A MODEL FOR THE MAGNETIC-FIELD IN THE MOLECULAR DISK AT THE GALACTIC-CENTER

The magnetic field configuration in the Galactic center disk is modeled in terms of an accretion disk that is threaded by open magnetic field lines. We suggest that this field is generated from the axial magnetic field observed at larger distances from the Galactic center by the differentially rotating radial inflow, and that the twisting and stretching of the field lines are balanced by ambipolar diffusion. A self-similar magnetized disk model is used to calculate the expected far-infrared polarization of thermal emission from dust grains and the circular polarization of absorption lines produced by Zeeman splitting within the circumnuclear disk. Recent 100 μm polarization measurements imply that the axial field component Bz is smaller than either the radial or the azimuthal component, and that Br and BΦ have comparable magnitudes and opposite signs, consistent with the generation of BΦ from Br by differential rotation. For the inferred disk parameters, the model reproduces the measurements and predicts that the polarization in the northwest and the southeast quadrants of the disk should be comparatively weak and roughly orthogonal to that measured along the ridge of dust emission. The circular polarization produced by Zeeman splitting is not proportional to the derivative of the absorption-line profile because the orientation of the field lines changes with height in the disk. The sense of polarization can change twice across the line profile: once because of the minimum in the profile and again as a result of the sign reversal of the line-of-sight field component. This effect may be observed in the northwest and southeast quadrants, which is also where the degree of circular polarization is predicted to peak. The field geometry indicated by the dust polarization measurements and the ∼1 mG field strength inferred from recent Zeeman splitting observations are consistent with the ordered magnetic field dominating the removal of angular momentum from the disk, in which case the disk would be the source of a bipolar outflow. The disk need not be highly clumped, contrary to previous interpretations. We suggest that the UV-heated emission regions are confined to the disk surfaces and are extended because of the flaring and warping of the disk, and that ambipolar diffusion heating inside the disk contributes to the observed CO and O I line emission.