Periodic activity on the (sub)millimetre surface of the asymptotic giant branch star Mira A

Evolved stars on the asymptotic giant branch (AGB) lose mass through stellar winds launched from their dynamic extended atmospheres. Recent high-angular-resolution observations of the archetypal AGB star Mira A (o Ceti) have revealed compact hotspots on its (sub)millimetre surface, whose origin remains unclear. We aim to characterise the (sub)millimetre photosphere of Mira A and investigate the properties and temporal behaviour of the compact hotspots detected on its surface. We analysed 19 epochs of high-angular-resolution observations obtained with the Atacama Large Millimetre/submillimetre Array in Bands 4, 6, 7, and 9, covering frequencies between ∼,131.30 and 669.25,GHz. The complex visibilities were fitted with parametric models to derive the flux density, size, morphology, and brightness temperature of the stellar disc and an additional, more compact component across multiple pulsation phases and observing frequencies. Our results show that, near maximum light, the flux density of the stellar disc increases with frequency while its apparent size decreases, in agreement with the expected stratified radio photosphere. In Band 6, both the flux density and the size of the stellar disc appear to vary with pulsation phase, with the largest measured values occurring close to optical maximum light and the lowest observed around minimum light. Compact hotspots are detected in Bands 6 and 7 near maximum light, with their brightness temperatures exceeding those of the stellar disc by several thousand kelvin and reaching a temperature excess of ∼ 8200,K in Band 6. They are highly energetic and are detected over timescales of weeks, while the available observations also suggest potential longer-term changes in both their position and intensity. Hotspot activity appears to occur repeatedly across multiple pulsation cycles and may be linked to strong shocks, magnetic activity, and/or episodic mass ejections and could therefore play an important role in the mass-loss process in AGB stars.