Zeeman splitting of 6.7 GHz methanol masers

Abstract

Context. To properly determine the role of magnetic fields during massive star formation, a statistically significant sample of field measurements probing different densities and regions around massive protostars needs to be established. However, relating Zeeman splitting measurements to magnetic field strengths needs a carefully determined splitting coefficient. Aims. Polarization observations of, in particular, the very abundant 6.7 GHz methanol maser, indicate that these masers appear to be good probes of the large scale magnetic field around massive protostars at number densities up to n H 2 ≈ 10 9 cm −3 . We thus investigate the Zeeman splitting of the 6.7 GHz methanol maser transition. Methods. We have observed of a sample of 46 bright northern hemisphere maser sources with the Effelsberg 100-m telescope and an additional 34 bright southern masers with the Parkes 64-m telescope in an attempt to measure their Zeeman splitting. We also revisit the previous calculation of the methanol Zeeman splitting coefficients and show that these were severely overestimated making the determination of magnetic field strengths highly uncertain. Results. In total 44 of the northern masers were detected and significant splitting between the right-and left-circular polarization spectra is determined in >75% of the sources with a flux density >20 Jy beam −1 . Assuming the splitting is due to a magnetic field according to the regular Zeeman effect, the average detected Zeeman splitting corrected for field geometry is ∼0.6 m s −1 . Using an estimate of the 6.7 GHz A-type methanol maser Zeeman splitting coefficient based on old laboratory measurements of 25 GHz E-type methanol transitions this corresponds to a magnetic field of ∼120 mG in the methanol maser region. This is significantly higher than expected using the typically assumed relation between magnetic field and density (B ∝ n 0.47 H 2 ) and potentially indicates the extrapolation of the available laboratory measurements is invalid. The stability of the right-and left-circular calibration of the Parkes observations was insufficient to determine the Zeeman splitting of the Southern sample. Spectra are presented for all sources in both samples.Conclusions. There is no strong indication that the measured splitting between right-and left-circular polarization is due to nonZeeman effects, although this cannot be ruled out until the Zeeman coefficient is properly determined. However, although the 6.7 GHz methanol masers are still excellent magnetic field morphology probes through linear polarization observations, previous derivations of magnetic fields strength turn out to be highly uncertain. A solution to this problem will require new laboratory measurements of the methanol Landé-factors.

The Zeeman Effect in the 44 GHZ Class I Methanol Maser Line Toward Dr21(oh)

“…If future measurements of the Zeeman splitting factor for methanol masers suggest fields that are much larger than plausible, it would be important to consider studies of the applicability of the phenomenon discussed by Houde (2014) to Class I CH 3 OH masers. Finally, another potential issue described in Vlemmings et al (2011) is that a rotation of the axis of symmetry for the molecular quantum states could cause an intensity-dependent circular polarization that could be mistakenly attributed to a Zeeman splitting. This happens when the maser stimulated emission rate R becomes larger than the Zeeman frequency shift gΩ.…”

“…Extrapolation of the Landé g-factor from lab measurements of several CH 3 OH transitions near 25 GHz, made by Jen (1951), could certainly be used to make an estimate of z for the 44 GHz CH 3 OH maser transition. However, such an extrapolation usually gives a value for the magnetic field that is at least an order of magnitude larger than expected (see, e.g., Vlemmings et al 2011;Momjian & Sarma 2012). Therefore, we will leave our results in terms of zB los (in units of Hz).…”

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

“…We selected a subgroup of five massive star-forming regions among the northern hemisphere sources observed with the Effelsberg 100-m telescope (Vlemmings 2008;Vlemmings et al 2011). Hereafter, we refer to this group as the second EVN group.…”

“…Magnetic fields have mainly been detected along outflows and in a few cases on surfaces of disk/tori (Vlemmings et al 2010;Surcis et al 2009Surcis et al , 2011bSurcis et al , 2012. Moreover, 6.7-GHz CH 3 OH masers are also ideal for measuring the Zeeman-splitting even though the exact proportionality between the measured splitting and the magnetic field strength is still uncertain (Vlemmings et al 2011). Therefore, enlarging the number of massive YSOs towards which observations in full polarization of 6.7-GHz CH 3 OH maser are made is of high importance.…”

Advances in Maser Theory

“…More recently, the magnetic interpretation of circular polarization observed in several 6.7 GHz and other methanol masers with the use of the Landé factors extrapolated from the only known laboratory measurements by Jen (1951)) for the 25 GHz transitions, with the correction for the old, recently revealed order-of-magnitude arithmetical error in the extrapolation, led to implausibly high values of the magnetic fields (e.g. Vlemmings, Torres & Dodson (2011);Fish et al (2011)). …”