AI 中文总结
本研究通过ALMA多波段观测，表征AGB星Mira A的亚毫米光球层，发现其表面存在周期性演化的高温致密热点，或与质量损失相关。
AI 中文摘要
渐近巨星分支（AGB）阶段的演化恒星通过从其动态延展大气中启动的恒星风损失质量。对典型AGB星Mira A（鲸鱼座ο）的近期高角分辨率观测在其亚毫米（submm）表面发现了致密热点，其起源尚不明确。本研究旨在表征Mira A的亚毫米光球层，并探究其表面探测到的致密热点的性质与时间行为。我们分析了阿塔卡马大型毫米/亚毫米阵列（ALMA）在4、6、7、9波段（频率范围134.34至669.25 GHz）获取的19个历元的高角分辨率观测数据，对复可见度进行参数模型拟合，以推导恒星盘及额外致密成分在多个脉动相位下的流量密度、大小、形态和亮温度。结果显示，恒星盘的流量密度随频率升高而增加，而其视大小随频率升高而减小，这与分层射电光球层的预期一致。在6波段，恒星盘的流量密度和大小均随脉动相位变化，在光学极大光附近达到最大值，向极小光方向减小。在6和7波段的极大光附近探测到致密热点，其亮温度比恒星盘高出数千开尔文，在6波段达到T>8600 K。这些热点能量极高，以周为尺度持续存在，但在更长时间尺度上演化，表现出位置和强度的变化。热点活动似乎在多个脉动周期中反复出现，可能与强激波、磁活动和/或间歇性质量抛射有关，因此可能在AGB星的质量损失过程中发挥重要作用。
英文摘要
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 submillimetre (submm) surface, whose origin remains unclear. We aim to characterise the submm 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, ranging from 134.34 to 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 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 vary with pulsation phase, reaching maximum values close to optical maximum light and decreasing toward 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 kelvins, reaching T>8600 K in Band 6. They are highly energetic and persist on timescales of weeks but evolve over longer timescales, showing changes in both 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.
CommentsSubmitted to A&A, with most comments from the 1st round of review implemented
