A giant magnetic eruption from the asymptotic-giant-branch star Mira A

Mass loss during the advanced evolutionary stages of low- and intermediate-mass stars is traditionally attributed to dust-driven winds, whereas the influence of magnetic fields remains poorly constrained. However, recent observations show massive directional mass ejections from the nearby asymptotic-giant-branch (AGB) star Mira A that do not appear to be driven by radiation pressure on the dust. We aim to observationally investigate whether magnetic fields are responsible for the mass-loss ejections from Mira A. We obtained full-polarisation ALMA observations of the SiO v=1, J=5̊ightarrow4 maser transition at 215 GHz with angular resolutions of 1.5--2.6 au. The observations reveal a highly magnetised loop-like mass ejection traced by strongly polarised SiO masers extending to approximately eight stellar radii. The morphology, polarisation pattern, energetics, and kinematics indicate a magnetically driven eruption analogous to a stellar coronal-mass ejection (CME). These observations demonstrate that magnetic eruptions can dominate mass loss from evolved stars and imply that magnetic fields must become an integral component of future models of AGB (and potentially other cool-giant) winds.