Sample Of Open Clusters Research Articles

Mapping radial abundance gradients with Gaia -ESO open clusters. Evidence of recent gas accretion in the Milky Way disk

Recent evidence from spectroscopic surveys points towards the presence of a metal-poor, young stellar population in the low-alpha , chemically thin disk. In this context, the investigation of the spatial distribution and time evolution of precise, unbiased abundances is fundamental to disentangle the scenarios of formation and evolution of the Galaxy. We study the evolution of abundance gradients in the Milky Way by taking advantage of a large sample of open star clusters, which are among the best tracers for this purpose. In particular, we used data from the last release of the Gaia -ESO survey. We performed a careful selection of open cluster member stars, excluding those members that may be affected by biases in spectral analysis. We compared the cleaned open cluster sample with detailed chemical evolution models for the Milky Way, using well-tested stellar yields and prescription for radial migration. We tested different scenarios of Galaxy evolution to explain the data, namely, the two-infall and the three-infall frameworks, which suggest the chemical thin disk is formed by one or two subsequent gas accretion episodes, respectively. With the performed selection in cluster member stars, we still find a metallicity decrease between intermediate-age ($1<$ Age/Gyr $<3$) and young (Age $<1$ Gyr) open clusters. This decrease cannot be explained in the context of the two-infall scenario, even by accounting for the effect of migration and yield prescriptions. The three-infall framework, with its late gas accretion in the last 3 Gyr, is able to explain the low metallic content in young clusters. However, we have invoked a milder metal dilution for this gas infall episode relative to previous findings. To explain the observed low metallic content in young clusters, we propose that a late gas accretion episode triggering a metal dilution would have taken place, extending the framework of the three-infall model for the first time to the entire Galactic disk.

Gaia DR3 reveals the complex dynamical evolution within star clusters

Context. Star clusters, composed of stars born from the same molecular cloud, serve as invaluable natural laboratories for understanding the fundamental processes governing stellar formation and evolution. Aims. This study aims to investigate correlations between the Mean Interdistance ($ \bar{D_{\mathrm{i}}} $), Mean Closest Interdistance ($ \bar{D_{\mathrm{c}}} $) and Median Weighted Central Interdistance ($ \bar{D_{\mathrm{cc}}} $) with the age of star clusters, examining their evolutionary trends and assessing the robustness of these quantities as possible age indicators. Methods. We selected a sample of open clusters in the solar region and with a representative number of members (e.g. well populated and without outliers). The interdistances are derived from the spatial distribution of member stars within a cluster. Their evolution over time allows us to use them as age indicators for star clusters. Results. Our investigation reveals a high-significant correlation between the interdistances and cluster age. Considering the full sample of clusters between 7 and 9 kpc, the relationship is very broad. This is due to uncertainties in parallax, which increase with increasing distance. In particular, we must limit the sample to a maximum distance from the Sun of about 200 pc to avoid artificial effects on cluster shape and on the spatial distribution of their stars along the line of sight. Conclusions. By conservatively restraining the distance to a maximum of ∼200 pc, we have established a relationship between the interdistances and the age of the clusters. In our sample, the relationship is mainly driven by the internal expansion of the clusters and is marginally affected by external perturbative effects. Such relation might enhance our comprehension of cluster dynamics and might be used to derive cluster dynamical ages.

A study on the metallicity gradients in the galactic disk using open clusters

We study the metallicity distribution and evolution in the galactic disk based on the largest sample of open star clusters in the galaxy. From the catalog of 1,879 open clusters in the range of galactocentric distance (RGC) from 4 to 20 kpc, we investigate the variation in metallicity in the galactic disk as functions of RGC, vertical distance (Z), and ages of the clusters. In the direction perpendicular to the galactic plane, the variation in metallicity is found to follow a stepped linear relation. We estimate a vertical metallicity gradient d[Fe/H]d|Z| of −0.545 ± 0.046 dex kpc−1 for |Z| < 0.487 kpc and −0.075 ± 0.093 dex kpc−1 for 0.487 < |Z| < 1.8 kpc. On average, metallicity variations above and below the galactic plane are found to change at similar rates. The change in metallicity in the radial direction is also found to follow a two-function linear relation. We obtain a radial metallicity gradient d[Fe/H]dRGC of −0.070 ± 0.002 dex kpc−1 for 4.0 ≲ RGC ≲ 12.8 kpc and −0.005 ± 0.018 dex kpc−1 for 12.8 ≲ RGC ≲ 20.5 kpc, which clearly shows a strong variation in the metallicity gradient when moving from the inner to the outer galactic disk. The age–metallicity relation (AMR) is found to follow a steeper negative slope of −0.031 ± 0.006 dex Gyr−1 for clusters older than 240 Myr; however, there is some hint of positive metallicity age gradient for younger clusters.

