We present a method to build a probability density function (pdf) for the age of a star based on its peculiar velocities $U$, $V$ and $W$ and its orbital eccentricity. The sample used in this work comes from the Geneva-Copenhagen Survey (GCS) which contains both the spatial velocities, orbital eccentricities and isochronal ages for about $14\,000$ stars. Using the GCS stars, we fitted the parameters that describe the relations between the distributions of kinematical properties and age. This parametrization allows us to obtain an age probability from the kinematical data. From this age pdf, we estimate an individual average age for the star using the most likely age and the expected age. We have obtained the stellar age pdf for the age of $9\,102$ stars from the GCS and have shown that the distribution of individual ages derived from our method is in good agreement with the distribution of isochronal ages. We also observe a decline in the mean metallicity with our ages for stars younger than 7 Gyr, similar to the one observed for isochronal ages. This method can be useful for the estimation of rough stellar ages for those stars that fall in areas of the HR diagram where isochrones are tightly crowded. As an example of this method, we estimate the age of Trappist-1, which is a M8V star, obtaining the age of $t(UVW) = 12.50(+0.29-6.23)$ Gyr.
Older Ages for 23 Pre-Main Sequence Stars in Upper Scorpius Using Dynamical Mass-Constrained Stellar Evolutionary Models
University of Arizona（亚利桑那大学）；National Radio Astronomy Observatory（国家射电天文台）；California Institute of Technology（加州理工学院）；National Taiwan Normal University（台湾师范大学）
We present a Bayesian implementation of isochrone fitting in deriving stellar ages and masses, incorporating absolute K magnitude ($\rm M_K$) derived from 2MASS photometry and Gaia DR1 parallax and differentiation between initial bulk metallicity and present day surface metallicity, with allowance for incorporating further constraints (e.g., asteroseismology) when available. As a test, we re-computed stellar ages and masses of $\sim 4000$ stars in the solar neighbourhood from six well-studied literature samples using both Hipparcos and TGAS parallaxes. Our ages are found to be compatible with …
In the era of large stellar spectroscopic surveys, there is emphasis on deriving not only stellar abundances but also ages for millions of stars. In the context of Galactic archeology, stellar ages provide a direct probe of the formation history of the Galaxy. We use the stellar evolution code MESA to compute models with atomic diffusion--with and without radiative acceleration--and extra mixing in the surface layers. The extra mixing consists of both density-dependent turbulent mixing and envelope overshoot mixing. Based on these models we argue that it is important to distinguish between ini…
Oxygen and carbon are important elements in stellar populations. Their behavior refers to the formation history of the stellar populations. C and O abundances would also obviously influence stellar opacities and the overall metal abundance $Z$. With observed high-quality spectroscopic properties, we construct stellar models with C and O elements, to give more accurate ages for 70 metal-poor dwarfs, which have been determined to be high-$α$ halo, low-$α$ halo and thick-disk stars. Our results show that high-$α$ halo stars are relatively older than low-$α$ halo stars by around 2.0 Gyr. The thick…
Stellar ages are key to improving our understanding of different astrophysical phenomena. However, many techniques to estimate stellar ages are highly model-dependent. The lithium depletion boundary (LDB), based on the presence or absence of lithium in low-mass stars, can be used to derive ages in stellar associations of between 20 and 500~Ma. The purpose of this work is to revise former LDB ages in stellar associations in a consistent way, taking advantage of the homogeneous \textit{Gaia} parallaxes as well as bolometric luminosity estimations that do not rely on monochromatic bolometric corr…
Towards breaking the degeneracy between stellar ages and unresolved multiplicity
Universitat de Barcelona（巴塞罗那大学）；Institut d’Estudis Espacials de Catalunya（加泰罗尼亚空间研究所）；Instituto de Astrofísica de Canarias（加那利天体物理研究所）；Universidad de La Laguna（拉古纳大学）；Leibniz-Institut für Astrophysik Potsdam（波茨坦阿诺德·爱因斯坦天体物理研究所）
The advent of Gaia has led to the discovery of nearly 300 elongated stellar associations (called `strings') spanning hundreds of parsecs in length and mere tens of parsecs in width. These newfound populations present an excellent laboratory for studying the assembly process of the Milky Way thin disk. In this work, we use data from GALAH DR3 to investigate the chemical distributions and ages of 18 newfound stellar populations, 10 of which are strings and 8 of which are compact in morphology. We estimate the intrinsic abundance dispersions in [X/H] of each population and compare them with those…
Accurate and precise stellar ages are best determined for stars which are strongly observationally constrained, that is which are intrinsically oscillating. We review here the seismic diagnostics which are sensitive to stellar ages and provide some illustrating examples of seismically age-dated stars.
