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VLE11 - Zeeman splitting of 6.7 GHz methanol masers. On the uncertainty of magnetic field strength determinations

Short title: Vlemmings et al. (2011)
Authors: Vlemmings, W. H. T.Torres, R. M.Dodson, R.
Journal: Astronomy & Astrophysics, Volume 529, id.A95, 12 pp.
Bibcode:

Target type: Known_Class_II
Transitions: CH3OH_6(II)

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 nH2 ≈ 109 cm𕒷. 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𕒵. 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 𕙘.6 m s𕒵. 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 � mG in the methanol maser region. This is signiﬁcantly higher than expected using the typically assumed relation between magnetic ﬁeld and density (B ∝ nH0.247) 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 non- Zeeman 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.

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 nH2 ≈ 109 cm𕒷. 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𕒵. 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 𕙘.6 m s𕒵. 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 � mG in the methanol maser region. This is signiﬁcantly higher than expected using the typically assumed relation between magnetic ﬁeld and density (B ∝ nH0.247) 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 non- Zeeman 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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