AAS 201st Meeting, January, 2003
Session 20. Star Formation I
Poster, Monday, January 6, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[20.22] Magnetic structure of a dark Bok globule

J.P. Vallee, J.D. Fiege (NRC Canada, HIA, Victoria, BC, Canada), J.S. Grieve (Royal Observatory, Edinburgh, Scot., UK)

We have mapped the polarization and magnetic field of a dark Bok globule, within a radius of 66" centered on the dust peak. The thermal dust continuum emission at 848 microns from the globule CB068, a Class-0 young stellar object (YSO), reveals strong linear polarization percentage values in two polarized-intensity regions located on both sides of the center, away from the twin CO outflows. At the 5 polarized intensity level, the two highly-polarized regions have a radial separation of 0.01 pc from the Bok center. The two polarized regions are located on both sides of the axis of the twin CO outflows, with polarization contours paralleling the outflows.

The observed linear polarization orientation leads us to deduce that, in the inner part of the polarized-intensity regions, the electric field vector roughly follows the boundary of a pseudodisk or bar (the pseudodisk or bar was previously detected both in dust and in ammonia emission). The linear polarization orientation, within a radius of 0.03 pc, can be matched roughly to an infalling halo-to-disk magnetic field morphology.

A magnetic field model was investigated for this Class 0 YSO. Within 5200 AU of the peak. On a 0.04pc scale, our model predictions for CB068 give a pseudodisk diameter of about 8800 AU, while a predicted magnetic field strength (over 0.03 pc) of 150 microGauss comes about from the Chandrasekhar & Fermi method (via the ratio of the line of sight velocity dispersion to the polarization angle dispersion).

Our model is a simple, radially concentrated hourglass-type model, with a helical twist of opposite sense on either side of the equator. Thus, it shares some features with both previous helical field prolate core models, and the more usual hourglass field models. The inspiration for this type of model is the idea that perhaps the central disk or pseudodisk is rotating on these small scales and dragging the poloidal flux surfaces around. This would produce a helical twist of opposite sense on either side of the equator. The density structure and magnetic field geometry are self consistent.

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