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Register a new userMeasurement of the magnetic field direction in the NGC2024-FIR5 protostellar outflow
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Derek Ward-Thompson
Publication date
2000
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Physics
and research's language is
English
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Molecular outflows from young protostars are widely believed to be collimated by magnetic fields, but there has been little observational evidence to support this hypothesis. Using the new technique of millimetre-wavelength spectro-polarimetry, we demonstrate the existence of a magnetic field in the NGC2024-FIR5 outflow lobe. The 1.3mm J=2-1 transition of carbon monoxide (CO) is polarized at a level of approximately 1%, in a direction within 10-15 degrees of the outflow axis. This agrees with theoretical models where the magnetic field channels the outflowing gas, and shows that the process can be effective as far as 0.1pc from the protostar.
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We present the first interferometric polarization maps of the NGC2024 FIR5 molecular core obtained with the BIMA array at approximately 2 arcsec resolution. We measure an average position angle of -60+-6 degrees in the main core of FIR5 and 54+-9 degrees in the eastern wing of FIR5. The morphology of the polarization angles in the main core of FIR5 suggests that the field lines are parabolic with a symmetry axis approximately parallel to the major axis of the putative disk in FIR5, which is consistent with the theoretical scenario that the gravitational collapse pulled the field lines into an hour-glass shape. The polarization percentage decreases toward regions with high intensity and close to the center of the core, suggesting that the dust alignment efficiency may decrease at high density. The plane-of-sky field strength can be estimated with the modified Chandrasekhar-Fermi formula, and the small dispersion of the polarization angles in FIR5 suggests that the magnetic field is strong ($gtrsim$ 2mG) and perhaps dominates the turbulent motions in the core.
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