Stellar Physics and Stellar Activity
Magnetic fields in massive stars
S. Hubrig, I. Ilyin (AIP), M. Schöller (ESO), N. Kharchenko (MAO,
Ukraine), M. Briquet (Universiteit Leuven), et al.
Very little is known about the existence, origin, and role of magnetic fields in massive OB stars.
During our recent observing run in visitor mode with FORS 2 at the VLT, we were able to detect
among the studied 36 massive stars in clusters and in the field five stars with mean longitudinal
magnetic fields in the range from 150 to 400 Gauss. Further, an additional spectropolarimetric study
of a sample of massive O-type stars and pulsating β Cephei stars was carried out using the SOFIN
echelle spectrograph at the 2.56 m Nordic Optical Telescope . The sample consisted of massive
stars already detected as magnetic in the course of our previous low-resolution spectropolarimetric
observations with FORS 2 and a few O-type stars with magnetic field detections reported in the
literature. The measurements of ξ1 CMa, separated by one year, =+386±39G and
=+297±26G, confirmed our previous detection of a slightly variable magnetic field in this star
(Hubrig et al. 2006). Below we present SOFIN I and V spectra of this stars in the spectral region
around the Si III (Mult. 2) lines.
Another β Cephei star, V1449 Aql, previously reported by
us as magnetic (Hubrig et al. 2009), was found to show the
strongest mean longitudinal magnetic field among the
magnetic β Cephei stars, of the order of -800G. The
detection of such a strong magnetic field in this massive
star is of special interest due to recently discovered solar-
like pulsations using CoRoT observations (Belkacem et al.
2009, Science).
Combining all our previous spectropolarimetric observations for a few β Cephei stars, we determined
their magnetic field properties, such as strength, magnetic field geometry, and time variability. In the
figures below we present magnetic field variations for two β Cephei stars, ξ1 CMa (left) with a period
of 2.2 days and 15 CMa (right) with the period of 12.6 days. We used a frequency analysis technique
to compute best-fit curves to the magnetic field variations. Apart from β Cephei itself, these are the
first magnetic field measurements for other massive pulsating stars with well-defined variations of
their magnetic fields. Such magnetic studies are important for the understanding of the impact of
magnetic fields on rotation, pulsation, and diffusion.
The carried out new magnetic field measurements also confirm the presence of longitudinal magnetic
fields in the massive O-type stars HD 36879, 15 Mon, and 9 Sgr, and in two Of?p stars, HD 191612
and HD 108, previously studied by Donati’s team (Donati et al. 2006, Martins et al. 2010).
Surprisingly, our recent calculations of space velocities of O-type stars with detected magnetic fields
imply that most magnetic massive stars can be considered as candidate runaway stars. This new
kinematical study indicates that it is possible that magnetic fields of massive stars were acquired
during the ejection from their parent clusters and protoclusters.