Be星III的短期变率和质量损失。天鹅座28颗卫星的BRITE和SMEI测光
Short-term variability and mass loss in Be stars III. BRITE and SMEI satellite photometry of 28 Cygni
摘要：
BRITE纳米卫星星座在2014-2016年的11个月里获得了28 Cygni的mmag测光。增加了2003-2010年太阳质量抛射成像仪的观测数据和118 H $α$谱线剖面。几十年来，天鹅座28已经显示出四个大振幅的频率：两个间隔很近的频率是光谱学证实的$g$模式，接近1.5 c/d，一个稍低的外光球（Stefl）频率，以及0.05 c/d的两个g模式之间的差频。这个顶层框架与eta Cen（论文I）没有区别，后者在光谱类型、旋转速率和视角方面也非常相似。Stefl频率是唯一不受差频影响的频率。后者的振幅经历了很大的变化;在最大值附近，近拱星物质的量增加，Stefl频率的振幅增长了一些因素。在这样的增亮过程中，几十个瞬态尖峰出现在频谱中，集中在三组中。只有11个频率是共同的所有年份的BRITE观测。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有规律，但却可以运行几十年。g模式与低差频的组合和/或大气对它的响应表现出显著的非线性。与eta Cen一样，差频变率似乎是28 Cyg星盘质量传递调制的主要原因。不同频率的分层集合可以达到Be现象已知的最长时间尺度。
英文摘要：
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$α$ 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.
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