A deep understanding of the Milky Way galaxy, its formation and evolution requires observations of huge numbers of stars. Stellar photometry, therefore, provides an economical method to obtain intrinsic stellar parameters. With the addition of distance information - deriving reliable ages from photometry is a possibility. We have developed a Bayesian method that generates 2D probability maps of a star's age and metallicity from photometry and parallax using isochrones. Our synthetic tests show that including a near-UV passband enables us to break the degeneracy between a star's age and metalli…
A major source of uncertainty in the age determination of old ($\sim10$ Gyr) integrated stellar populations is the presence of hot horizontal branch (HB) stars. Here, we describe a simple approach to tackle this problem, and show the performance of this technique that simultaneously models the age, abundances and HB properties of integrated stellar populations. For this we compare the results found during the fits of the integrated spectra of a sample of stellar population benchmarks, against the values obtained from the analysis of their resolved CMDs. We find that the ages derived from our s…
The formation of the extended thin disc is the most spectacular event of our Galaxy in the past $\sim8$\,Gyr. To unveil this process, obtaining precise and accurate stellar ages for a large sample of stars is essential although challenging. In this work, we present the asteroseismic age determination of 5306 red giant branch stars using \kepler{} and LAMOST data, with a thorough examination of how the age determination is affected by the choice of different temperature scales and stellar models. Thanks to the high precision of the asteroseismic and spectroscopic parameters of our sample stars,…
I review progress towards understanding the time-scales of star and cluster formation and of the absolute ages of young stars. I focus in particular on the areas in which Francesco Palla made highly significant contributions - interpretation of the Hertzsprung-Russell diagrams of young clusters and the role of photospheric lithium as an age diagnostic.
In this work we present chemical abundances of heavy elements (Z$>$28) for a homogeneous sample of 1059 stars from HARPS planet search program. We also derive ages using parallaxes from Hipparcos and Gaia DR1 to compare the results. We study the [X/Fe] ratios for different populations and compare them with models of Galactic chemical evolution. We find that thick disk stars are chemically disjunt for Zn and Eu. Moreover, the high-alpha metal-rich population presents an interesting behaviour, with clear overabundances of Cu and Zn and lower abundances of Y and Ba with respect to thin disk stars…
Gaia DR3 provides radial velocities for 33 million stars and spectroscopically derived atmospheric parameters for more than five million targets. When combined with the astrometric data, these allow us to derive orbital and stellar parameters that are key in order to understand the stellar populations of the Milky Way and perform galactic archaeology. We use the calibrated atmospheric parameters, 2MASS and Gaia-EDR3 photometry, and parallax-based distances to compute, via an isochrone fitting method, the ages, initial stellar masses and reddenings for the stars with spectroscopic parameters. W…
Although numerous photometric and spectroscopic studies of stars in Galactic globular clusters suggest the existence of several stellar generations in the majority of clusters studied so far, kinematical properties of these stellar generations are still relatively poorly know. In this contribution we present first results of our study of kinematical properties of chemically-tagged stellar populations in the Galactic globular cluster 47 Tuc. Our analysis reveals the existence of three stellar generations in 47 Tuc that are different in their [Li/Na] and [Na/O] ratios. Additionally, we find that…
The cornerstone mission Gaia, together with complementary surveys, will revolutionize our understanding of the formation and history of our Galaxy, providing accurate stellar masses, radii, ages, distances, as well as chemical properties for a very large sample of stars across different Galactic stellar populations.Using an improved population synthesis approach and new stellar evolution models we attempt to evaluate the possibility of deriving ages and masses of clump stars from their chemical properties.A new version of the Besancon Galaxy model (BGM) uses new stellar evolutionary tracks com…
In the present work we analyzed seven globular clusters selected from their location in the Galactic bulge and with metallicity values in the range $-1.30\lesssim\rm{[Fe/H]}\lesssim-0.50$. The aim of this work is first to derive cluster ages assuming single stellar populations, and secondly, to identify the stars from first (1G) and second generations (2G) from the main sequence, subgiant and red giant branches, and to derive their age differences. Based on a combination of UV and optical filters used in this project, we apply the Gaussian mixture models to distinguish the multiple stellar pop…
Stellar ages from [C/N] in giant stars: applicability and limitations
