A Curious Case of Circular Polarization in the 25 GHz Methanol Maser Line toward OMC-1

Abstract

We report the detection of a circular polarization signature in the Stokes $V$ profile of a 25 GHz Class I CH${}_3$OH maser toward the high mass star forming region OMC-1. Such a feature usually constitutes a detection of the Zeeman effect. If due to a magnetic field in OMC-1, this would represent the first detection and discovery of the Zeeman effect in the 25 GHz Class I CH${}_3$OH maser. The feature in Stokes $V$ is detected in two observations with different angular resolutions taken eight years apart with the Very Large Array (VLA); for our 2009 D-configuration observations, the fitted value for $z{B}_{\text{los}}$ is $152\pm 12$ Hz, where $z$ is the Zeeman splitting factor and $B_{\text{los}}$ is the line-of-sight magnetic field. For our 2017 C-configuration observations, the fitted value for $z{B}_{\text{los}}=149\pm 19$Hz, likely for the same maser spot. These correspond to $B_{\text{los}}$ in the range 171-214 mG, depending on which hyperfine transition is responsible for the maser line. While these $B_{\text{los}}$ values are high, they are not implausible. If the magnetic field increases in proportion to the molecular hydrogen density in shocked regions, then our detected fields predict values for the pre-shock magnetic field that are in agreement with observations. With $B_{\text{los}}$ =171-214 mG, the magnetic energy in the post-shocked regions where these 25 GHz Class I CH${}_3$OH masers occur would dominate over the kinetic energy density and be at least of the order of the pressure in the shock, implying that the magnetic field would exert significant influence over the dynamics of these regions.

11 pages, accepted for publication in The Astrophysical Journal

References in corpus (6)

• The Gould's Belt Distances Survey (GOBELINS) II. Distances and structure towards the Orion Molecular Clouds
• Magnetic Fields in Dark Cloud Cores: Arecibo OH Zeeman Observations
• The JCMT BISTRO Survey: The magnetic field strength in the Orion A filament
• The Zeeman Effect in the 44 GHz Class I Methanol Maser Line toward DR21(OH)
• Non-Zeeman Circular Polarization of Molecular Maser Spectral Lines
• The Zeeman Effect in the 44 GHz Class I Methanol (CH3OH) Maser Line Toward DR21W