Exploring the origin of magnetic fields in massive stars: a survey of O-type stars in clusters and in the field

Abstract

Context. Although the effects of magnetic fields in massive stars have been found to be substantial by recent models and observations, the magnetic fields of only a small number of massive O-type stars have so far been investigated. Additional observations are of the utmost importance to constraining the conditions that are conducive to magnetic fields and to determine the first trends about their occurrence rate and field strength distribution. Aims. To investigate statistically whether magnetic fields in massive stars are ubiquitous or appear only in stars with a specific spectral classification, certain ages, or in a special environment, we acquired 41 new spectropolarimetric observations for 36 stars. Among the observed sample, roughly half of the stars are probable members of clusters at different ages, whereas the remaining stars are field stars not known to belong to any cluster or association. Methods. Spectropolarimetric observations were obtained during three different nights using the low-resolution spectropolarimetric mode of FORS 2 (FOcal Reducer low dispersion Spectrograph) mounted on the 8-m Antu telescope of the VLT. To assess the membership in open clusters and associations, we used astrometric catalogues with the highest quality kinematic and photometric data currently available. Results. A field at a significance level of 3σ was detected in ten O-type stars. The strongest longitudinal magnetic fields were measured in two Of?p stars: B z = −381 ± 122 G for CPD−28 2561 and B z = −297 ± 62 G for HD 148937, the latter of which had previously been detected by ourselves as magnetic. The observations of HD 148937 obtained on three different nights indicate that the magnetic field is slightly variable. Our new measurements support our previous conclusion that large-scale organized magnetic fields with polar field strengths in excess of 1 kG are not widespread among O-type stars. Among the stars with a detected magnetic field, only one star, HD 156154, belongs to an open cluster at high membership probability. According to previous kinematic studies, four magnetic O-type stars in the sample are well-known candidate runaway stars.

THE FIRST SPECTROPOLARIMETRIC MONITORING OF THE PECULIAR O4 Ief SUPERGIANT ζ PUPPIS

“…The feasibility of longitudinal magnetic field measurements in massive stars using low-resolution spectropolarimetric observations was demonstrated by previous studies of O-and B-type stars (e.g., Hubrig et al 2006Hubrig et al , 2009Hubrig et al , 2008Hubrig et al , 2011cHubrig et al , 2013Hubrig et al , 2015b. A number of discrepancies in the published measurement accuracies using FORS 1 data were reported by Bagnulo et al (2012), who used the ESO FORS 1 pipeline to reduce the FORS 1 archive.…”

“…Recent detections of strong magnetic fields in very fast rotating early B-type stars indicate that the spindown timescale via magnetic braking can be much longer than the estimated age of these targets (e.g., Rivinius et al 2013). Furthermore, current studies of their kinematical status identified a number of magnetic O and Of?p stars as candidate runaway stars (e.g., Hubrig et al 2011c). Increasing the known number of magnetic objects with extreme rotation, which are probably products of a past binary interaction, is important to understanding the magnetic field origin in massive stars.…”

Photometric identification of the periods of the first candidate extragalactic magnetic massive stars

“…At the same time, new Of?p stars were identified in the Galaxy, with very similar characteristics (Walborn et al 2010): NGC 1624-2 (period of 158 d, Wade et al 2012b) and CPD−28 • 2561 (Hubrig et al 2011(Hubrig et al , 2013period of 73 d, Wade et al 2015). It must be stressed that, while about ten O stars are now known to be magnetic, the only actual class of magnetic massive stars is the Of?p category.…”

“…In this context, the recurrence timescale is the rotational period, and long rotation periods may then be explained by magnetic braking over the lifetime of the star (ud-Doula et al 2009). This implied that all Of?p stars are strongly magnetic, and HD 108 and HD 148937 were indeed soon found to be so (Hubrig et al 2008(Hubrig et al , 2011(Hubrig et al , 2013Martins et al 2010;Wade et al 2012a).…”

A search for the presence of magnetic fields in the two supergiant fast X-ray transients, IGR J08408−4503 and IGR J11215−5952

“…For best visibility of the Zeeman features, we overplot the Stokes V spectra of IGR J11215−5952 with the Stokes V spectra of the two well-known magnetic early B-type stars HD 96446 and ξ 1 CMa. Regarding the significance of the magnetic field detections in massive stars at significance levels around 3σ, we note that the two clearly magnetic Of?p stars HD 148937 and CPD −28 • 5104 have been for the first time detected as magnetic in our FORS 2 observations at significance levels of 3.1σ and 3.2σ, respectively (Hubrig et al 2008(Hubrig et al , 2011. The detection of the magnetic field in IGR J11215−5952 at significance levels of 3.2σ and 3.8σ indicates that this target likely possesses a kG magnetic field.…”