An X-ray activity cycle on the young solar-like star ɛ Eridani
An X-ray activity cycle on the young solar-like star ɛ Eridani
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年轻的类太阳恒星 Eridani 的 X 射线活动周期
DOI：
10.1051/0004-6361/201936479
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发表时间：
2020
影响因子：
6.5
通讯作者：
Ducci, L.
中科院分区：
文献类型：
--
作者：
Coffaro, M.;Stelzer, B.;Orlando, S.;Hall, J.;Metcalfe, T. S.;Wolter, U.;Mittag, M.;Sanz-Forcada, J.;Schneider, P. C.;Ducci, L.
色球Ca II活动周期经常在晚型恒星中发现，但还没有系统的程序来寻找它们的日冕X射线对应物。Ca II活动周期的典型时间尺度从几年到几十年不等。因此，需要长期的任务来探测日冕对应物。到目前为止，XMM-牛顿卫星已经探测到了五颗恒星的X射线周期。一个特别有趣的问题是，X射线周期在什么年龄（以及什么活动水平）开始。为此，在2015年，我们开始对年轻的类太阳星星波江座进行X射线监测，之前曾两次观察到：2003年和2015年初，都是由XMM-牛顿。它的年龄为4.4亿年，是最年轻的类太阳恒星之一，拥有已知的色球Ca II周期。从2002年开始，我们收集了波江座最新的威尔逊山S指数数据，包括以前未发表的数据。我们发现，Ca Ⅱ周期持续2.92 ± 0.02年，与过去的结果一致。从长期的XMM-牛顿光曲线，我们发现明确的和系统的X射线变化，我们的目标，与色球Ca II周期一致。X射线的平均光度为2 × 1028 erg s−1，振幅在整个周期中只有2倍。我们采用一种新的方法来描述的日冕发射测量分布的波江座的太阳磁结构的演变：活动区，活动区的核心，并在不同的填充分数覆盖恒星表面的耀斑。这三种类型的磁结构的组合只能描述观测到的波江座的X射线发射测量，如果太阳耀斑发射测量分布仅限于衰变阶段的事件。这种解释是，波江座日冕中的耀斑比太阳耀斑持续的时间更长。我们把这归因于波江座的金属丰度较低。我们的分析还表明，波江座的X射线循环强烈主导的核心活动区。在整个周期中，地核的覆盖率的变化与X射线光度的变化相同。周期的最大值的特点是由一个高百分比的覆盖分数的耀斑，一致的事实，即耀斑事件被认为是在相应的短期X射线光变曲线主要在周期的最大值。因此，波江座整个周期的高X射线发射是由其表面上高比例的磁性结构解释的。
Chromospheric Ca II activity cycles are frequently found in late-type stars, but no systematic programs have been created to search for their coronal X-ray counterparts. The typical time scale of Ca II activity cycles ranges from years to decades. Therefore, long-lasting missions are needed to detect the coronal counterparts. TheXMM-Newtonsatellite has so far detected X-ray cycles in five stars. A particularly intriguing question is at what age (and at what activity level) X-ray cycles set in. To this end, in 2015 we started the X-ray monitoring of the young solar-like starɛEridani, previously observed on two occasions: in 2003 and in early 2015, both byXMM-Newton. With an age of 440 Myr, it is one of the youngest solar-like stars with a known chromospheric Ca II cycle. We collected the most recent Mount Wilson S-index data available forɛEridani, starting from 2002, including previously unpublished data. We found that the Ca II cycle lasts 2.92 ± 0.02 yr, in agreement with past results. From the long-termXMM-Newtonlightcurve, we find clear and systematic X-ray variability of our target, consistent with the chromospheric Ca II cycle. The average X-ray luminosity is 2 × 1028erg s−1, with an amplitude that is only a factor of 2 throughout the cycle. We apply a new method to describe the evolution of the coronal emission measure distribution ofɛEridani in terms of solar magnetic structures: active regions, cores of active regions, and flares covering the stellar surface at varying filling fractions. Combinations of these three types of magnetic structures can only describe the observed X-ray emission measure ofɛEridani if the solar flare emission measure distribution is restricted to events in the decay phase. The interpretation is that flares in the corona ofɛEridani last longer than their solar counterparts. We ascribe this to the lower metallicity ofɛEridani. Our analysis also revealed that the X-ray cycle ofɛEridani is strongly dominated by cores of active regions. The coverage fraction of cores throughout the cycle changes by the same factor as the X-ray luminosity. The maxima of the cycle are characterized by a high percentage of covering fraction of the flares, consistent with the fact that flaring events are seen in the corresponding short-term X-ray lightcurves predominately at the cycle maxima. The high X-ray emission throughout the cycle ofɛEridani is thus explained by the high percentage of magnetic structures on its surface.
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DOI：
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期刊：
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