The Hanle Effect as a Diagnostic of Magnetic Fields in Stellar Envelopes. IV. Application to Polarized P Cygni Wind Lines

The Hanle effect has been proposed as a new diagnostic of circumstellarmagnetic fields for early-type stars, for which it is sensitive to fieldstrengths in the 1-300 G range. In this paper we compute the polarized P-Cygniline profiles that result from the Hanle effect. For modeling the polarization,we employ a variant of the ``last scattering approximation''. For cases inwhich the Sobolev optical depths are greater than unity, the emergent lineintensity is assumed to be unpolarized; while for smaller optical depths, theStokes source functions for the Hanle effect with optically thin linescattering are used. For a typical P Cygni line, the polarized emission formsin the outer wind, because the Sobolev optical depth is large at the innerwind. For low surface field strengths, weak P Cygni lines are needed to measurethe circumstellar field. For high values of the surface fields, both the Zeemanand Hanle diagnostics can be used, with the Zeeman effect probing thephotospheric magnetic fields, and the Hanle effect measuring the magnetic fieldin the wind flow. Polarized line profiles are calculated for a self-consistentstructure of the flow and the magnetic geometry based on the WCFields model,which is applicable to slowly rotating stellar winds with magnetic fields drawnout by the gas flow. For surface fields of a few hundred Gauss, we find thatthe Hanle effect can produce line polarizations in the range of a few tenths ofa percent up to about 2 percent.Comment: accepted to the Astrophysical Journa