We present the results from a systematic study of temporal variation of stellar activity in young late-type stars. We used multi-epoch LAMOST low-resolution spectra of over 300 member candidates in three young open clusters: Pleiades, Praesepe, and Hyades. The spectral measurements of TiO band strength near 7050 Å (TiO2) and equivalent width of Hα line (EW Hα ) are used as the tracers of cool-spot coverage and chromospheric emission strength, respectively. The analysis of time-variation patterns of these two tracers suggested there exist detectable variabilities in TiO2 and EW Hα , and their time-scales are in the wide range from days to years. Results showed that more active stars, younger and fast rotators, tend to have larger activity variations. There is a tendency of anti-correlation between temporal variations in TiO2 and EW Hα . Also, appreciable anti-correlation in the rotational phase between Hα emission and K2 brightness is detected in some M dwarfs, indicating spatial co-location of the plages with cool starspots, however, cool stars do not always show such co-location features. Furthermore, spot coverage and Hα emission were evident at all rotational phases of several M dwarfs, indicating a basal level of activity, perhaps due to many small and randomly located active regions in the atmosphere.
We analyze K2 light curves for 132 low-mass ($1\ \gtrsim\ M_*\ \gtrsim\ 0.1$~${M_{\odot}}$) members of the 600--800~Myr-old Hyades cluster and measure rotation periods ($P_{rot}$) for 116 of these stars. These include 93 stars with no prior $P_{rot}$ measurement; the total number of Hyads with known $P_{rot}$ is now 232. We then combine literature binary data with Gaia DR2 photometry and astrometry to select single star sequences in the Hyades and its roughly coeval Praesepe open cluster, and derive a new reddening value of $A_V = 0.035$$\pm$$0.011$ for Praesepe. Comparing the effective temper…
We analyze spectroscopic and photometric data to determine the projected inclinations of stars in three open clusters: the Pleiades, Praesepe, and M35. We determine the $\sin i$ values of 42, 35, and 67 stars in each cluster, respectively, and from their distributions we find that isotropic spins and moderate alignment are both consistent with the Pleiades and Praesepe data. While it is difficult to distinguish between these scenarios for a single cluster, an ensemble of such distributions may facilitate a distinction. The M35 inclination distribution is most consistent with a superposition of…
Flares, energetic eruptions on the surfaces of stars, are an unmistakable manifestation of magnetically driven emission. Their occurrence rates and energy distributions trace stellar characteristics such as mass and age. But before flares can be used to constrain stellar properties, the flaring-age-mass relation requires proper calibration. This work sets out to quantify flaring activity of independently age-dated main sequence stars for a broad range of spectral types using optical light curves obtained by the Kepler satellite. Drawing from the complete K2 archive, we searched 3435 $\sim 80$ …
TESS flares and chromospheric variability in the Pleiades, the Hyades, and Praesepe
Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences（孔科利天文台，匈牙利研究与创新学院地球科学与天文学研究中心）；CSFK, MTA Centre of Excellence（CSFK，匈牙利科学院卓越中心）；Eötvös Loránd University, Institute of Physics and Astronomy, Department of Astronomy（罗兰大学，物理与天文学研究所，天文学系）
Aims: Our scientific goal is to provide revised membership lists of the Alpha Per, Pleiades, and Praesepe clusters exploiting the second data release of Gaia and produce five-dimensional maps (ra, dec, parallax, pmRA, pmDEC) of these clusters. Methods: We implemented the kinematic method combined with the statistical treatment of parallaxes and proper motions to identify astrometric member candidates of three of the most nearby and best studied open clusters in the sky. Results: We cross-correlated the Gaia catalogue with large-scale public surveys to complement the astrometry of Gaia with mul…