The broadening of the main sequence in the open cluster M38

Abstract Our recent multi-band photometric study of the colour width of the lower main sequence of the open cluster M37 has revealed the presence of a sizeable initial chemical composition spread in the cluster. If initial chemical composition spreads are common amongst open clusters, this would have major implications for cluster formation models and the foundation of the chemical tagging technique. Here we present a study of the unevolved main sequence of the open cluster M38, employing Gaia DR3 photometry and astrometry, together with newly acquired Sloan photometry. We have analysed the distribution of the cluster’s lower main sequence stars with a differential colour-colour diagram made of combinations of Gaia and Sloan magnitudes, like in the study of M37. We employed synthetic stellar populations to reproduce the observed trend of M38 stars in this diagram, and found that the observed colour spreads can be explained simply by the combined effect of differential reddening across the face of the cluster and the presence of unresolved binaries. There is no need to include in the synthetic sample a spread of initial chemical composition as instead necessary to explain the main sequence of M37. Further photometric investigations like ours, as well as accurate differential spectroscopic analyses on large samples of open clusters, are necessary to understand whether chemical abundance spreads are common among the open cluster population.

GaiaData Release 3

Context.The motion of stars has been used to reveal details of the complex history of the Milky Way, in constant interaction with its environment. Nevertheless, to reconstruct the Galactic history puzzle in its entirety, the chemo-physical characterisation of stars is essential. PreviousGaiadata releases were supported by a smaller, heterogeneous, and spatially biased mixture of chemical data from ground-based observations.Aims.GaiaData Release 3 opens a new era of all-sky spectral analysis of stellar populations thanks to the nearly 5.6 million stars observed by the Radial Velocity Spectrometer (RVS) and parametrised by the GSP-Spec module. In this work, we aim to demonstrate the scientific quality ofGaia’s Milky Way chemical cartography through a chemo-dynamical analysis of disc and halo populations.Methods.Stellar atmospheric parameters and chemical abundances provided byGaiaDR3 spectroscopy are combined with DR3 radial velocities and EDR3 astrometry to analyse the relationships between chemistry and Milky Way structure, stellar kinematics, and orbital parameters.Results.The all-skyGaiachemical cartography allows a powerful and precise chemo-dynamical view of the Milky Way with unprecedented spatial coverage and statistical robustness. First, it reveals the strong vertical symmetry of the Galaxy and the flared structure of the disc. Second, the observed kinematic disturbances of the disc – seen as phase space correlations – and kinematic or orbital substructures are associated with chemical patterns that favour stars with enhanced metallicities and lower [α/Fe] abundance ratios compared to the median values in the radial distributions. This is detected both for young objects that trace the spiral arms and older populations. Severalα, iron-peak elements and at least one heavy element trace the thin and thick disc properties in the solar cylinder. Third, young disc stars show a recent chemical impoverishment in several elements. Fourth, the largest chemo-dynamical sample of open clusters analysed so far shows a steepening of the radial metallicity gradient with age, which is also observed in the young field population. Finally, theGaiachemical data have the required coverage and precision to unveil galaxy accretion debris and heated disc stars on halo orbits through their [α/Fe] ratio, and to allow the study of the chemo-dynamical properties of globular clusters.Conclusions.GaiaDR3 chemo-dynamical diagnostics open new horizons before the era of ground-based wide-field spectroscopic surveys. They unveil a complex Milky Way that is the outcome of an eventful evolution, shaping it to the present day.

Features of Gaia DR3 spectroscopic binaries I. Tidal circularization of main-sequence stars

ABSTRACT Previous studies pointed out that many observed samples of short-period binaries display a cut-off period, Pcut, such that almost all binaries with periods shorter than Pcut have circular orbits. This feature is probably due to long-term circularization processes induced by tidal interaction between the two stars of each binary. It seemed as if coeval main-sequence (MS) samples of open clusters display Pcut that depends on the sample age. Using the unprecedentedly large sample of MS spectroscopic orbits recently released by Gaia, we have found that the Pcut does not depend on the stellar age but, instead, varies with stellar temperature, decreasing linearly from 6.5 d at Teff ∼ 5700 K to ∼2.5 d at 6800 K. Pcut was derived by a new algorithm that relied on clear upper envelopes displayed in the period-eccentricity diagrams. Our Pcut determines both the border between the circular and eccentric binaries and the location of the upper envelope. The results are inconsistent with the theory, which assumes circularization occurs during the stellar MS phase, a theory that was adopted by many studies. The circularization has probably taken place at the pre-main-sequence phase, as suggested already in 1989 by Zahn and Bouchet, and later by Khaluillin and Khaluillina in 2011. Our results suggest that the weak dependence of Pcut on the cluster age is not significant, and/or might be due to the different temperatures of the samples. If indeed true, this has far-reaching implications for the theory of binary and exoplanet circularization, synchronization, and alignment.

