Short-term variability and mass loss in Be stars III. BRITE and SMEI satellite photometry of 28 Cygni
Short-term variability and mass loss in Be stars III. BRITE and SMEI satellite photometry of 28 Cygni
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Be 星 III 的短期变化和质量损失。
DOI：
10.1051/0004-6361/201731187
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发表时间：
2017
影响因子：
6.5
通讯作者：
K. Zwintz
中科院分区：
文献类型：
--
作者：
D. Baade;A. Pigulski;T. Rivinius;A. Carciofi;D. Panoglou;M. Ghoreyshi;G. Handler;R. Kuschnig;A. Moffat;H. Pablo;A. Popowicz;G. Wade;W. Weiss;K. Zwintz
BRITE 纳米卫星星座在 2014 年至 2016 年期间连续 11 个月获得了天鹅座 28 毫米级的光度测量。添加了 2003-2010 年太阳物质抛射成像仪的观测结果和 118 个 H$\alpha$ 线剖面。
几十年来，28 Cyg 表现出了四种大振幅频率：两个间隔紧密的光谱证实的 $g$ 模式频率，接近 1.5 c/d，一个稍低的外光层 (Stefl) 频率，以及在 0.05 c/d 时两种 g 模式之间的差频。这个顶层框架与eta Cen（论文一）没有什么区别，在光谱类型、旋转速率和视角方面也非常相似。 Stefl 频率是唯一似乎不受差频影响的频率。后者的幅度变化较大；在最大值附近，近恒星物质的数量增加，并且 Stefl 频率的幅度增加了一些倍数。在这种增亮过程中，频谱中会出现数十个瞬态尖峰，集中在三组中。所有年份的 BRITE 观测结果中只有 11 个频率是共同的。
Be恒星似乎是由几个耦合的时钟控制的，其中大多数在几周到几个月的时间尺度上不是很规则，但可以运行数十年。 g 模式与低差频率和/或大气对此的响应的组合表现出明显的非线性。与 eta Cen 一样，差频变化似乎是 28 Cyg 中恒星到盘质量传递调制的主要原因。一组不同频率的分层结构可能达到 Be 现象已知的最长时间尺度。
The BRITE Constellation of nanosatellites obtained mmag photometry of 28 Cygni for 11 months in 2014-2016. Observations with the Solar Mass Ejection Imager in 2003-2010 and 118 H$\alpha$ line profiles were added.
For decades, 28 Cyg has exhibited four large-amplitude frequencies: two closely spaced frequencies of spectroscopically confirmed $g$ modes near 1.5 c/d, one slightly lower exophotospheric (Stefl) frequency, and at 0.05 c/d the difference frequency between the two g modes. This top-level framework is indistinguishable from eta Cen (Paper I), which is also very similar in spectral type, rotation rate, and viewing angle. The Stefl frequency is the only one that does not seem to be affected by the difference frequency. The amplitude of the latter undergoes large variations; around maximum the amount of near-circumstellar matter is increased, and the amplitude of the Stefl frequency grows by some factor. During such brightenings dozens of transient spikes appear in the frequency spectrum, concentrated in three groups. Only eleven frequencies were common to all years of BRITE observations.
Be stars seem to be controlled by several coupled clocks, most of which are not very regular on timescales of weeks to months but function for decades. The combination of g modes to the low difference frequency and/or the atmospheric response to it appears significantly nonlinear. Like in eta Cen, the difference-frequency variability seems the main responsible for the modulation of the star-to-disc mass transfer in 28 Cyg. A hierarchical set of difference frequencies may reach the longest timescales known of the Be phenomenon.