Exploring the origin of magnetic fields in massive stars

Abstract

Context. Theories on the origin of magnetic fields in massive stars remain poorly developed, because the properties of their magnetic field as function of stellar parameters could not yet be investigated. Additional observations are of utmost importance to constrain the conditions that are conducive to magnetic fields and to determine first trends about their occurrence rate and field strength distribution. Aims. To investigate 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 67 new spectropolarimetric observations for 30 massive stars. Among the observed sample, roughly one third 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 four different nights using the low-resolution spectropolarimetric mode of FOcal Reducer low dispersion Spectrograph (FORS 2) mounted on the 8-m Antu telescope of the VLT. Furthermore, we present a number of follow-up observations carried out with the high-resolution spectropolarimeters SOFIN mounted at the Nordic Optical Telescope (NOT) and HARPS mounted at the ESO 3.6 m between 2008 and 2011. To assess the membership in open clusters and associations, we used astrometric catalogues with the highest quality kinematic and photometric data currently available. Results. The presence of a magnetic field is confirmed in nine stars previously observed with FORS

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

“…Later, additional nine FORS 2 spectropolarimetric observations of ζ Oph were obtained over four consecutive nights. The analysis of the FORS 2 observations showed the presence of a weak magnetic field with a reversal of polarity (Hubrig et al 2013) and an amplitude of about 100 G. The resulting periodogram for the magnetic field measurements using all available lines showed a dominating peak corresponding to a period of about 1.3 day, which is roughly double the period of 0.643 day determined by Pollmann (2012), who studied the variation of the equivalent width of the He I 6678 line. David-Uraz et al (2014) used ESPaDOnS and NARVAL observations of ζ Oph in 2011/12 with much larger uncertainties-typically around 350 G and between 100 and 800 G-and concluded that the upper limit of the dipole field strength is about 224 G and the maximum possible rotation period of 1.1 day.…”

On the use of the Fourier transform to determine the projected rotational velocity of line-profile variable B stars

“…The main difference between these two stars is that HD 181558 has one dominant g-mode while HD 74195 has several g-modes of similar amplitude. Also, while HD 181558 has no magnetic field detected (Aerts et al 2014), a weak field seems to be present in HD 74195 (Bagnulo et al 2012;Hubrig et al 2013), although it is not accompanied by a detectable signal of rotational modulation.…”

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.…”