AB Dor A的毫角秒射电结构
The milliarcsecond-scale radio structure of AB Dor A
摘要：
背景：快速旋转的前主序星AB Dor A是一个强大而持久的无线电发射体。异常的日冕耀斑活动被认为是致密射电辐射和其他相关现象的起源，如大弹弓发射。目的：研究AB D或A附近的射电辐射机制和毫角秒射电结构。方法：我们进行了相位参考VLBI观测在22.3 GHz，8.4 GHz和1.4 GHz超过十年使用澳大利亚VLBI阵列。结果如下：我们的8.4 GHz的图像显示了一个双核心晕形态，在所有的时代相似，发射延伸在5和18恒星半径之间的高度。此外，8.4 GHz地图的序列显示了观测时间内源结构的明显变化。然而，1.4 GHz和22.3 GHz的图像与紧凑的源兼容。8.4 GHz和1.4 GHz的相位参考位置与该星著名的毫角秒精度天体测量所预测的一致，而22.3 GHz的位置在西北方向偏离预测值4 $σ$。这一抵消的起源尚不清楚。结论：我们已经考虑了几个模型来解释的形态和演化的内部无线电结构检测到AB Dor A，其中包括从恒星极冠，耀斑，磁驱动的环结构，并存在头盔流光的发射。AB Dor A的一个可能的近伴星也被研究过。我们的研究结果证实了这颗恒星非凡的日冕磁活动，能够在非常大的高度产生紧凑的射电结构，到目前为止，只有在二元相互作用系统中才能看到。
英文摘要：
Context: The fast rotator, pre-main sequence star AB Dor A is a strong and persistent radio emitter. The extraordinary coronal flaring activity is thought to be the origin of compact radio emission and other associated phenomena as large slingshot prominences. Aim: We aim to investigate the radio emission mechanism and the milliarcsecond radio structure around AB Dor A. Methods: We performed phase-referenced VLBI observations at 22.3 GHz, 8.4 GHz, and 1.4 GHz over more than one decade using the Australian VLBI array. Results: Our 8.4 GHz images show a double core-halo morphology, similar at all epochs, with emission extending at heights between 5 and 18 stellar radii. Furthermore, the sequence of the 8.4 GHz maps shows a clear variation of the source structure within the observing time. However, images at 1.4 GHz and 22.3 GHz are compatible with a compact source. The phase-reference position at 8.4 GHz and 1.4 GHz are coincident with those expected from the well-known milliarcsecond-precise astrometry of this star, meanwhile the 22.3 GHz position is 4$σ$ off the prediction in the north-west direction. The origin of this offset is still unclear. Conclusions: We have considered several models to explain the morphology and evolution of the inner radio structure detected in AB Dor A which include emission from the stellar polar caps, a flaring, magnetically-driven loop structure, and the presence of helmet streamers. A possible close companion to AB Dor A has been also investigated. Our results confirm the extraordinary coronal magnetic activity of this star, able to produce compact radio structures at very large heights, so far only seen in binary interacting systems.
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