Discovery of polarizations from ground state absorption lines: tracer of sub-Gauss magnetic field on 89Her

Abstract

We report the identification of the polarization of ground state absorption lines from post-AGB 89~Hercules. Two ground state neutral iron lines are found to have counterintuitive high-amplitude polarizations and an unchanged polarization direction through the orbital period, as opposed to the pattern of polarizations of absorption lines from excited states, which are synchronized with the orbital phase owing to optical pumping. This can be explained with magnetic realignment on the ground state. The 3D mean magnetic field is thereby unveiled from the degree and direction of the polarizations of the two iron lines. The field strength is also constrained to be $\lesssim 100\,{\rm mG}$. Our result has thus improved the accuracy by orders of magnitude compared to the previous $10\,{\rm G}$ upper limit set by non-detection of the Zeeman effect.