Periodic activity on the (sub)mm surface of the AGB star Mira A
Abstract
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 ALMA in Bands 4, 6, 7, and 9. 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.
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BibTeX
@misc{andriantsaralaza2026periodic,
title = {{Periodic activity on the (sub)mm surface of the AGB star Mira A}},
author = {M. Andriantsaralaza and B. Bojnordi Arbab and W. Vlemmings and T. Khouri and L. Planquart and K. T. Wong and J. Alcolea and E. Humphreys and S. del Palacio and M. Wittkowski},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2608.23724},
eprint = {2608.23724},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.23724},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Periodic activity on the (sub)mm surface of the AGB star Mira A
AU - M. Andriantsaralaza
AU - B. Bojnordi Arbab
AU - W. Vlemmings
AU - T. Khouri
AU - L. Planquart
AU - K. T. Wong
AU - J. Alcolea
AU - E. Humphreys
AU - S. del Palacio
AU - M. Wittkowski
PY - 2026
DA - 2026/10/19
PB - arXiv
DO - 10.48550/arxiv.2608.23724
UR - https://arxiv.org/abs/2608.23724
AB - 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 ALMA in Bands 4, 6, 7, and 9. 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.
ER -