Discovery of Ground-state Absorption Line Polarization and Sub-Gauss Magnetic Field in the Post-AGB Binary System 89 Her

We report the polarization of ground-state absorption lines from the post-asymptotic giant branch (post-AGB) binary star system 89 Herculis (89 Her). Two ground-state neutral iron lines are found to have counterintuitive high-amplitude polarizations and an unchanged polarization direction through the orbital period. This is contrary to the pattern of polarization of absorption lines from excited states, which are synchronized with the orbital phase due to optical pumping. This can be explained by magnetic realignment of the ground state, whereby the 3D mean magnetic field is unveiled from the degree and direction of the polarizations of the two iron lines. The field strength is also constrained to be ≲100 mG. Our result improves the accuracy by orders of magnitude compared to the previous 10 G upper limit set by non-detection of the Zeeman effect.