Polarization Data in Perseus Reveal a Core without a Polarization Hole

Observations toward the centre of the dark cloud Barnard 1 in Perseus using the SCUBA polarimeter at 850 microns reveal several dusty cores, labelled B1-a through B1-d on Figure 1 (Matthews & Wilson 2001). The greyscale traces the column density in dust emission while the vectors show the net polarization percentage and position angle summed through the optically thin cloud. (The blue cross marks the position of IRAS 03301+3057.) The polarization vectors are binned to 6" spacing within the cores and 12" in the lower column density dust. Interestingly, the vectors in the cores are strongly aligned but each core exhibits a different position angle. These angles are in turn different from the position angle of 90^o E of N measured toward the faint dust. Rotation of these vectors to infer a magnetic field direction would produce an extremely disjointed field with sharp changes in projected field direction at the core boundaries.

In Figure 2, we have plotted the behavior of the polarization percentage versus the dust intensity for several of the cores. The B1-d core is quite faint and exhibits the typical decline in polarization percentage with intensity to its peak. However, the B1-b and B1-c cores show a deviation from this behavior; beyond an intensity of 1 Jy/beam, the polarization percentage ceases to decline. (We have truncated our data sample at p<1%, resulting in the removal of 5 vectors [shown in red on Figure 1] from B1-b and 1 vector from B1-c. This could be creating an artificial threshold for B1-b, but cannot account for the behavior for B1-c.) The threshold does not appear to be due to optical depth effects, since even at the B1-c peak, we estimate tau < 1. Additionally, no polarized emission would be expected from an optically thick source, since contributions from orthogonal orientations would be equal.

Based on the B1-c data, the polarized intensity (p x I) in fact increases to the peak of that core. This means the high density dust is likely contributing to the polarized emission. In other observations of star-forming (e.g. Greaves et al., 1999; Greaves et al. 1995) or starless cores (Ward-Thompson et al., 2000), the constant decline in polarization with intensity has been interpreted as evidence that only dust grains up to a nominal extinction contribute to the polarization we measure. Since all grains contribute to the total emission, the polarization percentage (polarized/total intensity) drops toward core peaks.

References

Greaves, J.S., Holland, W.S., Minchin, N.R., Murray, A.G., & Stevens, J.A. 1999, A&A, 344, 668
Greaves, J.S., Holland, W.S., Murray, A.G., & Nartallo, R. 1995, MNRAS, 272, L1
Matthews, B.C., & Wilson, C.D. 2001, submitted to ApJ.
Ward-Thompson, D., Kirk, J.M., Crutcher, R.M., Greaves, J.S., Holland, W.S., & Andre, P. 2000, ApJ, 537, 135

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