The Australian National University（澳大利亚国立大学）；INAF - Osservatorio di Astrofisica e Scienza dello Spazio（意大利国家天体物理研究所-天文与空间科学观测站）；Università degli Studi di Catania（卡塔尼亚大学）；Università degli Studi di Bologna（博洛尼亚大学）；Vilnius University（维尔纽斯大学）；INAF - Osservatorio Astrofisico di Arcetri（意大利国家天体物理研究所-阿切特里天文台）
The recent Gaia Data Release 1 of stellar parallaxes provides ample opportunity to find metal-poor main-sequence stars with precise parallaxes. We select 21 such stars with parallax uncertainties better than $σ_π/π\leq0.10$ and accurate abundance determinations suitable for testing metal-poor stellar evolution models and determining the distance to Galactic globular clusters. A Monte Carlo analysis was used, taking into account uncertainties in the model construction parameters, to generate stellar models and isochrones to fit to the calibration stars. The isochrones which fit the calibration …
The vast majority of Milky Way stellar halo stars were likely accreted from a small number ($\lesssim$3) of relatively large dwarf galaxy accretion events. However, the timing of these events is poorly constrained, relying predominantly on indirect dynamical mixing arguments or imprecise age measurements of stars associated with debris structures. Here, we aim to infer robust stellar ages for stars associated with galactic substructures to more directly constrain the merger history of the Galaxy. By combining kinematic, asteroseismic, and spectroscopic data where available, we infer stellar ag…
We present estimates of stellar age and mass for 0.93 million Galactic disk main sequence turn-off and sub-giant stars from the LAMOST Galactic Spectroscopic Surveys. The ages and masses are determined by matching with stellar isochrones using Bayesian algorithm, utilizing effective temperature $T_{\rm eff}$, absolute magnitude ${\rm M}_V$, metallicity [Fe/H] and $α$-element to iron abundance ratio [$α$/Fe] deduced from the LAMOST spectra. Extensive examinations suggest the age and mass estimates are robust. The overall sample stars have a median error of 34 per cent for the age estimates, and…
Stellar magnetic activity decays over the main-sequence life of cool stars due to the stellar spin-down driven by magnetic braking. The evolution of chromospheric emission is well-studied for younger stars, but difficulties in determining the ages of older cool stars on the main sequence have complicated such studies for older stars in the past. Here we report on chromospheric Ca II H and K line measurements for 26 main-sequence cool stars with asteroseismic ages older than a gigayear and spectral types F and G. We find that for the G stars and the cooler F-type stars which still have convecti…
Among the various methods used to age-date stars, methods based on stellar model predictions are widely used, for nearly all kind of stars in large ranges of masses, chemical compositions and evolutionary stages. The precision and accuracy on the age determination depend on both the precision and number of observational constraints, and on our ability to correctly describe the stellar interior and evolution. The imperfect input physics of stellar models as well as the uncertainties on the initial chemical composition of stars are responsible for uncertainties in the age determination. We prese…
Solar twins are stars with similar stellar (surface) parameters to the Sun that can have a wide range of ages. This provide an opportunity to analyze the variation of their chemical abundances with age. Nissen (2015) recently suggested that the abundances of the s-process element Y and the $α$-element Mg could be used to estimate stellar ages. This paper aims to determine with high precision the Y, Mg, and Fe abundances for a sample of 88 solar twins that span a broad age range ($0.3-10.0$\,Gyr) and investigate their use for estimating ages. We obtained high-quality Magellan Inamori Kyocera Ec…
Detailed modelling of stellar oscillations is able to give precise estimates for stellar ages, but the inferred results typically depend on the adopted model parameters used for the age inference. High-quality asteroseismic data with precise measurements of mixed modes are available for 36 Kepler subgiants. To obtain a handle on the robustness of the ages for these stars, we first study the dependencies of seismic ages on three model input parameters. We find that inferred ages do not change systematically with the helium fraction (Y) or the mixing-length parameter but depend strongly on the m…
Age Gradients in the Stellar Populations of Massive Star Forming Regions Based on a New Stellar Chronometer
理解大质量恒星形成区（MSFR）恒星形成的一个主要障碍是缺乏可靠的恒星计时器来解开它们复杂的恒星形成历史。我们提出了一个新的估计恒星年龄的新方法，采用近红外（NIR）和X射线光度学，XNUJX。恒星的质量是从金牛座云的X射线光度中使用Lx -质量关系推导出来的。J-波段光度进行比较，以质量依赖的主序前的演化模型来估计年龄。BJX对广泛的演化阶段都很敏感，从嵌入云中的盘状恒星到广泛分散的较老的前主序星。MYStIX（Massive Young Star-Forming Complex Study in Infrared and X-ray）计划使用X-射线、中红外和NIR星表描述了20个OB-dominated MSFR。MYSTIX可能的复杂成员31，784人中，有5525人使用了MYSTIX方法。我们提供了一个均匀的一组中位数年龄超过100个子集群在15 MSFR，中位数子集群年龄范围在0.5百万年和5百万年之间。重要的科学成果是发现了MYStIX地区的年龄梯度。MSFR的年龄分布呈空间分异结构，年龄分布较宽。在分子云中的模糊位置，年龄最小，在显示的星团中居中，在分布的种群中最老。近红外颜色指数J-H，一个替代措施的灭绝，可以作为一个近似的年龄预测年轻的嵌入集群。