Rotation periods from Kepler K2 are combined with projected rotation velocities from the WIYN 3.5-m telescope, to determine projected radii for fast-rotating, low-mass ($0.15 \leq M/M_{\odot} \leq 0.6$) members of the Praesepe cluster. A maximum likelihood analysis that accounts for observational uncertainties, binarity and censored data, yields marginal evidence for radius inflation -- the average radius of these stars is $6\pm4$ per cent larger at a given luminosity than predicted by commonly-used evolutionary models. This over-radius is smaller (at 2-sigma confidence) than was found for sim…
X-ray observations of low-mass stars in open clusters are critical to understanding the dependence of magnetic activity on stellar properties and their evolution. Praesepe and the Hyades, two of the nearest, most-studied open clusters, are among the best available laboratories for examining the dependence of magnetic activity on rotation for stars with masses lower than $\approx 1\ M_{\odot}$. We present an updated study of the rotation X-ray activity relation in the two clusters. We updated membership catalogs that combine pre-Gaia catalogs with new catalogs based on Gaia Data Release 2. The …
The presence and strength of a stellar magnetic field and activity is rooted in a star's fundamental parameters such as mass and age. Can flares serve as an accurate stellar "clock"? To explore if we can quantify an activity-age relation in the form of a flaring-age relation, we measured trends in the flaring rates and energies for stars with different masses and ages. We investigated the time-domain photometry provided by Kepler's follow-up mission K2 and searched for flares in three solar metallicity open clusters with well-known ages, M45 (0.125 Gyr), M44 (0.63 Gyr), and M67 (4.3 Gyr). We u…
Open clusters exquisitely track the Galactic disc chemical properties and its time evolution; a substantial number of studies and large spectroscopic surveys focus mostly on the chemical content of relatively old clusters (age $\gtrsim$ 1 Gyr). Interestingly, the less studied young counterpart populating the solar surrounding has been found to be solar (at most), with a notable surprising lack of young metal-rich objects. While there is wide consensus about the moderately above-solar composition of the Hyades cluster, the metallicity of Praesepe is still controversial. Recent studies suggest t…
In this study, we continue our project to search for unresolved binary and multiple systems in open clusters exploiting the photometric diagram (H-W2)-W1 vs W2-(BP-K) firstly introduced in \citet{Malofeeva+2022}. In particular, here we estimate the binary and multiple star ratios and the distribution of the component mass ratio $q$ in the Galactic clusters Alpha Persei, Praesepe, and NGC 1039. We have modified the procedure outlined in our first study \citep{Malofeeva+2022} making star counts automatic and by introducing bootstrapping for error estimation. Basing on this, we re-investigated th…
Radius inflation continues to be explored as a peculiar occurrence among magnetically active, low-mass stars. Recently Somers et. al. 2017 showed that radius inflation among low-mass stars in the young open cluster M45 (Pleiades Cluster) is correlated to the rotation rate: faster rotators are more inflated. Here we extend that work to a sample of 68 stars of the older open Hyades Cluster. We derive the stars' spectral energy distributions to measure their bolometric fluxes. With spectroscopically defined $T_{\rm eff}$ and {\it Gaia\/} distances we calculate stellar radii using the Stefan-Boltz…
Astrometric and photometric initial mass functions from the UKIDSS Galactic Clusters Survey: III Praesepe
在过去的几十年里，疏散星团一直是许多研究的主题。考虑到初始质量函数的普适性仍然是当前研究的主题，这样的研究是至关重要的。Praesepe是一个有趣的疏散星团，可以用来研究恒星和亚恒星的质量函数（MF），考虑到它的中间年龄和附近的距离。在这里，我们提出了一个广泛的领域，近红外研究的Praesepe使用数据发布9（DR 9）的UKIRT红外深空调查（UKIDSS）星系团调查（GCS）的结果。我们获得了集群候选人的Praesepe基于3sigma天体测量和5波段光度选择。我们推导出Praesepe在0.2-0.45Msol质量范围内的二进制频率为25.6+/-3.0%，0.1-0.2Msol为19.6+/-3.0%，0.07-0.1Msol为23.2+/-5.6%。我们还研究了Praesepe的集群候选人的可变性，我们得出结论，七个对象可以是可变的。我们推导了Praesepe在Z-和J-波段的光度函数，并导出了它的MF.我们观察到，我们确定的MF Praesepe不同于以前的研究：虽然以前的MF呈现从0.6到0.1Msol的增加，我们的MF显示出下降。我们在星团的两个不同区域，即1.25度以内和1.25度以外，观察了Praesepe的MF，我们观察到这两个区域的MF都降低到较低的质量。在0.072-0.6Msol的质量范围内，我们将我们的结果与毕宿星、昴宿星和α Per MF进行了比较，…
