The polarization of mm methanol masers

Abstract

We present a survey of the polarization properties of mm-wavelength methanol masers, comprising both classes, and transitions from 84.5 to 157.0 GHz. Linear polarization is found in more than half of the observed objects, and circular polarization is tentatively detected in two sources. Class I and Class II CH_3OH masers show similar polarization properties.The largest linear polarization is found in the 133 GHz Class I maser towards L379 (39.5 %), and in the 157 GHz Class II maser towards G9.62+0.19 (36.7 %). The spectral profiles of the polarization angle of Class I masers are mostly flat, except for two sources showing a linear slope. Since the mm-line methanol masers are expected to be weakly (or not) saturated, we suggest that the stronger fractional polarizations found by us are enhanced by anisotropic pumping and radiative losses. In NGC 7538, we find, for both maser classes, a good agreement between our polarization angles, and those measured for the submillimeter dust continuum. This can be taken as evidence for magnetic alignment of dust grains. It is also possible that an unsaturated maser with equally populated magnetic substates simply amplifies polarized continuum seed radiation. For Class II masers, the polarization properties of the various velocity components towards a given source with detectable polarization are quite homogeneous. A possible explanation is discussed. Since methanol is non-paramagnetic, the circular polarization of the unsaturated maser emission can only be due to variations of the angle between the magnetic field and the line of sight along the maser propagation path.

arXiv commentAstronomy & Astrophysics, in press, received 27 February 2004, accepted 27 July 2004

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• A Non-Zeeman Interpretation for Polarized Maser Radiation and the Magnetic Field at the Atmospheres of Late-Type Giants
• The Relationship Between the Circular Polarization and the Magnetic Field for Astrophysical Masers with Weak Zeeman Splitting
• Submillimeter Imaging Polarimetry of the NGC 7538 Region
• The Magnetic Field Structure in W51A
• Detection of a new methanol maser line with the Kitt Peak 12-m telescope by remote observing from Moscow
• Molecular Hydrogen Outflows in W51

Cited by in corpus (13)

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• The Relationship between Class I and Class II Methanol Masers
• Review of Zeeman Effect Observations of Regions of Star Formation
• XPOL - the correlation polarimeter at the IRAM 30m telescope
• Online Database of Class I Methanol Masers
• First Interferometric Images of the 36 GHz Methanol Masers in the DR21 Complex
• The detection of Class I methanol masers towards regions of low-mass star formation
• Linear Polarization of Class I Methanol Masers in Massive Star-Forming Regions
• A Review of Maser Polarization and Magnetic Fields
• The Zeeman Effect in the 44 GHz Class I Methanol Maser Line toward DR21(OH)
• NGC7538 IRS1 -- an O star driving an ionized jet and giant N-S outflow
• A Catalog of CH_3OH 7_0-6_1 A^+ Maser Sources in Massive Star-Forming Regions. II. Masers in NGC 6334F, G8.67-0.36, and M17