Accurate determination of stellar rotation periods is important for estimating stellar ages as well as for understanding stellar activity and evolution. While for about thirty thousand stars in the Kepler field rotation periods can be determined, there are over hundred thousand stars, especially with low photometric variability and irregular pattern of variations, for which rotational periods are unknown. Here, we investigate the effect of metallicity on the detectability of rotation periods. This is done by synthesising light curves of hypothetical stars, which are identical to our Sun, with the exception of the metallicity. These light curves are then used as an input to the period determination algorithms. We find that the success rate for recovering the rotation signal has a minimum close to the solar metallicity value. This can be explained by the compensation effect of facular and spot contributions. In addition, selecting solar-like stars with near-solar effective temperature, near solar photometric variability, and with metallicity between M/H = -0.35 and M/H = 0.35 from the Kepler sample, we analyse the fraction of stars for which rotational periods have been detected as a function of metallicity. In agreement with our theoretical estimate we found a local minimum for the detection fraction close to the solar metallicity. We further report rotation periods of 87~solar-like Kepler stars for the first time.
We examine how the fraction $f$ of stars for which rotational modulation has been detected in Kepler light curves depends on the stellar mass $M_\star$ and age $t_\star$. Our sample consists of $\approx 850$ FGK stars hosting transiting planet candidates detected from the prime Kepler mission. For these stars, atmospheric parameters have been derived using high-resolution spectra from the California-Kepler survey, and rotational modulation has been searched in Kepler light curves homogeneously. We fit stellar models to the atmospheric parameters, Gaia parallax, and 2MASS magnitude of these sta…
The Be star phenomenon is related to fast rotation, although the cause of this fast rotation is not yet clearly established. The basic effects of fast rotation on the stellar structure are reviewed: oblateness, mixing, anisotropic winds. The processes governing the evolution of the equatorial velocity of a single star (transport mechanisms and mass loss) are presented, as well as their metallicity dependence. The theoretical results are compared to observations of B and Be stars in the Galaxy and the Magellanic Clouds.
Stellar rotation is a complex function of mass, metallicity, and age and can be altered by binarity. To understand the importance of these parameters in main sequence stars, we have assembled a sample of observations that spans a range of these parameters using a combination of observations from The Transiting Exoplanet Survey Satellite (TESS) and the Kepler Space Telescope. We find that while we can measure rotation periods and identify other classes of stellar variability (e.g., pulsations) from TESS lightcurves, instrument systematics prevent the detection of rotation signals longer than th…
The rotation rates and magnetic activity of Sun-like and low-mass (< 1.4 Msun) main-sequence stars are known to decline with time, and there now exist several models for the evolution of rotation and activity. However, the role that chemical composition plays during stellar spin-down has not yet been explored. In this work, we use a structural evolution code to compute the rotational evolution of stars with three different masses (0.7, 1.0, and 1.3Msun and six different metallicities, ranging from [Fe/H]=-1.0 to [Fe/H]=+0.5. We also implement three different wind-braking formulations from the …
