THE HANLE EFFECT AS A DIAGNOSTIC OF MAGNETIC-FIELDS IN STELLAR ENVELOPES .1. THEORETICAL RESULTS FOR INTEGRATED LINE-PROFILES

The Hanle effect concerns the modification of polarized resonance-line scattering by magnetic fields; thus, it can be used as a diagnostic o f stellar magnetic fields. The Hanle effect has been used to determine the held strength and distribution of magnetic structures present in prominences of the Sun. To investigate its potential use in stellar as tronomy, the simplified case of an optically thin axisymmetric ring il luminated by a stellar point source is considered. The results are the n used to derive the polarization from polar plumes, equatorial disks, and spherical shells. The integrated line polarization is calculated for axisymmetric rings with a variety of magnetic field orientations, and in every case the polarization is proportional to sin(2) i (where i is the viewing inclination), just as in the zero field case. It is a lso found that the Hanle effect can significantly alter the integrated line polarization. In some cases the position angle of the polarizati on in the line can be rotated by 90 degrees relative to the zero field case. We consider the Hanle effect as a possible diagnostic of magnet ic fields in stellar winds with prominent ultraviolet and visible reso nance lines. For these lines the diagnostic has sensitivity in the ran ge of 1-1000 G. The Zeeman effect is not normally applicable for diagn osing magnetic fields in stellar winds in the subkilogauss range; thus , the Hanle effect should provide an especially useful new method of d etermining magnetic fields in stars other than the Sun. Possibilities for measuring the fields in early-type stars using ultraviolet observa tions is discussed.