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Session 44 - Circumstellar Material.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center
The Hanle Effect describes resonant line scattering polarization in a magnetic medium. Classically, the polarization process is described by a damped oscillator that is caused to precess about a magnetic field. The Hanle Effect has been used to study the magnetic structures present in prominences of the sun to determine the field strength and distribution. To investigate its potential use in stellar astronomy, we consider several simplified cases associated with an optically thin axisymmetric ring illuminated by a stellar point source. The ring is assumed to be expanding or rotating. We derive the Stokes parameters for the following magnetic field distributions: radial, toroidal, polar radial, latitudinal, and poloidal. It is shown that for a radial field, the scattering at each point of the ring is effectively non-magnetic. For each of the other field distributions, the magnetic fields produce effects on the polarization within the line profile. For the poloidal case, the magnetic effects vanish when integrated over the line profile frequencies. One promising application of the Hanle effect is as a diagnostic of magnetic fields in hot stars with prominent ultraviolet resonance lines, where it is sensitive to fields in the range 10-1000 Gauss. A sounding rocket payload under development at the University of Wisconsin, the Far Ultraviolet Spectropolarimeter, will begin exploring this area in 1997.
