The Zeeman Effect in the 44 GHz Class I Methanol Maser Line toward DR21(OH)

TL;DR

This study reports the first detection of the Zeeman effect in the 44 GHz Class I methanol maser line in DR21(OH), providing insights into magnetic field strengths in star-forming regions.

Contribution

It presents the first empirical measurement of Zeeman splitting in the 44 GHz methanol maser line, estimating the Zeeman splitting factor without laboratory data.

Findings

Detected Zeeman effect with 20-sigma significance in the 44 GHz methanol maser

Estimated magnetic field strength of ~10 mG in DR21(OH)

Inferred Zeeman splitting factor z ~ 5 Hz mG^{-1}

Abstract

We report the detection of the Zeeman effect in the 44 GHz Class I methanol maser line toward the star forming region DR21(OH). In a 219 Jy/beam maser centered at an LSR velocity of 0.83 km s$^{-1}$, we find a 20-$\sigma$ detection of $zB_{\text{los}} = 53.5 \pm 2.7$ Hz. If 44 GHz methanol masers are excited at $n \sim 10^{7-8}$ cm$^{-3}$, then the $B~vs.~n^{1/2}$ relation would imply from comparison with Zeeman effect detections in the CN($1-0$) line toward DR21(OH) that magnetic fields traced by 44 GHz methanol masers in DR21(OH) should be $\sim$10 mG. Together with our detected $zB_{\text{los}} = 53.5$ Hz, this would imply that the value of the 44 GHz methanol Zeeman splitting factor $z$ is $\sim$5 Hz mG$^{-1}$. Such small values of $z$ would not be a surprise, as the methanol molecule is non paramagnetic, like H$_2$O. Empirical attempts to determine $z$, as demonstrated, are important because currently there are no laboratory measurements or theoretically calculated values of $z$ for the 44 GHz methanol transition. Data from observations of a larger number of sources are needed to make such empirical determinations robust.