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arXiv 1402.0073astro-ph.SR
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冷星紫外线发射的波动和耀斑：系外行星凌日观测的意义

Fluctuations and Flares in the Ultraviolet Line Emission of Cool Stars: Implications for Exoplanet Transit Observations

R. O. Parke Loyd, Kevin France

备注：
28 pages, 18 figures, 7 tables
发表信息：
The Astrophysical Journal Supplement 211 (2014) 9
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摘要：

恒星通量的变化可能会掩盖过境行星的光度信号。这种变化性以前没有在用于探测热彗星周围膨胀大气的紫外线中得到很好的表征。因此，我们调查了38颗F-M星在四个狭窄的光谱波段的强度变化：两个封闭的强线来自已知居住在热木星大气中的物种，CII $λλ$ 1334，1335和SiIII $λ$ 1206;一个封闭的SiIV $λλ$ 1393，1402;和36.5埃的散布连续谱。对于每个星/波段组合，我们从档案HST COS和STIS时间标记的光子数据生成60秒的节奏光变曲线。在这些光变曲线中，我们将耀斑和随机波动描述为不同形式的变化。耀斑：我们使用互相关方法检测到116个耀斑。这些事件在时间序列中平均每2.5小时发生一次，超过50%持续4分钟或更短，并且大多数在SiIV中产生最强的响应。如果耀斑发生在60分钟的凌日测量期间，90/116将破坏地球，27/116海王星和7/116木星的信号，通量的向上偏差范围为静止水平的1-109%。波动：光子噪声和潜在的恒星波动在静态数据中产生散射。我们将恒星波动建模为具有标准差$σ_x$的高斯白噪声。对于60 s测量，$σ_x$的极大似然值范围为1-41%。这些值表明，许多冷星将只允许过境检测到高置信度的紫外线共振线，如果掩星盘的半径是$\gtrsim$ 1 R $_J$。然而，对于某些M矮星，这个极限可以低至几个$R_{Earth}$。

英文摘要：

Variations in stellar flux can potentially overwhelm the photometric signal of a transiting planet. Such variability has not previously been well-characterized in the ultraviolet lines used to probe the inflated atmospheres surrounding hot Jupiters. Therefore, we surveyed 38 F-M stars for intensity variations in four narrow spectroscopic bands: two enclosing strong lines from species known to inhabit hot Jupiter atmospheres, CII $λλ$1334,1335 and SiIII $λ$1206; one enclosing SiIV $λλ$1393,1402; and 36.5 angstroms of interspersed continuum. For each star/band combination, we generated 60 s cadence lightcurves from archival HST COS and STIS time-tagged photon data. Within these lightcurves, we characterized flares and stochastic fluctuations as separate forms of variability. Flares: We used a cross-correlation approach to detect 116 flares. These events occur in the time-series an average of once per 2.5 h, over 50% last 4 min or less, and most produce the strongest response in SiIV. If the flare occurred during a transit measurement integrated for 60 min, 90/116 would destroy the signal of an Earth, 27/116 Neptune, and 7/116 Jupiter, with the upward bias in flux ranging from 1-109% of quiescent levels. Fluctuations: Photon noise and underlying stellar fluctuations produce scatter in the quiescent data. We model the stellar fluctuations as Gaussian white noise with standard deviation $σ_x$. Maximum likelihood values of $σ_x$ range from 1-41% for 60 s measurements. These values suggest that many cool stars will only permit a transit detection to high confidence in ultraviolet resonance lines if the radius of the occulting disk is $\gtrsim$ 1 R$_J$. However, for some M dwarfs this limit can be as low as several $R_{Earth}$.

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