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Submillimetre Polarimetry of NGC 2024
Magnetic fields play a significant, even crucial role in the process of star formation (Heiles 1993), through magnetic support of molecular clouds, dissipation of angular momentum in accretion disks, and the generation of jets and outflows. Polarized thermal emission at submillimeter wavelengths from aligned dust grains directly traces the magnetic field structure projected onto the plane of the sky (Hildebrand 1988). With the recent development of focal plane bolometer arrays equipped with polarimeters, sensitive imaging polarimetry in the submillimeter is now possible. The Orion B (L1630) molecular cloud, at ~415 pc (Anthony-Twarg 1982), is one of the nearest giant molecular clouds and is an active site of low- to high-mass star formation. It was one of the first to be systematically studied for dense cores by Lada et al. (1991), who found that massive star formation took place only in the five largest clumps, which together make up more than 50% of the mass of dense gas Lada et al. (1991). NGC 2024 is an HII region and the most prominent star formation region in Orion B, associated with a massive cluster, ionizing B stars, and stars at all phases of evolution (Mezger et al. 1988; Lada et al. 1991; Chandler et al. 1996). The submillimeter emission, discovered by Mezger et al. (1988) & Mezger et al. (1992), is located behind the HII region (see cartoon in Barnes (1989)) and consists of at least seven sources aligned along a ridge, similar to OMC-3 in Orion A (Johnstone & Bally 1999; Matthews & Wilson 2000). Two of these cores are the origins of unipolar molecular outflows, one of which is very highly collimated and very extended (Chandler & Carlstrom 1996; Sanders & Wilner 1985; Richer, Hills & Padman 1992), while another core contains a water maser (Genzel & Downes 1977), indicating intermediate-mass protostars. The rest of the cores show no sign of star formation activity (Visser et al. 1998).
(Figure 1 left) Using the imaging polarimeter for the Submillimeter Common User Bolometric Array at the James Clerk Maxwell Telescope, we have detected polarized thermal emission at 850 um from dust toward the NGC 2024 star-forming core system. Here we show the 850 um dust emission as a ``grey''-scale, on which are plotted the 12''-binned polarization vectors. Vectors are plotted where I>0.001 V, p>1%, dp<1.5% and p/dp>4. Bold vectors show those where p/dp>7. The polarization patterns are not indicative of those expected for the case of uniform fields, and exhibit depolarization toward the highest intensity peaks. NGC 2024 exhibits an organized polarization pattern which is structured consistently along the length of a chain of 7 far-infrared sources and may be dominated by the filamentary structure rather than the cores. (Figure 2 right) We've modelled the polarization pattern of NGC 2024 with a ``bent filament'' model of Fiege & Pudritz (2000). The length of the filament is 6 times the radius and the ends have been rounded. We have bent the entire filament into a circular arc perpendicular to the plane of the sky and toward the observer, keeping the top of the filament parallel to the original orientation. The radius of the arc is 3 times the filament length. We then rotated the entire structure by 20deg and inclined it relative to the plane of the sky by -15deg. The magnetic field threading the region in this model is helical and the ratio of ``toroidal'' to ``poloidal'' magnetic flux is about 2. Although this model is not unique, in that other field geometries could duplicate the observed polarization pattern, the excellence of the fit supports a helical field surrounding a bent filament. This work is part of the Canadian Consortium for Star Formation Studies. Bibliography Anthony-Twarog, B. J., 1982, AJ, 87, 1213 Barnes, P.J., Crutcher, R.M., Bieging, J.H., Storey, J.W.V., & Willner, S.P. 1989, ApJ, 342, 883 Chandler, C. J., & Carlstrom, J. E. 1996, ApJ, 466, 338 Fiege, J.D., & Pudritz, R.E. 2000, MNRAS, 311, 85 Genzel, R. & Downes, D. 1977, A&ASuppl, 30, 145 Heiles, C., Goodman, A. A., McKee, C. F., & Zweibel, E. G. 1993, in Protostars & Planets III, ed. E. H. Levy and J. I. Lunine (Tucson, University of Arizona Press), 279 Hildebrand, R. H. 1988, QJRAS, 29, 327 Johnstone, D., & Bally, J. 1999, ApJ, 510, L49 Lada, E. A., DePoy, D. L., Evans, N. J., & Gatley, I. 1991, ApJ, 371, 171 Matthews, B. C., & Wilson, C. D. 2000, ApJ, 531, 868 Mezger, P. G., Chini, R., Kreysa, E., Wink, J. W., & Salter, C. J. 1988, A&A, 191, 44 Mezger, P. G., Sievers, A. W., Haslam, C. G. T., Kreysa, E., Lemke, R., Mauersberger, R., & Wilson, T. L. 1992, A&A, 256, 631 Richer, J. S., Hills, R. E., & Padman, R. 1992, MNRAS, 254, 525 Sanders, D. B., & Willner, S. P. 1985, ApJ, 293, L39 Visser, A. E., Richer, J. S., Chandler, C. J., & Padman, R. 1998,
MNRAS, 301, 585
Gerald Moriarty-Schieven |