Coma Berenices (Coma Ber), an open cluster about the same age as Praesepe and the Hyades (700-800 Myr) is, despite being only 85 pc away, less well studied than its famous cousins. This is due principally to its sparseness and low proper motion, which together made Coma Ber's membership challenging to establish pre-Gaia. We have curated a new list of its members based on Gaia DR2 astrometry, derived its metallicity and interstellar reddening using LAMOST data, and inferred the cluster's age by fitting PARSEC isochrones to its color$-$magnitude diagram. We then measured rotation periods for Com…
We have analysed K2 light curves for more than 3,000 low mass stars in the $\sim$8 Myr old Upper Sco association, the $\sim$125 Myr age Pleiades open cluster and the $\sim$700 Myr old Hyades and Praesepe open clusters to determine stellar rotation rates. Many of these K2 targets show two distinct periods, and for the lowest mass stars in these clusters virtually all of these systems with two periods are photometric binaries. The most likely explanation is that we are detecting the rotation periods for both components of these binaries. We explore the evolution of the rotation rate in both comp…
The Age-Rotation-Activity Relation: From Myrs to Gyrs
在过去的40年里，观测调查已经建立了恒星的年龄，自转周期和磁场活动之间的紧密关系。这种年龄-自转-活动关系记录了恒星的磁发电机和角动量演化之间的相互作用，并为孤立场恒星提供了一个有价值的年龄估计。虽然年龄旋转活动的关系已被广泛研究，在集群小于5亿年，经验测量的旋转周期是稀缺的老年人。利用帕洛玛瞬变工厂（PTF），我们已经开始了一项恒星自转巡天，以绘制出年龄-自转-活动关系的后期演化：哥伦比亚/康奈尔/加州理工PTF（CCCP）对疏散星团的巡天。第一个CCCP目标是附近的约6亿年Hyades-类似Praesepe，PTF已经产生了超过3个月的光变曲线，包含约650个集群成员的>150个测量值。通过分析这些光变曲线，我们测量了40个K & M星团成员的自转周期，填补了以前报道的太阳型毕宿星团（Radick et al. 1987，Prosser et al. 1995）和少数低质量Praesepe成员（Scholz et al. 2007）的周期之间的空白。我们的测量结果表明，Praesepe的周期颜色关系在过渡的特征光谱类型的~M1 -从一个定义良好的奇异关系在较高的质量，在较低的质量更分散的分布的快速和慢速旋转。这一转变的位置与基于对年轻星团的观测的预期大致一致，并且假设恒星自旋下降是影响约6亿年角动量演化的主要机制。除了介绍我们的Praesepe光度监测的结果，我们总结了CCCP…
We apply for the first time a two-dimensional fitting statistic, $τ^2$, to rotational evolution models (REMs) of stars (0.1 to 1.3 $M_{\odot}$) on the period-mass plane. The $τ^2$ statistic simultaneously considers all cluster rotation data to return a goodness of fit, allowing for data-driven improvement of REMs. We construct data sets for Upper Sco, the Pleiades and Praesepe, to which we tune our REMs. We use consistently determined stellar masses (calculated by matching $K_\textrm{s}$ magnitudes to isochrones) and literature rotation periods. As a first demonstration of the $τ^2$ statistic,…
An investigation of chromospheric activity spanning the Vaughan--Preston gap: impact on stellar ages
色球活动被广泛用作太阳型恒星的年龄指标，其依据是早期证据表明，从年轻和活跃的恒星到年老和不活跃的恒星会有一个平稳的演化过程。我们分析了两个疏散星团中五颗太阳型恒星的色球活动，以研究色球活动如何随时间演变。我们拍摄了IC 4756中3颗星和NGC 5822中2颗星的UVES高分辨率、高信噪比光谱，并结合以前研究的数据集进行了重新分析。深层光球Ca II K线的发射核心被用作色球活动的探测器。新样本中的所有5颗恒星，包括1.2 Gyr-old NGC 5822中的恒星，其活动水平与毕宿二和Praesepe相当。对我们的数据的一个可能的解释是，太阳型恒星的色球活动，从毕星团的年龄到太阳的年龄，并没有平稳地演变。恒星从活跃到不活跃的变化是在很短的时间尺度上发生的。这种转变前后的演变远不如周期性和长期变化那么重要。我们表明，文献中提出的数据，以支持年龄和活动之间的相关性，也可以根据我们的研究结果进行不同的解释。
The Ages of Early-Type Stars: Strömgren Photometric Methods Calibrated, Validated, Tested, and Applied to Hosts and Prospective Hosts of Directly Imaged Exoplanets
Determination of stellar age is a crucial task in astrophysics research. Different methods are nowadays used either model dependent or based on calibrated empirical relations. The most reliable results are generally obtained when different methods are used in complementary way. We propose a new method for the age determination of young stellar associations and open clusters (ages < 125 Myr), which can allow to further constrain the age when used together with other methods. We explore the amplitude of the photometric variability in bins of color and rotation period of five young associations a…
Identifying and characterizing young stars in the Solar neighbourhood is essential to find and describe planets in the early stages of their evolution. This work seeks to identify nearby young stars showing a Lithium 6707.78$\,Å$ absorption line in the GALAH survey. A robust, data-driven approach is used to search for corresponding templates in the pool of 434,215 measured dwarf spectra in the survey. It enables a model-free search for best-matching spectral templates for all stars, including M dwarfs with strong molecular absorption bands. 3147 stars have been found to have measurable Lithium…
