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

Abstract

Context. The importance of the magnetic field in high-mass-star formation is not yet fully clear and there are still many open questions concerning its role in the accretion processes and generation of jets and outflows. In the past few years, masers have been successfully used to probe the magnetic field morphology and strength at scales of a few au around massive protostars, by measuring linear polarisation angles and Zeeman splitting. The massive protostar IRAS 18089-1732 is a well studied high-mass-star forming region, showing a hot core chemistry and a disc-outflow system. Previous SMA observations of polarised dust revealed an ordered magnetic field oriented around the disc of IRAS 18089-1732. Aims. We want to determine the magnetic field in the dense region probed by 6.7 GHz methanol maser observations and compare it with observations in dust continuum polarisation, to investigate how the magnetic field in the compact maser region relates to the large-scale field around massive protostars. Methods. We reduced MERLIN observations at 6.7 GHz of IRAS 18089-1732 and we analysed the polarised emission by methanol masers. Results. Our MERLIN observations show that the magnetic field in the 6.7 GHz methanol maser region is consistent with the magnetic field constrained by the SMA dust polarisation observations. A tentative detection of circularly polarised line emission is also presented. Conclusions. We found that the magnetic field in the maser region has the same orientation as in the disk. Thus the large-scale field component, even at the au scale of the masers, dominates over any small-scale field fluctuations. We obtained, from the circular polarisation tentative detection, a field strength along the line of sight of 5.5 mG which appeared to be consistent with the previous estimates.

Characterization of methanol as a magnetic field tracer in star-forming regions

“…Such reversals have usually been interpreted as a change in field direction. However, reversals on au-scales would be surprising if one considers the agreement between the fields probed by methanol masers and dust emission [25]. A more plausible explanation favored by our results is that in the masers with opposite signs of polarization, the masing process itself is due to the dominance of different hyperfine transitions.…”

Disk Wind Feedback from High-mass Protostars

“…We find magnetic field strengths at the base of the outflow approaching 100 mG, in approximate agreement with the findings from Cep A, and the high density and near disk region of IRAS 18089. Surcis et al (2009) andDall'Olio et al (2017) also found that the small scale field probed by the masers are consistent with large scale fields traced by dust.…”

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

“…From an observational point of view, the presence of molecular outflows in massive star-forming regions (SFRs) is nowadays a fact (e.g., Tan et al 2014 and references therein). Also, measuring the morphology of the magnetic field around massive YSOs is now regularly done by using polarized dust emission (on a scale of 10 3 au; e.g., Zhang et al 2014a;Girart et al 2016) and polarized maser emission (on a scale of tens of au; e.g., Vlemmings et al 2010;Surcis et al 2012Sanna et al 2017;Dall'Olio et al 2017). However, the findings at the two different scales conflict.…”