A curious rotation period distribution in the Color-Magnitude-Period Diagram (CMPD) of the Kepler field was recently revealed, thanks to data from Gaia and Kepler spacecraft. It was found that redder and brighter stars are spinning slower than the rest of the main sequence. On the theoretical side, it was demonstrated that metallicity should affect the rotational evolution of stars as well as their evolution in the Hertzprung-Rüssel or Color-Magnitude diagram. In this work we combine this dataset with medium and high resolution spectroscopic metallicities and carefully select main sequence sin…
Over 2,000 stars were observed for one month with a high enough cadence in order to look for acoustic modes during the survey phase of the Kepler mission. Solar-like oscillations have been detected in about 540 stars. The question of why no oscillations were detected in the remaining stars is still open. Previous works explained the non-detection of modes with the high level of magnetic activity. However, the studied stars contained some classical pulsators and red giants that could have biased the results. In this work, we revisit this analysis on a cleaner sample of 1,014 main-sequence solar…
Exoplanet and brown dwarf atmospheres commonly show signs of disequilibrium chemistry. In the James Webb Space Telescope era high resolution spectra of directly imaged exoplanets will allow the characterization of their atmospheres in more detail, and allow systematic tests for the presence of chemical species that deviate from thermochemical equilibrium in these atmospheres. Constraining the presence of disequilibrium chemistry in these atmospheres as a function of parameters such as their effective temperature and surface gravity will allow us to place better constrains in the physics govern…
Evolution of massive Be and Oe stars at low metallicity towards the Long Gamma Ray bursts\footnote{Based on ESO runs 069.D-0275(A), 072.D-0245(A) and (C).}
Winds of massive stars are an important ingredient in determining their evolution, final remnant mass, and feedback to the surrounding interstellar medium. We compare empirical results for OB star winds at low metallicity with theoretical predictions. Observations suggest very weak winds at SMC metallicity, but there are exceptions. We identified promising candidates for rotationally enhanced mass-loss rates with two component wind and partially stripped stars hiding among OB stars with slow but dense wind in the SMC. A preliminary analysis of these systems, derived parameters, and their impli…
Ultra-precise astrometry from the Gaia mission is expected to lead to astrometric detections of more than 20,000 exoplanets in our Galaxy. One of the factors that could hamper such detections is the astrometric jitter caused by the magnetic activity of the planet host stars. In our previous study, we modeled astrometric jitter for the Sun observed equator-on. In this work, we generalize our model and calculate the photocenter jitter as it would be measured by the Gaia and Small-JASMINE missions for stars with solar rotation rate and effective temperature, but with various values of the inclina…
Binarity at LOw Metallicity (BLOeM): Projected rotational velocities
Instituto de Astrofísica de Canarias（加那利天体物理研究所）；Universidad de La Laguna（拉各纳大学）；Centro de Astrobiologìa (CSIC–INTA)（天体生物学中心（西班牙高等科学研究理事会-国家航空航天技术研究所））；Royal Observatory of Belgium（比利时皇家天文台）；Astrophysics Research Centre, School of Mathematics & Physics, Queen’s University, Belfast（贝尔法斯特女王大学数学与物理学院天体物理研究中心）；Argelander-Institut für Astronomie, Universität Bonn（波恩大学阿尔冈德天文学研究所）；Max-Planck-Institute for Astrophysics（马克斯·普朗克天体物理学研究所）
Tidal dissipation in rotating low-mass stars and implications for the orbital evolution of close-in planets I. From the PMS to the RGB at solar metallicity
