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arXiv 1503.07647astro-ph.SR
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用ALMA解算Mira AB双星的恒星活动

Resolving the stellar activity of the Mira AB binary with ALMA

W. H. T. Vlemmings, S. Ramstedt, E. O'Gorman, E. M. L. Humphreys, M. Wittkowski, A. Baudry, M. Karovska

摘要:

我们根据ALMA科学验证观测结果,在波段3(~94.6 GHz)和波段6(~228.7 GHz),从二进制Mira A(o Ceti)和Mira B中,给出了毫米连续波长的大小,形状和通量密度。Mira AB系统已被ALMA观测到,空间分辨率低至~25 mas。解析了Mira A的扩展大气层和Mira B源周围的风场,并导出了Mira A和Mira B周围电离区的大小。还导出了频带3内(89 - 100 GHz之间)和频带3与频带6之间的谱指数。发现Mira A的光谱指数从波段3内的1.71 ± 0.05变化到波段3和6之间的1.54 ± 0.04。Mira B在波段3的光谱指数为1.3 ± 0.2,与较长波长的测量结果非常一致。然而,它在频带之间上升到1.72 +-0.11。这是第一次在这些频率下解析恒星的扩展大气层,Mira A在波段3的直径为~3.8x3.2 AU(亮度温度Tb~5300 K),在波段6的直径为~4.0x3.6 AU(Tb~2500 K)。此外,在Mira A的星盘上还发现了一个亮度约为0.4AU、Tb约为10000 K的亮热点。发现Mira B的吸积盘周围的电离区大小约为2.4 AU。在Mira B周围的辐射与靠近Mira B的吸积盘的Mira A引力束缚物质的部分电离风的辐射一致。Mira A的大气层并不完全符合预测,波段3的亮度温度明显高于预期,可能是由于冲击加热。热点可能是由于磁场活动,并可能与以前观察到的Mira A的X射线耀斑有关。

英文摘要:

We present the size, shape and flux densities at millimeter continuum wavelengths, based on ALMA science verification observations in Band 3 (~94.6 GHz) and Band 6 (~228.7 GHz), from the binary Mira A (o Ceti) and Mira B. The Mira AB system has been observed with ALMA at a spatial resolution of down to ~25 mas. The extended atmosphere of Mira A and the wind around Mira B sources are resolved and we derive the size of Mira A and of the ionized region around Mira B. The spectral indices within Band 3 (between 89-100 GHz) and between Band 3 and Band 6 are also derived. The spectral index of Mira A is found to change from 1.71+-0.05 within Band 3 to 1.54+-0.04 between Band 3 and 6. The spectral index of Mira B is 1.3+-0.2 in Band 3, in good agreement with measurements at longer wavelengths. However it rises to 1.72+-0.11 between the bands. For the first time the extended atmosphere of a star is resolved at these frequencies and for Mira A the diameter is ~3.8x3.2 AU in Band 3 (with brightness temperature Tb~5300 K) and ~4.0x3.6 AU in Band 6 (Tb~2500 K). Additionally, a bright hotspot of ~0.4 AU and with Tb~10000 K is found on the stellar disc of Mira A. The size of the ionized region around the accretion disk of Mira B is found to be ~2.4 AU. The emission around Mira B is consistent with that from a partially ionized wind of gravitationally bound material from Mira A close to the accretion disk of Mira B. The Mira A atmosphere does not fully match predictions, with brightness temperatures in Band 3 significantly higher than expected, potentially due to shock heating. The hotspot is likely due to magnetic activity and could be related to the previously observed X-ray flare of Mira A.

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