Linear Polarization of Class I Methanol Masers in Massive Star-Forming Regions

Abstract

Class I methanol masers are found to be good tracers of the interaction between outflows from massive young stellar objects with their surrounding media. Although polarization observations of Class II methanol masers have been able to provide information about magnetic fields close to the central (proto)stars, polarization observations of Class I methanol masers are rare, especially at 44 and 95 GHz. We present the results of linear polarization observations of 39 Class I methanol maser sources at 44 and 95 GHz. These two lines are observed simultaneously with one of the 21 m Korean VLBI Network telescopes in single-dish mode. Approximately 60% of the observed sources have fractional polarizations of a few percent in at least one transition. This is the first reported detection of linear polarization of the 44 GHz methanol maser. The two maser transitions show similar polarization properties, indicating that they trace similar magnetic environments, although the fraction of the linear polarization is slightly higher at 95 GHz. We discuss the association between the directions of polarization angles and outflows. We also discuss some targets having different polarization properties at both lines, including DR21(OH) and G82.58+0.20, which show the 90°polarization angle flip at 44 GHz.

The Zeeman Effect in the 44 GHz Class I Methanol (CH3OH) Maser Line toward DR21W

“…Wiebe & Watson (1998) found that in masers with strong linear polarization, changes in the orientation of the magnetic field along the line of sight could cause rotation of the linear polarization vector to produce circular polarization. Observing with the Korean VLBI network telescopes in single dish mode, Kang et al (2016) measured (2.0±0.2) % linear polarization in the 44 GHz CH 3 OH masers toward DR21W. Given this low value, it is unlikely that linear polarization is causing the observed Stokes V in our observations.…”

Review of Zeeman Effect Observations of Regions of Star Formation

“…They argue that the masers are unsaturated and that the theory of Nedoluha and Watson (1992) applies; specifically that the linear polarization arises from anisotropic pumping and anisotropic radiation losses and the circular polarization from non-Zeeman effects (Watson, 2009). Kang et al (2016) present a survey of linear polarization in 44 GHz and 95 GHz Class I masers conducted with a single dish from the Korean VLBI Network (KVN). Their results yielded a lower fractional linear polarization and they argue accordingly for a diminished influence of anisotropic pumping and losses than Wiesemeyer et al (2004).…”

“…There remain broad open questions in both the micophysics and macrophysics of star formation theory (McKee and Ostriker, 2007). Maser polarization observations in SFR probe the fine-scale magnetic field in dense regions n ∼ 10 5−11 cm −3 of obscured HMSFR (Kang et al, 2016) and therefore provide constraints on the theory of high-mass as opposed to low-mass star formation (Zinnecker and Yorke, 2007). In addition, masers are confined to certain evolutionary phases of HMSF, specifically between the formation of hot dense molecular cores (HDMC) (Zinnecker and Yorke, 2007), equivalently highmass protostellar objects (HMPO) (McKee and Ostriker, 2007), and their subsequent evolution into ultra-compact HII regions (UCHII) (Churchwell, 2002).…”

Linear polarisation of Class I methanol masers in massive star formation regions

“…In this paper, we report on single-dish measurements of linear polarisation toward 44 and 95 GHz methanol maser sources. The full results are published in Kang et al (2016). We also report on the preliminary results of follow-up VLBI observations, especially on the G10.34-0.14.…”