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

Left: the 2D magnetic field direction is deduced from polarization angles for the two magnetically realigned Fe I lines. The error bars mark the 3σ uncertainty range;
Right: The POS view of the system. The orbital phases are marked along the secondary track. The color scale denotes the line-of-sight (henceforth LOS) velocity of the medium. The POS projection of the symmetric axis of the outflow is ~45° to the East-West direction.

Sub-Gauss magnetic fields are of great importance to many physical processes from Galactic kpc scales down to stellar scales. Such fields, however, are nontrivial to detect. Present magnetic diagnoses of weak fields trace only one component of a magnetic field directly. Here we report the first discovery of 3D sub-Gauss magnetic field beyond our solar system. We find that two weak neutral iron absorption lines from the ground state in the atmosphere of $89~Her$ produced counterintuitive high-amplitude polarizations and consistent polarization angles, exactly in line with our theoretical prediction from ground state alignment (GSA, Yan & Lazarian 2006, ApJ, 653,1292). Our analysis provides the first precise magnetic field measurement on $89~Her$, which is found to have a 3D orientation aligned to the stellar outflow axis and a strength ~< 100mG. This result substantially improves the accuracy by 2 orders of magnitude compared to the previous 10G upper limit set by non-detection of Zeeman effect. This long-awaited discovery is anticipated to usher in a new era of precise measurement of sub-gauss magnetic fields in the Universe.

journal reference: ApJL, 902, L7

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