A search for the favored hyperfine transition of a 6.7 GHz methanol maser line

Abstract

The polarized emission of astrophysical masers, especially OH and methanol lines, is an effective tool to study the magnetic field in high-mass star-forming regions. The magnetic field strength measurement via the Zeeman effect of OH maser emission is well established, but that of the methanol maser emission is still under debate because of its complex hyperfine structure. We aim to identify the dominating hyperfine transition of the Class II methanol maser emission by comparing the magnetic field strength measured with the excited OH maser emission and the Zeeman splitting of the methanol maser emission. We used quasi-simultaneous EVN observations of the two maser emissions at 6.035 GHz ex-OH and 6.668 GHz methanol toward two well-known high-mass young stellar objects: ON 1 and W75N. The observations were performed in full polarimetric mode and in phase-referencing mode to couple the maser features of the two maser emissions in each source. We detected linearly and circularly polarized emission in both maser transitions and HMYSOs. Specifically, we measured the magnetic field strength in twelve and five ex-OH maser features toward ON1 and W75N, respectively, and the Zeeman splitting of the methanol maser spectra in one and three maser features toward ON 1 and W75N, respectively. We determined that the two maser emissions likely probe the same magnetic field but at different densities. Indeed, a direct comparison of the magnetic field strength and the Zeeman splitting as measured with the ex-OH and methanol maser spots, respectively, provided values of the Zeeman splitting coefficient for the 6.7 GHz maser that do not match any of the table values present in the literature. We are not able to uniquely identify the dominating hyperfine transition; however, through density considerations, we can narrow the choice down to three hyperfine transitions: 3->4, 6->7A, and 7->8.