The structure of the magnetic field in the massive star-forming region W75N

Abstract

Context. A debated topic in star formation theory is the role of magnetic fields during the protostellar phase of high-mass stars. It is still unclear how magnetic fields influence the formation and dynamics of massive disks and outflows. Most current information on magnetic fields close to high-mass protostars comes from polarized maser emissions, which allows us to investigate the magnetic field on small scales by using very long-baseline interferometry. Aims. The massive star-forming region W75N contains three radio continuum sources (VLA 1, VLA 2, and VLA 3), at three different evolutionary stages, and associated masers, while a large-scale molecular bipolar outflow is also present. Very recently, polarization observations of the 6.7 GHz methanol masers at milliarsecond resolution have been able to probe the strength and structure of the magnetic field over more than 2000 AU around VLA 1. The magnetic field is parallel to the outflow, suggesting that VLA 1 is its powering source. The observations of H 2 O masers at 22 GHz can give more information about the gas dynamics and the magnetic fields around VLA 1 and VLA 2. Methods. The NRAO Very Long Baseline Array was used to measure the linear polarization and the Zeeman-splitting of the 22 GHz water masers in the star-forming region W75N. Results. We detected 124 water masers, 36 around VLA 1 and 88 around VLA 2 of W75N, which indicate two different physical environments around the two sources, where VLA 1 is in a more evolved state. The linear polarization of the masers confirms the tightly ordered magnetic field around VLA 1, which is aligned with the large-scale molecular outflow, and also reveals an ordered magnetic field around VLA 2, which is not parallel to the outflow. The Zeeman-splitting measured on 20 of the masers indicates strong magnetic fields around both sources (the averaged values are |B VLA1 | ∼ 700 mG and |B VLA2 | ∼ 1700 mG). The high values of the magnetic field strengths, which come from the shock compression of the gas, are consistent with the methanol and OH magnetic field strengths. Moreover, by studying the maser properties we were also able to determine that the water masers are pumped in C-shocks in both sources.

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

“…Because the massive star-forming regions investigated so far are at a distance of a few kpc, Φ f is estimated to be within the errors of the linear polarization angles of the CH 3 OH maser emission (Surcis et al 2009(Surcis et al , 2011b(Surcis et al , 2012. This The red full line is the limit of emerging brightness temperature above which the CH 3 OH masers are considered saturated (T b ΔΩ > 2.6 × 10 9 K sr; Surcis et al 2011a), and the dotted line gives the lower limit to the linear polarization fraction for saturated masers (P l ≈ 4.5%, Surcis et al 2012).…”

“…The CH 3 OH maser features were identified by following the same procedure described in Surcis et al (2011a). We made use of the adapted full radiative transfer method (FRTM) code for 6.7-GHz CH 3 OH masers (Vlemmings et al 2010;Surcis et al 2011b; to model the total intensity and the linearly polarized spectrum of every maser feature for which we were able to detect linearly polarized emission.…”

“…Nowadays, although using the same observing setup, the EVN SFXC at JIVE enables us to correlate the spectral data with a larger number of channels than previously possible on the JIVE hardware correlator (Schilizzi et al 2001), providing a better spectral resolution (i.e., ∼0.05 km s −1 ). Because of this higher spectral resolution we can now determine the ΔV Z by using the adapted FRTM code for 6.7-GHz CH 3 OH maser, as was successfully done for H 2 O masers by Surcis et al (2011a). The best values for T b ΔΩ and ΔV i are included in the code to produce the I and V models that were used for fitting the total intensity and circular polarized spectra of the CH 3 OH masers (Fig.…”

“…Over the last years, the high importance of using masers as probes of magnetic fields on the smallest scales (10-100 AU) has been proven (e.g., Vlemmings et al 2006Vlemmings et al , 2010Surcis et al 2011aSurcis et al , 2011bSurcis et al , 2012. In particular, the 6.7-GHz CH 3 OH masers, which are among the most abundant maser species in massive star-forming regions, are playing a crucial role in determining the magnetic field morphology (e.g., Surcis et al 2009Surcis et al , 2011bSurcis et al , 2012.…”

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

“…The Faraday rotation has been suggested as an explanation for depolarization at longer wavelengths (Elitzur 2002). The effects of Faraday rotation on the linear polarization of the masers are well described by Fish & Reid (2006) and Surcis et al (2011). According to their explanations, internal Faraday rotation due to the free electrons in the path of maser amplification can decrease the linear polarization fraction of the radiation, sometimes completely circularizing it if the Faraday rotation is strong enough (Goldreich et al 1973).…”