Magnetic Field Orientation in Galactic Filaments
© Lloyd

I think CC said magnetic fields are parallel to spiral arms, so they aren't pinching and compressing the arms. This article seems to say that the magnetic field of this Orion filament is perpendicular to it, so I'm guessing that means the field does compress the filament. Is that right? [CC says No, the B-perpendicular is perpendicular, so B is parallel. But Brant makes some statements too to follow.]


Magnetic Fields in Star-Forming Molecular Clouds: JCMT Polarimetry of OMC-3 in Orion A Brenda C. Matthews & Christine D. Wilson Dept. of Physics & Astronomy, McMaster University, Hamilton, ON, Canada
Using the new imaging polarimeter mounted on SCUBA at the JCMT, we have detected polarized thermal emission at 850 micro-m from dust along the 6min length of the dense filament known as OMC-3 in Orion A. The polarization pattern is highly ordered and is aligned with the long axis of the filament throughout most of the region, diverging only near the southern boundary by 30-60° (see Figures 1 and 2). This configuration indicates a plane-of-sky magnetic field, B-perpendicular, oriented normal to the filament along most of its length (if the polarization arises from thermal emission of dust grains aligned by the magnetic field via the Davis-Greenstein mechanism). The mean percentage polarization measured is 4.2% with a 1-sigma dispersion of 1%. OMC-3 is part of the integral-shaped filament, first identified in ^13 CO J = 1 - 0 by Bally et al. (1987). It is a region of active star formation, with only two of nine dust condensations in our field lac king evidence of outflow activity. The outflow directions do not appear to be correlated with the direction of B-perpendicular or the filament itself. Thus, the field of the filament is unlikely to be the dominant factor in determining the configuration of the protostellar systems embedded within it. Depolarization toward regions of higher density, previously detected in many other star forming cores and protostars, is also evident in our data (see Figure 3). [See next post for images.]

MMS1 MMS4 MMS5 MMS2/3 MMS6 MMS7 MMS8 MMS9 10% polarized
Figure 1. 850 micro-m polarization structure. The coloured greyscale indicates variations in uncalibrated I, ranging from -2 to 3-sigma; polarization vectors (up to a maximum of 20%) are overlain. The polarization structures are highly ordered and follow the filament's orientation along most of its length (see Figure 2). Therefore, the orientation of the B-perpendicular-field is predominantly orthogonal to the filament. Only vectors with sigma-p > 2.5 are plotted, where sigma-p is the signal-to-noise of the polarized flux. The accuracy of the position angle, theta, of each vector is related to sigma-p, so no vector has d-theta > 12°. Blue contours indicate bounds of sigma-p = 6 ( d-theta < 5°) and sigma-p = 10 (d-theta < 3°) respectively. These data contain 6 sigma-p detections on sources as weak as 0.5 Jy. The dust condensations MMS1-9 are those first identified at 1.3 mm by Chini et al. (1997). The orientations of known outflows have been labelled in green (see Yu et al. 1997; Chini et al. 1997; Castets & Langer 1995). The central coordinate is alpha-B1950 =5h 32m 55.6s, delta-B1950 = -5° 3m 25s.

Figure 2. Variations in the Orientation of theta. On each histogram, the dashed line represents the position angle of the filament, while the polarization data have been binned to 6° widths and fit with 1 or 2 Gaussians (red) as appropriate by minimizing X2. The resultant values for the mean and dispersion of the position angle distributions are summarized in green for each region. The data reveal the changing orientation of the polarization pattern with respect to the filament. F rom MMS1 to MMS7, it is clear that the polarization vectors lie predominantly along the filament to within 10°. However, around MMS8 and MMS9, the filament runs roughly north-south, while the polarization vectors are offset by 30-60°. Since an evolutionary sequence has been suggested for the region, with source ages decreasing as one moves north, one might wonder whether the outflows were influencing the measured magnetic field direction where they are the most evolved. Reipurth et al. (1999) have identified MMS9 as the source of the most powerful outflow in OMC-3, yet Figure 1 reveals that the outflow direction is offset from B-perpendicular by 30-60°.

Figure 3. Depolarization across OMC-3. These data represent a perpendicular cut through the filament centered on alpha-1950 =5h 32m 46.9s and delta-1950 = 5° 1m 43.6s (the position of the MMS2/3 binary). The plot clearly shows a reduction in polarization toward the center of the filament. The error bars represent the uncertainty asso ciated with each data point. Measurements of decreased p toward regions of high density could be due to changes in physical conditions or averaging of small scale variations in a large beam, but they are also suggested as an observable signature of helical fields (Fiege & Pudritz 1999). In OMC-1, Schleuning (1998) has measured a decreased polarization toward the core center at 350 micro-m and high-resolution data by Rao et al. (1998) with BIMA revealed substantial changes in polarization angle near the source which would tend to average to zero in a larger beam. Interpretation of the depolarization in OMC-3 will require modelling of field structures.

[See next post below for images.]

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