Variability observed in photometric lightcurves of late-type stars (on timescales longer than a day) is a dominant noise source in exoplanet surveys and results predominantly from surface manifestations of stellar magnetic activity, namely faculae and spots. The implementation of faculae in lightcurve models is an open problem, with scaling typically based on spectra equivalent to hot stellar atmospheres or assuming a solar-derived facular contrast. We modelled rotational (single period) lightcurves of active G2, K0, M0 and M2 stars, with Sun-like surface distributions and realistic limb-dependent contrasts for faculae and spots. The sensitivity of lightcurve variability to changes in model parameters such as stellar inclination, feature area coverage, spot temperature, facular region magnetic flux density and active band latitudes is explored. For our lightcurve modelling approach we used actress, a geometrically accurate model for stellar variability. actress generates 2-sphere maps representing stellar surfaces and populates them with user-prescribed spot and facular region distributions. From this, lightcurves can be calculated at any inclination. Quiet star limb darkening and limb-dependent facular contrasts were derived from MURaM 3D magnetoconvection simulations using ATLAS9. 1D stellar atmosphere models were used for the spot contrasts. We applied actress in Monte-Carlo simulations, calculating lightcurve variability amplitudes in the Kepler band. We found that, for a given spectral type and stellar inclination, spot temperature and spot area coverage have the largest effect on variability of all simulation parameters. For a spot coverage of 1%, the typical variability of a solar-type star is around 2 parts-per-thousand. The presence of faculae clearly affects the mean brightness and lightcurve shape, but has relatively little influence on the variability.
Surfaces of the Sun and other cool stars are filled with magnetic fields, which are either seen as dark compact spots or more diffuse bright structures like faculae. Both hamper detection and characterisation of exoplanets, affecting stellar brightness and spectra, as well as transmission spectra. However, the expected facular and spot signals in stellar data are quite different, for instance they have distinct temporal and spectral profiles. Consequently, corrections of stellar data for magnetic activity can greatly benefit from the insight on whether the stellar signal is dominated by spots …
University of Graz（格拉茨大学）；Max-Planck-Institut für Sonnensystemforschung（马克斯·普朗克太阳系统研究所）；Massachusetts Institute of Technology（麻省理工学院）；Kyung Hee University（庆熙大学）；Imperial College London（伦敦帝国学院）
Center-to-limb variations of solar active regions: Observations of spots, faculae, and network in the 6173 Å continuum
Leibniz-Institut für Astrophysik Potsdam (AIP)（波茨坦阿斯特拉菲斯研究所）；Universität Potsdam, Institut für Physik und Astronomie（波茨坦大学物理与天文学系）；Evangelisches Gymnasium Kleinmachnow（克莱因马霍瑙福音中学）
How global faculae and network coverage relates to that of sunspots is relevant to the brightness variations of the Sun and Sun-like stars. We extend earlier studies that found the facular-to-sunspot-area ratio diminishes with total sunspot coverage. Chromospheric indices and the total magnetic flux enclosed in network and faculae, referred to here as 'facular indices', are modulated by the amount of facular and network present. We probed the relationship between various facular and sunspot indices through an empirical model that takes into account how active regions evolve. This model was inc…
Stellar rotation periods can be determined by observing brightness variations caused by active magnetic regions transiting visible stellar disk as the star rotates. The successful stellar photometric surveys stemming from the Kepler and TESS observations led to the determination of rotation periods in tens of thousands of young and active stars. However, there is still a lack of information about rotation periods of older and less active stars, like the Sun. The irregular temporal profiles of light curves caused by the decay times of active regions, which are comparable to or even shorter than…
Precise measurements of exoplanets radii are of key importance for our understanding of the origin and nature of these objects. Measurement of the planet radii using the transit method have reached a precision that the effects of stellar surface features have to be taken into account. While the effects from spots have already been studied in detail, our knowledge of the effects caused by faculae is still limited. This is particularly the case for M-stars. Faculae can pose a problem if they are inhomogeneously distributed on the stellar surface. Using the eclipse mapping method, we study the di…
The spotted detached eclipsing binary (DEB) offers insights into starspots on the binary. Three spotted DEBs, KIC 8097825, KIC 6859813, and KIC 5527172, which were observed by the Kepler photometry and LAMOST spectroscopy, are studied in this work. The physical parameters of binaries are determined by binary modeling. The sizes, lifetimes, and single/double-dip ratio (SDR) of starspots are derived by starspot analysis. KIC 8097825 has large starspots. KIC 6859813 has a spot rotation period shorter than its orbital period but the system should be synchronized inferred from timescale estimation.…
