoothly according to this same period. Using the most recent epochs of OGLE photometry, we derived a new ephemeris for SMC 159-2, with T 0 = 2457416.997 and P = 14.915 ± 0.003 d. This ephemeris is sufficiently precise to allow us to phase our new EWs, measured from He ii 4686, Hβ and Hα, with those of Bagnulo et al (2017). The resultant phase curves, illustrated in Fig.…”

“…Using the most recent epochs of OGLE photometry, we derived a new ephemeris for LMC 164-2, with T 0 = 2457506.140 1, from Bagnulo et al (2017), and the references therein. Photometric data are from OGLE.…”