Hotspot Activity in the Extended Atmosphere of Mira: Insights from Multi-Epoch ALMA Observations

Evolved stars on the Asymptotic Giant Branch (AGB) play a crucial role in the cycle of cosmic matter through their massive stellar winds. The onset of these AGB winds occurs in their dynamic extended atmospheres, which have long remained observationally inaccessible. In recent years, high-angular resolution interferometric observations have opened a new window into the intricate atmospheres of these cool giants by directly resolving their stellar disks, revealing asymmetries, variability, and localised surface activity. Mira A (omi Cet), a well-studied AGB star, has been found to exhibit bright, compact hotspots on its surface at millimetre wavelengths. While their origin, physical nature, and prevalence remain poorly understood and are not predicted by current 3D atmospheric AGB models, these hotspots have been suggested to be linked to shocks induced by convective motions and pulsations, or to magnetic activity. Investigating stellar hotspot activity is therefore essential for constraining models of dynamic AGB stellar atmospheres and for improving our understanding of their mass-loss mechanisms. In this work, we present a multi-epoch, multi-frequency study of Mira A using high-angular-resolution ALMA continuum observations to investigate the temporal and spectral variability of its hotspots and stellar disk. Our analysis, encompassing 19 epochs spanning ALMA Bands 4, 6, 7, and 9, reveals distinct variations in the properties and occurrence of hotspots between epochs and across atmospheric depths, highlighting a complex, time-variable environment in the extended atmosphere of Mira A.