牛郎星-X射线中“最热”的磁活动恒星
Altair - the 'hottest' magnetically active star in X-rays
摘要：
A7恒星牛郎星是最热的磁活动恒星之一。它靠近太阳，可以详细研究具有浅对流区的恒星的X射线日冕。我们使用了一个深XMM牛顿观测牛郎星和分析X射线光变曲线，光谱和发射线，调查的时间行为和性质的X射线发射等离子体和研究全球日冕结构。我们发现牛郎星的日冕主要位于低纬地区，其X射线光度L_X =1.4 × 10^27 erg/s，活动水平很低，log L_X/L_bol =-7.4。在低密度下，X射线发射主要由冷等离子体（1-4 MK）主导，元素丰度表现出类似太阳的FIP效应和Ne/O比。X射线亮度在观测过程中变化了30%，很可能是由于旋转调制和微小的活动;相比之下，没有检测到强烈的耀斑或大量的热等离子体。牵牛星的X射线活动水平显然接近饱和水平，与晚型恒星相比，大约减少了四个数量级。凭借其快速旋转，牛郎星提供了一个效率低下，但非常稳定的发电机，主要在赤道周围“较冷”表面区域下方的对流层中运行。这种发电机机制导致了磁活动，并导致了X射线的性质，总体上与静止的太阳非常相似，尽管底层恒星非常不同。
英文摘要：
The A7 star Altair is one of the hottest magnetically active stars. Its proximity to the Sun allows a detailed investigation of a corona in X-rays for a star with a shallow convection zone. We used a deep XMM-Newton observation of Altair and analyzed X-ray light curves, spectra, and emission lines, investigated the temporal behavior and properties of the X-ray emitting plasma and studied the global coronal structure. We find that Altair's corona with an X-ray luminosity of L_X =1.4 x 10^27 erg/s and a very low activity level of log L_X/L_bol = -7.4, is located predominantly at low latitude regions. The X-ray emission is dominated by cool plasma (1-4 MK) at low density, and elemental abundances exhibit a solar-like FIP effect and Ne/O ratio. The X-ray brightness varies by 30 % over the observation, most likely due to rotational modulation and minor activity; in contrast, no strong flares or significant amounts of hot plasma were detected. The X-ray activity level of Altair is apparently close to the saturation level, which is reduced by roughly four orders of magnitude when compared to late-type stars. With its fast rotation, Altair provides an inefficient, but very stable dynamo that mainly operates in convective layers below its 'cooler' surface areas around the equator. This dynamo mechanism results in magnetic activity and leads to X-ray properties that are overall very similar to those of the quiescent Sun, despite very different underlying stars.
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