A Curious Case of Circular Polarization in the 25 GHz Methanol Maser
  Line toward OMC-1
  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 $zB_\text{los} = 149 \pm
19~$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.
