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

TL;DR

This paper reports the first detection of the Zeeman effect in the 25 GHz Class I CH$_3$OH maser, revealing strong magnetic fields in OMC-1 that influence shock dynamics and star formation processes.

Contribution

It presents the first observation of Zeeman splitting in the 25 GHz CH$_3$OH maser, indicating significant magnetic fields in high-mass star forming regions.

Findings

Detected circular polarization in the 25 GHz CH$_3$OH maser in OMC-1.

Estimated magnetic field strength of 171-214 mG from Zeeman splitting.

Magnetic energy dominates over kinetic energy in post-shocked regions.

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 $zB_\text{los} = 149 \pm 19~$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.