Stellar magnetic activity, just like that of the Sun, manifests itself in the form of flares and spots on the surface of the star. In the solar case, the largest flares originate from large active regions. In this work, we present a study of the activity of the star Kepler-411, including spot modeling from planetary transits. Our goal was to search for a connection between the area of starspots with the energy of superflares produced by this star. Kepler-411 is a K2V-type star with an average rotation period of 10.52 days, radius of 0.79 $R_{\odot}$ and a mass of 0.83 $M_{\odot} $, which was o…
Magnetic activity analysis for a sample of G-type main sequence \emph{Kepler} targets
恒星光变曲线的变化是由恒星表面的磁特征（星点和光斑）的旋转调制引起的，可以用来研究宿主恒星的磁性。在本文中，我们使用光变曲线变化的周期性和幅度作为两个代理，由（He et al. 2015）建议，以研究G型主序星\emph{Kepler}目标的大样本的恒星磁性，其旋转周期最近由（McQuillan et al. 2014）确定。通过对两个磁代理的相关性分析，发现：（1）样本中绝大多数恒星的磁代理都是正相关的，负相关、零相关和正相关的百分比分别为$4.27\%$、$6.81\%$和$88.91\%$;（3）随着自转周期的增加，正相关星的相对数减少，负相关星的相对数增加。这些结果表明，自转周期较短的恒星往往在两个代理星之间具有正相关性，并且很大一部分正相关恒星比负相关恒星具有更大的光变幅度（因此磁活动更强烈）。
We developed a dedicated statistical test for a massive detection of spot- and facula-crossing anomalies in multiple exoplanetary transit lightcurves, based on the frequentist $p$-value thresholding. This test was used to augment our algorithmic pipeline for transit lightcurves analysis. It was applied to $1598$ amateur and professional transit observations of $26$ targets being monitored in the EXPANSION project. We detected $109$ statistically significant candidate events revealing a roughly $2:1$ asymmetry in favor of spots-crossings over faculae-crossings. Although some candidate anomalies…
CSI2264: Simultaneous optical and X-ray variability in the pre-main sequence stars of NGC2264. II: Photometric variability, magnetic activity, and rotation in classIII objects and stars with transition disks
We present an analysis of the starspot evolution, the surface differential rotation (SDR), the correlation between chromospheric activity indicators and the spatial connection between chromospheric and photospheric activities on the active star Kepler-411, using time series photometry over 4 years from Kepler, and spectroscopic data from Keck I 10-m telescope and Lijiang 2.4-m telescope. We constructed the light curve by re-performing photometry and reduction from the Target Pixel Files and Cotrending Basis Vectors with a manually redefined aperture using the software PyKE3. An efficient progr…
Technological advances in instrumentation have led to an exponential increase in exoplanet detection and scrutiny of stellar features such as spots and faculae. While the spots and faculae enable us to understand the stellar dynamics, exoplanets provide us with a glimpse into stellar evolution. While the ubiquity of noise (e.g., telluric, instrumental, or photonic) is unavoidable, combining this with increased spectrographic resolution compounds technological challenges. To account for these noise sources and resolution issues, we use a temporal multifractal framework to study data from the SO…
How do the characteristics of starspots influence the triggering of stellar flares? Here we investigate the activity of two K-type stars, similar in every way from mass to rotation periods and planetary systems. Both stars exhibit about a hundred spots, however, Kepler-411 produced 65 superflares, while Kepler-210 presented none. The spots of both stars were characterized using the planetary transit mapping technique, which yields the intensity, temperature, and radius of starspots. The average radius was $(17\pm7) \times 10^3$ km and $(58 \pm 23) \times 10^3$ km, while the intensity ratio wit…
Calibration of Differential Light Curves for Physical Analysis of Starspots
本文提出了详细考虑的方法来校准差分光变曲线精确的物理星点建模。我们使用太阳和星点模型作为测试平台，以突出一些因素，在这种校准，尚未被小心对待。一个明确的成功的程序转换为光赤字曲线的差异光曲线似乎难以实现，但方法，在许多情况下工作。开普勒望远镜长达数年的时间覆盖提供了一个强大的优势，但尚未解决的问题有关的竞争，有时类似的影响，表面差异旋转与斑点数量和大小的演变可以防止正确的斑点覆盖分数的信心恢复在某些情况下。我们还考虑是否光斑检测开普勒和/或必须占。我们得出结论，他们的影响是这样的，绝对测光是不需要现场赤字校准。为了阐明它们的特征，我们重新研究了绝对亮度，差分变化和数百颗开普勒恒星的表观斑点覆盖率之间的相关性，并从Montet et al.（2017）进行了绝对校准。结果与他们的相似，但我们得出的结论有些不同。正如预期的那样，这个活跃的太阳型样本中的大多数恒星都是斑点主导的。部分原因是缺乏较长周期的恒星，在这个样本中光斑优势的证据既稀少又相对薄弱。光斑人口表现出令人费解的缺乏依赖于旋转周期，这引起了问题的明显检测的“光斑”信号在短时间内。
The LAMOST spectra and $\it{Kepler}$ light curves are combined to investigate the detached eclipsing binary KIC 8098300, which shows the O'Connell effect caused by spot/facula modulation. The radial velocity (RV) measurements are derived by using the tomographic spectra disentangling technology. The mass ratio $q = K_1/K_2 = 0.812 \pm 0.007$, and the orbital semi-major axis $a\mathrm{\sin}i = 14.984 \pm 0.048\ R_\odot$ are obtained by fitting the RV curves. We optimize the binary model concerning the spot/facula activity with the code PHOEBE and obtain precise parameters of the orbit including…
Superflares on Solar-Type Stars Observed with Kepler II. Photometric Variability of Superflare-Generating Stars : A Signature of Stellar Rotation and Starspots
