EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions

Abstract

Context. Theoretical simulations and observations at different angular resolutions have shown that magnetic fields have a central role in massive star formation. Like in low-mass star formation, the magnetic field in massive young stellar objects can either be oriented along the outflow axis or randomly. Aims. Measuring the magnetic field at milliarcsecond resolution (10-100 au) around a substantial number of massive young stellar objects permits determining with a high statistical significance whether the direction of the magnetic field is correlated with the orientation of the outflow axis or not. Methods. In late 2012, we started a large VLBI campaign with the European VLBI Network to measure the linearly and circularly polarized emission of 6.7 GHz CH 3 OH masers around a sample of massive star-forming regions. This paper focuses on the first seven observed sources, G24.78+0.08, G25.65+1.05, G29.86-0.04, G35.03+0.35, G37.43+1.51, G174.20-0.08, and G213.70-12.6. For all these sources, molecular outflows have been detected in the past. Results. We detected a total of 176 CH 3 OH masing cloudlets toward the seven massive star-forming regions, 19% of which show linearly polarized emission. The CH 3 OH masers around the massive young stellar object MM1 in G174.20-0.08 show neither linearly nor circularly polarized emission. The linear polarization vectors are well ordered in all the other massive young stellar objects. We measured significant Zeeman splitting toward both A1 and A2 in G24.78+0.08, and toward G29.86-0.04 and G213.70-12.6. Conclusions. By considering all the 19 massive young stellar objects reported in the literature for which both the orientation of the magnetic field at milliarcsecond resolution and the orientation of outflow axes are known, we find evidence that the magnetic field (on scales 10-100 au) is preferentially oriented along the outflow axes.

Methanol masers reveal the magnetic field of the high-mass protostar IRAS 18089-1732

“…4) and it is consistent with the strength of our measured polarisation fraction (e.g. Surcis et al 2015).…”

“…Previous works showed that the circular polarisation fraction in methanol masers is typically very weak (<1%; e.g Surcis et al 2015), so circular polarisation can be observed more easily in the brightest maser features. However, as shown by Vlemmings et al (2001Vlemmings et al ( , 2002, an increase of the noise, a narrowing and re-broadening, or a change in shape of the maser line can also occur in hyperfine interactions or in 3D maser propagation effect.…”

“…However, as shown by Vlemmings et al (2001Vlemmings et al ( , 2002, an increase of the noise, a narrowing and re-broadening, or a change in shape of the maser line can also occur in hyperfine interactions or in 3D maser propagation effect. Therefore, as suggested by Surcis et al (2015), a detection of circular polarisation should only be considered real if it presents a V peak flux density at least five times higher than the RMS.…”

“…The first full polarisation observations of 6.7 GHz methanol masers were made by Ellingsen (2002). Thereafter, Green et al (2007) Vlemmings et al (2006); Vlemmings (2008) ;Vlemmings et al (2010), Dodson & Moriarty (2012) and Surcis et al (2012Surcis et al ( , 2014bSurcis et al ( , 2015 have demonstrated that maser emission (from e.g. methanol and water) allows us to probe magnetic fields.…”

VLA Survey of Dense Gas in Extended Green Objects: Prevalence of 25 GHz Methanol Masers

“…Cyganowski et al (2011a) resolved five distinct sources at 3.6 cm (denoted CM1,..., CM5). The two strongest (CM1 and CM2) were also detected by these authors at 1.3 cm, and CM2 is associated with 6.7 GHz CH 3 OH masers (Cyganowski et al 2009(Cyganowski et al , 2011aSurcis et al 2015). The morphology of the 1.3 cm continuum emission in our VLA image is consistent with multiple, unresolved centimeter sources, and is spatially coincident with CM1, CM2, CM4, and CM5 from Cyganowski et al (2011a).…”