Understanding the formation and evolution of our Galaxy requires accurate distances, ages and chemistry for large populations of field stars. Here we present several updates to our spectro-photometric distance code, that can now also be used to estimate ages, masses, and extinctions for individual stars. Given a set of measured spectro-photometric parameters, we calculate the posterior probability distribution over a given grid of stellar evolutionary models, using flexible Galactic stellar-population priors. The code (called {\tt StarHorse}) can acommodate different observational datasets, pr…
Owing to their simplicity and ease of application, seismic scaling relations are widely used to determine the properties of stars exhibiting solar-like oscillations, such as solar twins and red giants. So far, no seismic scaling relations for determining the ages of red giant stars have been developed. Such relations would be desirable for galactic archaeology, which uses stellar ages to map the history of the Milky Way. The ages of red giants must instead be estimated with reference to grids of theoretical stellar models, which can be computationally intensive. Here I present an exhaustive se…
The C+N+O abundance of Omega Centauri giant stars: implications on the chemical enrichment scenario and the relative ages of different stellar populations
Age determination is undertaken for nearby early-type (BAF) stars, which constitute attractive targets for high-contrast debris disk and planet imaging surveys. Our analysis sequence consists of: acquisition of uvbyβ photometry from catalogs, correction for the effects of extinction, interpolation of the photometry onto model atmosphere grids from which atmospheric parameters are determined, and finally, comparison to the theoretical isochrones from pre-main sequence through post-main sequence stellar evolution models, accounting for the effects of stellar rotation. We calibrate and validate o…
1I/'Oumuamua is the first interstellar object observed passing through the Solar System. Understanding the nature of these objects will provide crucial information about the formation and evolution of planetary systems, and the chemodynamical evolution of the Galaxy as a whole. We obtained the galactic orbital parameters of this object, considering 8 different models for the Galaxy, and compared it to those of stars of different ages from the Geneva-Copenhagen Survey (GCS). Assuming that the galactic orbital evolution of this object is similar to that of stars, we applied a Bayesian analyses a…
We present a new age-dating technique that combines gyrochronology with isochrone fitting to infer ages for FGKM main-sequence and subgiant field stars. Gyrochronology and isochrone fitting are each capable of providing relatively precise ages for field stars in certain areas of the Hertzsprung-Russell diagram: gyrochronology works optimally for cool main-sequence stars, and isochrone fitting can provide precise ages for stars near the main-sequence turnoff. Combined, these two age-dating techniques can provide precise and accurate ages for a broader range of stellar masses and evolutionary st…
In this first work attempts to analytically explain the effects on the magnetic braking index, $q$, caused by the evolution of stellar velocity in main-sequence stars, and estimated by de Freitas \& De Medeiros (2013). We have found that the effect of $q$ is here a determining factor for understanding the delicate mechanisms that control the spin-down of stars as a function of the mass of stars. We note that our models predict that the calculated ages are distinct from gyrochronology ages. Indeed, the gyro-ages are measured considering only the canonical value of the Skumanich relation ($q$=3)…
The present work attempts to analytically explain the effects on the moment of inertia, $I$, caused by the evolution of stellar velocity in main-sequence stars. We have found that the effects linked to stellar oblateness can be estimated from the variation of the magnetic braking index denoted by $Δq$. We have also found that the effect of $\dot{I}$ is here a determining factor for understanding the delicate mechanisms that control the spin-down of stars in this evolutionary phase. We note that our models predict that the behaviour of the variations of the braking index is distinct when the st…
Estimating stellar ages is important for advancing our understanding of stellar and exoplanet evolution and investigating the history of the Milky Way. However, ages for low-mass stars are hard to infer as they evolve slowly on the main sequence. In addition, empirical dating methods are difficult to calibrate for low-mass stars as they are faint. In this work, we calculate ages for Kepler F, G, and crucially K and M dwarfs, using their rotation and kinematic properties. We apply the simple assumption that the velocity dispersion of stars increases over time and adopt an age--velocity--dispers…
Calibration of the [C/N] and [Y/Mg] chemical clocks with asteroseismic ages from the TESS space mission
Institute of Theoretical Physics and Astronomy, Vilnius University（维尔纽斯大学理论与天体物理研究所）；Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University（奥胡斯大学物理学与天文学系恒星天体物理学中心）
