Polarization properties of OH masers in AGB and post-AGB stars

Abstract

Context. Ground-state OH maser emission from late-type stars is usually polarized and remains a powerful probe of the magnetic field structure in the outer regions of circumstellar envelopes if observed with high angular and spectral resolutions. Observations in all four Stokes parameters are quite sparse and this is the most thorough, systematic study published to date. Aims. We aim to determine polarization properties of OH masers in an extensive sample of stars that show copious mass loss and search for candidate objects that are well-suited for high angular resolution studies. Methods. Full-polarization observations of the OH 1612 and 1667 MHz maser transitions were carried out for a sample of 117 AGB and post-AGB stars. Several targets were also observed in the 1665 MHz line. Results. A complete set of full-polarization spectra together with the basic polarization parameters are presented. Polarized features occur in more than 75% of the sources in the complete sample and there is no intrinsic difference in the occurrence of polarized emission between the three classes of objects of different infrared characteristics. The highest fractional polarization occurs for the post-AGB+PN and the Mira+SR classes at 1612 and 1667 MHz, respectively. Differences in the fractional polarization between the sources at different evolutionary stages appear to be related to depolarization caused by blending. The alignment of the polarization angles at the extreme sides of the shell implies a regular structure of the magnetic field of a strength of 0.3-2.3 mG. Conclusions. Polarized OH maser features are widespread in AGB and post-AGB stars. The relationship between the circular and linear fractional polarizations for a representative sample are consistent with the standard models of polarization for the Zeeman splitting higher than the Doppler line width, whereas the polarized features are the σ components.

Maser polarization and magnetic fields

“…Recent polarization observations of CSE 1.6 GHz OH masers confirm a regular structure and few milliGauss magnetic field strength in AGB OH maser envelopes (Wolak et al 2012) as found in previous observations. Recent years have also seen an increase in 22 GHz H 2 O and 43 GHz SiO maser observations of Mira variables, OH/IR stars and supergiants.…”

Magnetic fields around evolved stars: further observations of H2O maser polarization

“…However, we emphasize that the 1665/7 MHz OH maser emission originates in inner regions when compared to the 1612 MHz OH maser transition. Therefore, it is expected that magnetic field strength measurements based on the first line to be stronger than the second (Wolak et al 2012). (Vlemmings et al 2002(Vlemmings et al , 2005Herpin et al 2006;Rudnitski et al 2010).…”

Linear polarization of hydroxyl masers in circumstellar envelope outer regions

“…Improvements of spectral resolution up to 0.1 km s −1 enabled detection circularly polarized features in several objects (Zell & Fix 1991), whereas linear polarization was usually weak, if any (Olnon et al 1980). Our recently published polarimetric survey of AGB and post-AGB stars revealed widespread occurrence of polarized OH maser emission (Wolak et al 2012). More than 75% of the objects in the complete sample have polarized features.…”

“…The linearly polarized flux density p = (Q 2 + U 2 ) 0.5 , fractional linear polarization, m L = p/I, and polarization position angle, χ = 0.5 tan −1 (U/Q), were derived from the Stokes parameters. The properties of the star sample are given in Wolak et al (2012). In the following we limit the discussion to the sources that have both extreme peaks at 1612 MHz brighter than 10σ, i.e.…”