Toruń Centre for Astronomy, Nicolaus Copernicus University,
2 GEPI, UMR 8111, Observatoire de Paris, 5 place J. Janssen, 92195 Meudon Cedex, France
3 Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, M13 9PL, UK
Received: 16 April 2010
Accepted: 6 August 2010
Context. OH masers from the envelopes of M-type supergiants show a significant degree of polarization, implying magnetic fields of a few mG. Nothing is known about the temporal characteristics of the magnetic fields or how such changes may affect stellar mass loss.
Aims. We therefore observed two supergiant stars in order to quantify the long-term polarization behaviour of the maser emission.
Methods. Full-polarization spectra at 1612 and 1667 MHz were obtained with the Nançay radio telescope at intervals over periods of 4 and 6 years for S Per and VX Sgr, respectively.
Results. Time series of OH maser full polarization spectra are presented. Semiregular variations of the integrated flux densities generally follow the visual light curves as expected for radiative pumping cycles. For both sources the variability indices of individual features are higher at 1667 MHz than at 1612 MHz and their extreme values occur for the blue-shifted emission. The degrees of polarization at 1667 MHz show diverse behaviours usually uncorrelated with the total flux, whereas those at 1612 MHz are commonly stable on time scales of 4 − 6 yr. Several outbursts of the 1667 MHz emission on time scales of 0.5 − 2 yr were found in both targets. The bursting features are highly polarized and show drifts in velocity. Small changes of the degrees of polarization and smooth variations of position angle of linear polarization during the bursts were observed in S Per but they are more dramatic in VX Sgr.
Conclusions. The OH outbursts do not seem to have any direct link to stellar events, but seem to be localized in the wind.
Key words: masers / polarization / circumstellar matter / supergiants / stars: individual: S Per / stars: individual: VX Sgr
Appendices A and B are only available in electronic form via http://www.edpsciences.org
A complete catalogue of the data in all four Stokes parameters is only available in electronic form at CDS via anonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or via http://cdsweb.u-strasbg.fr/viz-bin/qcat?J/A+A/524/A99
© ESO, 2010