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

Abstract

Context. Although the role of magnetic fields in launching molecular outflows in massive young stellar objects has been convincingly demonstrated by theoretical arguments, observationally, the alignment of the magnetic field lines with the molecular outflows is still under debate. Aims. We aim to complete the measurements of the direction of the magnetic fields at milliarcsecond resolution around a sample of massive star-forming regions to determine whether the magnetic field and outflows are aligned. Methods. In 2012, we started a large very long baseline interferometry campaign with the European VLBI Network to measure the magnetic field orientation and strength toward a sample of 31 massive star-forming regions (called the flux-limited sample) by analyzing the polarized emission of 6.7 GHz CH3OH masers. In the previous papers of the series, we have presented 80% of the sample. Here, we report the linearly and circularly polarized emission of 6.7 GHz CH3OH masers toward the last five massive star-forming regions of the flux-limited sample. The sources are G30.70-0.07, G30.76-0.05, G31.28+0.06, G32.03+0.06, and G69.52-0.97. Results. We detected a total of 209 CH3OH maser cloudlets, 15% of which show linearly polarized emission (0.07–16.7%), and 2% of which show circularly polarized emission (0.2–4.2%). As reported in previous papers, in the last five sources of the flux-limited sample, we also measured well-ordered linear polarization vectors. Zeeman splitting was measured toward G30.70-0.07, G32.03+0.06, and G69.52-0.97. Conclusions. The statistical analysis of the entire flux-limited sample shows that the observations are consistent with a bimodal distribution in the difference between the 3D magnetic field direction and the outflow axis, with half the magnetic field directions being perpendicular and the other half being parallel to the outflow. In addition, we determined that typical values of the linear and circular polarization fractions for 6.7 GHz CH3OH masers are Pl = 1.0–2.5% and PV = 0.5–0.75%, respectively. From the circularly polarized spectra of the CH3OH maser features, we found that a typical Zeeman splitting is in the range between 0.5 m s−1 and 2.0 m s−1. This would correspond to 9 mG < |B||| < 40 mG if F = 3 → 4 is the most favored of the eight hyperfine transitions that might contribute to the maser emission.

6.7 GHz CH$_3$OH masers polarization in massive star-forming regions: the Flux-Limited Sample

“…We detected linearly polarized emission of 6.7 GHz CH 3 OH maser toward all the sources of the Flux-Limited sample but one, for a total of 233 CH 3 OH maser features, and circularly polarized emission toward 14 sources (33 maser features). We reported the detailed results of each source in a number of papers [1,[13][14][15][16][17][18][19]. To enlarge further the sample we also included in it all the massive YSOs of the southern hemisphere (declination < -9 • ) that were observed with an interferometer and that showed polarized 6.7 GHz CH 3 OH maser emission.…”

“…Therefore, they concluded that their data were consistent with a random distribution. To provide a statistical study with a higher number of sources and with a better spatial resolution (scales < 100 au), since 2008, we started to observe with the European VLBI Network1 (EVN) the polarized emission of 6.7 GHz CH 3 OH masers from a number of massive star-forming regions (SFRs) that in 2011 become part of the Flux-Limited sample [1,[13][14][15][16][17][18][19]. From these observations, it is possible to determine both the orientation and the strength of the magnetic field close to the massive YSOs.…”

“…Here we present a summary of the final statistical analysis of the Flux-Limited sample, that is briefly introduced in the next section; a more detailed description can be found in [1].…”

“…More than 10 years ago we started a large EVN campaign to measure the magnetic field orientation and strength toward a sample of 30 massive star-forming regions, called the "Flux-Limited sample", by observing the polarized emission of 6.7 GHz CH 3 OH masers. Here, we present a summary of the final statistics of the Flux-Limited sample, extensively reported in [1], which are focused on the relative orientation of the outflows with the magnetic fields and on the polarized characteristics of 6.7 GHz CH 3 OH masers.…”

Stellar Evolution Studies Using Masers

“…The size of the maser cloud(let)s has been measured to be ∼10 AU and they occur within 1000 AU of the HMYSOs. The EVN has been critical to their study e.g., [18][19][20][21][22][23] (Figure 1). 6.7 GHz methanol masers occur in a range of morphologies, but an interesting morphological sub-set is the ring-type targets.…”