在恒星模型中必须考虑恒星与行星的相互作用，以理解近距离行星的动力学演化。潮汐相互作用对恒星结构和自转演化的依赖性具有特殊的重要性，应正确对待。我们量化的潮汐耗散的对流信封旋转的低质量恒星的演变，从主序前的红巨星分支取决于初始恒星质量。我们调查的后果，这种演变对行星轨道的演变。我们将Mathis（2015）中描述的潮汐耗散形式与恒星演化代码STAREVOL相结合，并将其应用于质量在0.3和1.4 M $_\odot$之间的旋转恒星。此外，我们通过使用恒星演化高级阶段的新潮汐耗散预测来跟踪近距离行星的轨道演化，从而推广了Bolmont & Mathis（2016）的工作。在PMS上，潮汐耗散的演化受控于收缩星内部结构的演化。在MS上，它受到表面旋转变化的强烈驱动，而表面旋转变化受到磁化恒星风制动的影响。考虑到恒星的旋转演化的主要影响是降低潮汐耗散强度约四个数量级的主序列，相比之下，归一化的耗散率，只考虑到结构的变化。耗散的演化强烈依赖于恒星内部结构和自转的演化。从前主序星到红巨星分支的顶端，它的变化有几个数量级，对近距离大质量行星的轨道演化有很大的影响。这些效应在主序前最强，意味着行星主要对恒星的早期历史敏感。
Using the stellar evolution code---Modules for Experiments in Stellar Astrophysics (MESA), we investigate the evolution of massive stars with different rotational velocities and metallicities towards Wolf-Rayet stars. In our simulations, the initial rotating velocities are taken as 0, 250, 500 and 650 km s$^{-1}$, and the metallicities equal to 0.02, 0.014, 0.008, 0.006, 0.004 and 0.002. We show our rapid rotation models in the HR diagram compared with the observations. We find that the rotational mixing is less efficient at high metallicity, and these stars become Wolf-Rayet (WR) stars when t…
The lithium abundances for a large sample of red giants
在考虑非LTE校正的情况下，推导了378 G/K巨星的锂丰度。其中，有23颗恒星拥有行星系统。锂丰度的研究，作为金属丰度，有效温度和旋转速度的函数，以及一个巨大的行星对G/K巨星的影响。结果表明，锂丰度是金属丰度和有效温度的函数。锂丰度与vsini $<$ 10 km s $^{-1}$处的自转速度没有相关性。具有行星的巨行星存在较低的锂丰度和缓慢的旋转速度（vsini $<$ 4 km s $^{-1}$）。我们的样本包括3颗富锂G/K巨星，36颗锂正常星和339颗锂耗尽星。该样品中富锂恒星的比例与文献中小于1 $\%$的一般比例一致，并且显示正常Li量的恒星被认为在当前星际介质中具有相同的丰度。对于贫锂巨星，许多中等质量（1.5-5 M $_{\odot}$）的G/K巨星在主序星阶段可能已经发生了缺锂。最后，我们提出了锂丰度和运动学参数的一个扩大的样本的565巨星使用的文献汇编，并确认锂丰度是金属丰度和有效温度的函数。利用放大的样品，我们研究了薄盘巨星和厚盘巨星中锂丰度的差异，发现厚盘巨星中锂丰度比薄盘巨星中的锂丰度更贫化。
Metal-Poor Stars as Metal-Rich Impostors via Planet Engulfment
Yunnan Observatories, Chinese Academy of Sciences（中国科学院云南天文台）；University of Chinese Academy of Sciences（中国科学院大学）；International Centre of Supernovae, Yunnan Key Laboratory of Supernova Research, Yunnan Observatories, Chinese Academy of Sciences（中国科学院云南天文台超新星研究中心（国际超新星中心、云南省超新星研究重点实验室））；School of Physics and Astronomy, Sun Yat-sen University（中山大学物理学院）
Understanding the nature of the first stars is key to understanding the early universe. With new facilities such as JWST we may soon have the first observations of the earliest stellar populations, but to understand these observations we require detailed theoretical models. Here we compute a grid of stellar evolution models using the Geneva code with the aim to improve our understanding of the evolution of zero-metallicity stars, with particular interest in how rotation affects surface properties, interior structure, and metal enrichment. We produce a range of models of initial masses (Mini) f…
Most neutron capture elements have a double production by r- and s-processes, but the question of production sites is complex and still open. Recent studies show that including stellar rotation can have a deep impact on nucleosynthesis. We studied the evolution of Sr and Ba in the Milky Way. A chemical evolution model was employed to reproduce the Galactic enrichment. We tested two different nucleosynthesis prescriptions for s-process in massive stars, adopted from the Geneva group and the Rome group. Rotation was taken into account, studying the effects of stars without rotation or rotating w…
Exoplanet surveys of evolved stars have provided increasing evidence that the formation of giant planets depends not only on stellar metallicity ([Fe/H]), but also the mass ($M_\star$). However, measuring accurate masses for subgiants and giants is far more challenging than it is for their main-sequence counterparts, which has led to recent concerns regarding the veracity of the correlation between stellar mass and planet occurrence. In order to address these concerns we use HIRES spectra to perform a spectroscopic analysis on an sample of 245 subgiants and derive new atmospheric and physical …
