S. Ellingsen (University of Tasmania)

The strongest class II methanol maser emission is produced by the 6.7 GHz transition, which was discovered in 1991. These methanol masers are only found in high-mass star-formation regions and are often closely associated with main-line OH masers. From the first high resolution observations of class II methanol masers many sources have been found to show a simple spatial and velocity distribution, which contrasts with the complex morphology usually observed in OH masers. Some of the methanol masers have a linear spatial distribution and monotonic velocity gradient, consistent with the maser arising in an edge-on rotating disk (Figure 1). However, to date, this hypothesis has not been conclusively demonstrated for any source. If some methanol masers do originate in disks then they would represent a unique opportunity to study accretion in high-mass star formation at unprecedented resolution.

One aspect of methanol masers that has not previously been investigated in detail is their polarization properties. Polarized emission from masers occurs when the orientation of the molecules producing the maser radiation is not random. Radio-frequency molecular lines are produced between rotational quantum levels and these only arise in molecules that have a dipole moment. Thus magnetic fields are able to align masing molecules, and the degree to which this occurs depends upon the strength of the field and the rate at which collisions and radiation interactions disrupt the alignment process. Methanol is a diamagnetic molecule and the separation of the Zeeman components produced by magnetic fields of 1-10 milligauss (typical of star-formation regions) is much less than the width of the maser line. In these circumstances modest linear polarization can occur, but any circular polarization is expected to be very weak.

The exact role that magnetic fields play in the star-formation process is still a matter of heated debate, and as the polarization properties of masers depend critically upon the magnetic field they are potentially useful probes. In addition, the magnetic field associated with a circumstellar accretion disk is expected to be well ordered, and so if the methanol masers do originate in a disk then the polarization should show regular structure. If we naively assume that the magnetic field will be oriented perpendicular to the disk (from an edge-on perspective) then the masers are propagating perpendicular to the magnetic field (Figure 2). In this situation the position angle of the linear polarization will also be perpendicular to the magnetic field (i.e. parallel to the position angle of the disk). This ignores the effects of Faraday rotation, which will not mask the presence of regular structure, but may shift the position angle of the polarization vectors.

The Australia Telescope Compact Array has been used to make the first full polarization observations of 6.7-GHz methanol masers. Four strong sources were observed and linear polarization at levels between a few and 10% were detected towards all sources. None of the sources show circular polarization stronger than 1.5%. The majority of spectral features show linear polarization at levels of 2-3%, with a small number of (usually) strong features exhibiting levels of linear polarization of up to 10%.

G339.88-1.26 is one of the best methanol maser disk candidates. The maser emission is coincident with a weak ultra-compact HII region and there is some evidence for an outflow perpendicular to the line traced by the masers (Figure 1). The greatest degree of linear polarization in this source is 10%, which is exhibited by one of the weaker maser features (Figure 3). The position angle shows smooth variation across the velocity range decreasing from near 90 degrees at the lowest velocities to approximately 50 degrees at the highest velocities. For a number of features the position angle changes significantly across the line, perhaps indicating some spectral blending. Polarization VLBI observations which resolve the individual maser spots are required to determine if this is the case. This result is not consistent with a simple model of the maser emission arising in an edge-on disk, as that should produce polarization vectors with a small range of position angles, even if they are not parallel to the disk due to Faraday rotation.

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