年轻极低质量恒星的变化：来自分光光度监测的两个惊喜
Variability in young very low mass stars: Two surprises from spectrophotometric monitoring
摘要：
我们提出了同步的光度和光谱观测的7个年轻的和高度可变的M矮星在恒星形成区域的猎户座，进行了4个观测之夜与FORS 2在ESO/VLT。所有七个目标显示出显着的光度变化，在I波段，与振幅之间的0.1-0.8马格，光谱，但是，保持显着不变，与光谱类型的变化小于0.5亚型。因此，亮度变化不是由“填充”吸收特征的遮蔽引起的。在$σ$猎户座星系团（年龄$\sim$ 3百万年）中的三个天体除了连续变化外，还表现出强烈的H $α$发射和H $α$变化。它们的行为大多与光球层上方温度为$\sim300$ K，填充因子在0.2-0.4之间的斑点相一致，而不是在更大质量的恒星中观察到的典型热点。$ε$ Ori附近的剩余目标可能更老，显示出类似日食的光变曲线，没有明显的H $α$活动，并且由于拱星物质而更好地表现为可变消光。有趣的是，其中两个没有红外过量发射的证据。我们的研究表明，年轻的极低质量恒星中的高振幅变化可能是由与质量更大的金牛座T星不同的现象引起的，并且在盘消失且吸积停止时可以持续存在。
英文摘要：
We present simultaneous photometric and spectroscopic observations of seven young and highly variable M dwarfs in star forming regions in Orion, conducted in 4 observing nights with FORS2 at ESO/VLT. All seven targets show significant photometric variability in the I-band, with amplitudes between 0.1-0.8 mag, The spectra, however, remain remarkably constant, with spectral type changes less than 0.5 subtypes. Thus, the brightness changes are not caused by veiling that 'fills in' absorption features. Three objects in the $σ$ Ori cluster (age $\sim$3 Myr) exhibit strong H$α$ emission and H$α$ variability, in addition to the continuum variations. Their behaviour is mostly consistent with the presence of spots with temperature of $\sim300$ K above the photosphere and filling factors between 0.2-0.4, in contrast to typical hot spots observed in more massive stars. The remaining targets near $ε$ Ori, likely to be older, show eclipse-like lightcurves, no significant H$α$ activity and are better represented by variable extinction due to circumstellar material. Interestingly, two of them show no evidence of infrared excess emission. Our study shows that high-amplitude variability in young very low mass stars can be caused by different phenomena than in more massive T Tauri stars and can persist when the disk has disappeared and accretion has ceased.
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