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arXiv 0712.2120astro-ph
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仙女座Z巨星的活动周期及其自转周期

Activity cycle of the giant star of Z Andromedae and its spin period

Elia M. Leibowitz, Liliana Formiggini

摘要:

我们重新分析了共生星Z仙女座的长期光学光变曲线(LC),涵盖了112年的主要视觉观测。在这个LC中可以识别出两个严格周期和一个准周期循环。一个P1=7550 D准周期性的特点是重复时间的爆发事件的共生星。到目前为止,已记录了六起此类事件。在系统的静止状态期间,即在爆发之间的时间间隔中,LC明显地被P2=759.1 d的稳定相干周期调制。这是众所周知的轨道周期的Z和双星系统,已测量光谱。第三个相干周期P3=658.4 d调制爆发期间系统的光学亮度的剧烈波动。我们将爆发现象的触发器和驱动它的时钟归因于太阳型磁发电机循环,该循环在对流和系统巨星的外层中运行。我们认为,在巨恒星磁周期的最大阶段,其表面的强烈活动在一个或两个固定在恒星大气层中并随其旋转的对极区中特别增强,这类斑点可能是围绕恒星一般磁偶极子场的南北两极的活动区。P3周期是双星轨道周期和巨星自转周期差拍的一半。后者则为482或1790 d。如果巨星表面只有一个磁极活动,那么P3就是拍频周期本身,自转周期为352 d。如果这颗巨星以逆行的方向旋转,它也可能是5000 d。我们简要地比较了Z和LC中的这些发现与其他两种原型共生体BF Cyg和YY Her的LC中发现的类似调制。

英文摘要:

We have reanalyzed the long-term optical light curve (LC) of the symbiotic star Z Andromedae, covering 112--yr of mostly visual observations. Two strictly periodic and one quasi-periodic cycles can be identified in this LC. A P1=7550 d quasi periodicity characterizes the repetition time of the outburst episodes of this symbiotic star. Six such events have been recorded so far. During quiescence states of the system, i.e. in time intervals between outbursts, the LC is clearly modulated by a stable coherent period of P2=759.1 d. This is the well known orbital period of the Z And binary system that have been measured also spectroscopically. A third coherent period of P3=658.4 d is modulating the intense fluctuations in the optical brightness of the system during outbursts. We attribute the trigger of the outbursts phenomenon and the clock that drives it, to a solar type magnetic dynamo cycle that operates in the convection and the outer layers of the giant star of the system. We suggest that the intense surface activity of the giant star during maximum phases of its magnetic cycle is especially enhanced in one or two antipode regions, fixed in the atmosphere of the star and rotating with it. Such spots could be active regions around the North and the South poles of a general magnetic dipole field of the star. The P3 periodicity is half the beat of the binary orbital period of the system and the spin period of the giant. The latter is then either 482 or 1790 d. If only one pole is active on the surface of the giant, P3 is the beat period itself, and the spin period is 352 d. It could also be 5000 d if the giant is rotating in retrograde direction. We briefly compare these findings in the LC of Z And to similar modulations that were identified in the LC of two other prototype symbiotics, BF Cyg and YY Her.

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