In the search of a sample of metal-poor bright giants using Str{ö}mgren photometry, we serendipitously found a sample of 26 young (ages younger than 1 Gyr) metal-rich giants, some of which have high rotational velocities.We determined the chemical composition and rotational velocities of these stars in order to compare them with predictions from stellar evolution models. These stars where of spectral type A to B when on the main sequence, and we therefore wished to compare their abundance pattern to that of main-sequence A and B stars.Stellar masses were derived by comparison of the position o…
Context. Radial velocity (RV) measurements induced by the presence of planets around late-type stars are contaminated by stellar signals that are of the order of a few meters per second in amplitude, even for the quietest stars. Those signals are induced by acoustic oscillations, convective granulation patterns, active regions co-rotating with the stellar surface, and magnetic activity cycles. Aims. This study investigates the properties of all coherent stellar signals seen on the Sun on timescales up to its sidereal rotational period. By combining HARPS and HARPS-N solar data spanning several…
NGC1850 is the nearest Young Massive Cluster of the Local Group with a mass similar to those of Galactic globular clusters. Recent studies have revealed an extended morphology of its MSTO, which can be interpreted as a spread in either age or internal rotation. An accurate spectroscopic determination of its chemical properties is still missing. We analyse spectra obtained with MUSE in adaptive optics mode of 1167 stars in both components of this cluster (NGC1850A and NGC1850B). Thanks to this dataset, we measure an average metallicity of <[M/H]>=-0.31 +/- 0.01, a mean Ba abundance of <[Ba/Fe]>…
Spectroscopy for 247 stars towards the young cluster NGC 346 in the Small Magellanic Cloud has been combined with that for 116 targets from the VLT-FLAMES Survey of Massive Stars. Spectral classification yields a sample of 47 O-type and 287 B-type spectra, while radial-velocity variations and/or spectral multiplicity have been used to identify 45 candidate single-lined systems, 17 double-lined systems, and one triple-lined system. Atmospheric parameters (T$_eff$ and log$g$) and projected rotational velocities ($v_e$sin$i$) have been estimated using TLUSTY model atmospheres; independent estimat…
Low-metallicity massive single stars with rotation. Evolutionary models applicable to I Zwicky 18
大规模的旋转的单星的初始金属成分适合矮星系I Zw 18（[Fe/H]= $-$ 1.7）的氢燃烧过程中的初始质量为9-300 M $_{\odot}$和0-900 km s $^{-1}$的旋转速度。在这些模型中的内部混合过程进行了校准的基础上观测到的样本中的麦哲伦星云的OB型恒星。即使是中等速度的旋转，这可能是丰富的金属丰度，发现进行有效的混合诱导旋转导致准化学均匀的演变。这些不断发展的模型在核心氢燃烧期间达到高达90 kK的有效温度。这一点，加上它们的中等质量损失率，使它们成为透明风紫外线强恒星（TWUIN星），它们的预期数量可能解释在I Zw 18和其他低金属量HeII星系中观察到的HeII电离光子通量。在$\sim$ 80 M $_{\odot}$以上的缓慢旋转恒星在核心氢燃烧过程中演化成B-到M-型超巨星，在I Zw 18的距离上，视星等高达$^{\mathrm{m}}$。这两种类型的恒星，TWUIN星和发光的晚型超巨星，都只在低金属丰度下被预测。在低金属丰度的大规模恒星演化的性质不同，从富含金属的环境。我们的网格可以用来解释本地恒星形成矮星系和高红移星系的观测，以及我们的银河系和它的球状星团的贫金属成分。
From the Milky Way to the Early Universe: How Metallicity Shapes the Formation of Double Neutron Stars
University of Florida（佛罗里达大学）；Harvard Society of Fellows（哈佛学会）；Center for Astrophysics | Harvard & Smithsonian（天体物理中心（哈佛大学与史密森尼学会））；Université de Genève（日内瓦大学）；The Adler Planetarium（阿德勒天文馆）；Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA)（天体物理学跨学科探索与研究中心）
Mass-loss rates and terminal wind velocities are key parameters that determine the kinetic wind energy and momenta of massive stars. Furthermore, accurate mass-loss rates determine the mass and rotational velocity evolution of mass stars, and their fates as neutron stars and black holes in function of metallicity (Z). Here we update our Monte Carlo mass-loss Recipe with new dynamically-consistent computations of the terminal wind velocity -- as a function of Z. These predictions are particularly timely as the HST ULLYSES project will observe ultraviolet spectra with blue-shifted P Cygni lines …
