Full‐Polarization Observations of OH Masers in Massive Star‐forming Regions. II. Maser Properties and the Interpretation of Polarization

Fish, Vincent L.; Reid, M. J.

doi:10.1086/502650

Cited by 89 publications

(87 citation statements)

References 119 publications

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“…The fractions could be lower limits, because there are several effects reducing the original degree of linear polarization. Linear polarization can be reduced by field line curvature along the propagation direction by destroying the phase coherence of polarization (Elitzur 2002), or by internal Faraday rotation due to the presence of electrons in the magnetized medium along the maser amplification path (Fish & Reid 2006). However, the effects of internal Faraday rotation are expected to be small (Surcis et al 2011), so our results are not likely affected by the Faraday rotation.…”

Section: Resultsmentioning

confidence: 88%

“…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).…”

Section: Comparison Of Polarization Properties At 44 and 95 Ghzmentioning

confidence: 99%

“…For a long time, masers were thought to trace the isolated pockets of a compressed field rather than the field of an ambient medium. However, some recent studies of 1.6 GHz hydroxyl and 6.7 GHz methanol masers showed that they possibly probe the large-scale magnetic field (e.g., Fish & Reid 2006;Vlemmings et al 2010;Surcis et al 2012).…”

Section: Introductionmentioning

confidence: 99%

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

Kang

Byun

Kim

et al. 2016

ApJS

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Class I methanol masers are found to be good tracers of the interaction between outflows from massive young stellar objects with their surrounding media. Although polarization observations of Class II methanol masers have been able to provide information about magnetic fields close to the central (proto)stars, polarization observations of Class I methanol masers are rare, especially at 44 and 95 GHz. We present the results of linear polarization observations of 39 Class I methanol maser sources at 44 and 95 GHz. These two lines are observed simultaneously with one of the 21 m Korean VLBI Network telescopes in single-dish mode. Approximately 60% of the observed sources have fractional polarizations of a few percent in at least one transition. This is the first reported detection of linear polarization of the 44 GHz methanol maser. The two maser transitions show similar polarization properties, indicating that they trace similar magnetic environments, although the fraction of the linear polarization is slightly higher at 95 GHz. We discuss the association between the directions of polarization angles and outflows. We also discuss some targets having different polarization properties at both lines, including DR21(OH) and G82.58+0.20, which show the 90°polarization angle flip at 44 GHz.

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Section: Resultsmentioning

confidence: 88%

Section: Comparison Of Polarization Properties At 44 and 95 Ghzmentioning

confidence: 99%

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

Kang

Byun

Kim

et al. 2016

ApJS

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“…Additionally, the internal Faraday rotations introduce large scattering of polarization angle (Fish & Reid 2006). Therefore, it is not possible to determine the absolute geometry of the magnetic field in the OH maser region of W43A.…”

Section: The Role Of the Magnetic Fieldmentioning

confidence: 99%

The magnetic field of the evolved star W43A

Amiri

Vlemmings

Langevelde

2009

A&A

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Context. The majority of the observed planetary nebulae exhibit elliptical or bipolar structures. Recent observations have shown that asymmetries already start during the last stages of the AGB phase. Theoretical modeling has indicated that magnetically collimated jets may be responsible for the formation of the non-spherical planetary nebulae. Direct measurement of the magnetic field of evolved stars is possible using polarization observations of different maser species occurring in the circumstellar envelopes around these stars. Aims. The aim of this project is to measure the Zeeman splitting caused by the magnetic field in the OH and H 2 O maser regions occurring in the circumstellar envelope and bipolar outflow of the evolved star W43A. We compare the magnetic field obtained in the OH maser region with the one measured in the H 2 O maser jet. Methods. We used the UK Multi-Element Radio Linked Interferometer Network (MERLIN) to observe the polarization of the OH masers in the circumstellar envelope of W43A. Likewise, we used the Green Bank Telescope (GBT) observations to measure the magnetic field strength obtained previously in the H 2 O maser jet. Results. We report a measured magnetic field of approximately 100 μG in the OH maser region of the circumstellar envelope around W43A. The GBT observations reveal a magnetic field strength B || of ∼30 mG changing sign across the H 2 O masers at the tip of the red-shifted lobe of the bipolar outflow. We also find that the OH maser shell shows no sign of non-spherical expansion and that it probably has an expansion velocity that is typical for the shells of regular OH/IR stars. Conclusions. The GBT observations confirm that the magnetic field collimates the H 2 O maser jet, while the OH maser observations show that a strong large-scale magnetic field is present in the envelope surrounding the W43A central star. The magnetic field in the OH maser envelope is consistent with the one extrapolated from the H 2 O measurements, confirming that magnetic fields play an important role in the entire circumstellar environment of W43A.

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“…These typically display a few mG field strength (e.g. Fish & Reid 2006;Bartkiewicz et al 2005) and a coherent magnetic field structure. For example the polarization of the OH masers of W75N matches a toroidal field in a massive torus or disk (e.g.…”

Section: Introductionmentioning

confidence: 99%

Zeeman splitting of 6.7 GHz methanol masers

Vlemmings

Torres

Dodson

2011

A&A

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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.

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Full‐Polarization Observations of OH Masers in Massive Star‐forming Regions. II. Maser Properties and the Interpretation of Polarization

Cited by 89 publications

References 119 publications

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

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

The magnetic field of the evolved star W43A

Zeeman splitting of 6.7 GHz methanol masers

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