Systematic detection of Zeeman splitting of 6.7 GHz class II methanol masers by long-term monitoring observations using Hitachi 32 m radio telescope

Abstract Magnetic fields in massive star-forming regions (MSFRs) play crucial roles, such as supporting massive molecular cloud cores and/or driving strong outflows and jets. However, the strength of the magnetic fields in MSFRs remains largely uncertain because high-sensitivity observations are required for their detection. The Zeeman effect of the 6.7 GHz class II methanol maser is one of the most appropriate tools for detecting magnetic fields in the upper layer of the circumstellar disk in MSFRs. In this study, we averaged each circularly polarized spectral data obtained by high-cadence simultaneous dual circular polarization monitoring observations of the 6.7 GHz methanol maser at the Hitachi 32 m radio telescope over 5 years (2017–2022). The typical $1\sigma$ rms noise of the averaged spectra was ${\sim }20\,\,{\rm mJy}$. We attempted to detect Zeeman splitting in 103 sources by applying a method called the Cross–Correlation method. Consequently, we detected Zeeman splitting in 62 sources, 38 of which were newly identified in this study. The average and median values of the strength of the line-of-sight components of the magnetic fields detected in this study were $1.8\times 10^2$ and $1.3\times 10^2$ mG, respectively. These results are consistent with those of previous studies by Vlemmings (2008, A&A, 484, 773) and Vlemmings, Torres, and Dodson (2011, A&A, 529, A95). For 17 sources in which magnetic fields were detected in multiple velocity components, different signs and strengths of the magnetic fields were derived for each source. The cause of these differences can be explained by the helical magnetic field.