  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.
