More than half of the dust and heavy element enrichment in galaxies originates from the winds and outflows of evolved, low-to-intermediate mass stars on the asymptotic giant branch (AGB). However, numerous details of the physics of late-stage stellar mass loss remain poorly understood, ranging from the wind launching mechanism(s) to the geometry and timescales of the mass loss. One of the major challenges to understanding AGB winds is that the AGB evolutionary phase is characterized by the interplay between highly complex and dynamic processes, including radial pulsations, shocks, magnetic fields, opacity changes due to dust and molecule formation, and large-scale convective flows. Collectively, these phenomena lead to changes in the observed stellar properties on timescales of days to years. Probing the complex atmospheric physics of AGB stars therefore demands exquisite spatial resolution, coupled with temporal monitoring over both short and long timescales. Observations of the molecular maser lines that arise in the winds and outflows of AGB stars using very long baseline interferometry (VLBI) offer one of the most powerful tools available to measure the atmospheric dynamics, physical conditions, and magnetic fields with ultra-high spatial resolution (i.e., up to tens of microarcseconds, corresponding to ~0.002R* at d~150pc), coupled with the ability to track features and phenomena on timescales of days to years. Observational advances in the coming decade will enable contemporaneous observations of an unprecedented number of maser transitions spanning centimeter to submillimeter wavelengths. In evolved stars, observations of masers within the winds and outflows are poised to provide groundbreaking new insights into the atmospheric physics and mass-loss process.
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