A Gaia EDR3 search for tidal tails in disintegrating open clusters

ABSTRACT We carry out a search for tidal tails in a sample of open clusters with known relatively elongated morphology. We identify the member stars of these clusters from the precise astrometric and deep photometric data from Gaia Early Data Release 3 using the robust membership determination algorithm, ML-MOC. We identify 46 open clusters having a stellar corona beyond the tidal radius, 20 of which exhibit extended tails aligned with the cluster orbit direction in Galactocentric coordinates. Notably, we find NGC 6940 (at a distance of ∼1 kpc) is the furthest open cluster, exhibiting tidal tails that are ∼50 pc from its centre, while also identifying ∼40 pc long tidal tails for the nearby Pleiades. Using the minimum spanning tree length for the most massive stars relative to all cluster members, we obtain the mass segregation ratio ($\rm \lambda _{MSR}$) profiles as a function of the number of massive stars in each cluster. From these profiles, we can classify the open clusters into four classes based on the degree of mass segregation experienced by the clusters. We find that clusters in the most mass segregated classes are the oldest on average and have the flattest mass function slope. Of the 46 open clusters studied in this work, 41 exhibit some degree of mass segregation. Furthermore, we estimate the initial masses (M$\rm _{i}$) of these open clusters, finding that some of them, having M$\rm _{i}\gtrsim 10^{4} M_{\odot }$, could be the dissolving remnants of young massive clusters.

New Open-cluster Candidates Found in the Galactic Disk Using Gaia DR2/EDR3 Data

We report 541 new open-cluster candidates in Gaia EDR3 through revisiting the cluster results from an earlier analysis of Gaia DR2, which revealed nearly 1000 open-cluster candidates in the solar neighborhood (mostly d <3 kpc) residing at Galactic latitudes ∣b∣ < 20°. A subsequent comparison with lists of known clusters shows a large increase of the cluster samples within 2 kpc from the Sun. We assign membership probabilities to the stars through the open-source pyUPMASK algorithm, and also estimate the physical parameters through isochrone fitting for each candidate. Most of the new candidates show small total-proper-motion dispersions and clear features in the color–magnitude diagrams. Besides, the metallicity gradient of the new candidates is consistent with those found in the literature. The cluster parameters and member stars are available at CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via https://cdsarc.unistra.fr/viz-bin/cat/J/ApJS/260/8. The discovery of these new objects shows that the open-cluster samples in Gaia data is still not complete, and more discoveries are expected in future research.

The Gaia-ESO survey: Age-chemical-clock relations spatially resolved in the Galactic disc

Context. The last decade has seen a revolution in our knowledge of the Galaxy thanks to the Gaia and asteroseismic space missions and the ground-based spectroscopic surveys. Aims. To complete this picture, it is necessary to map the ages of its stellar populations. During recent years, the dependence on time of abundance ratios involving slow (s) neutron-capture and α elements (called chemical-clocks) has been used to provide estimates of stellar ages, usually in a limited volume close to the Sun. We aim to analyse the relations of chemical clocks in the Galactic disc extending the range to RGC ∼ 6–20 kpc. Methods. Using the sixth internal data release of the Gaia-ESO survey, we calibrated several relations between stellar ages and abundance ratios [s/α] using a sample of open clusters, the largest one so far used with this aim (62 clusters). Thanks to their wide galactocentric coverage, we investigated the radial variations of the shape of these relations, confirming their non-universality. Results. The multi-variate relations allowed us to infer stellar ages for field stars. We estimated our accuracy (ranging from 0.0 to −0.9 Gyr) and precision (from 0.4 to 2.3 Gyr) in recovering the global ages of open clusters, and the ages of their individual members. We applied the relations with the highest correlation coefficients to the field star population, finding an older population at lower metallicity and higher [α/Fe] in the thin disc, and a younger one at higher [Fe/H] and low [α/Fe], as expected. Conclusion. We confirm that there is no single age-chemical clock relationship valid for the whole disc, but that there is a dependence on the galactocentric position, which is related to the radial variation of the star formation history combined with the non-monotonic dependence on metallicity of the yields of the s-process elements from low- and intermediate-mass stars. Finally, the abundance ratios [Ba/α] are more sensitive to age than those with [Y/α] for young disc stars, and their slopes vary less with galactocentric distance. We remind the reader that the application of such relationships to field stars is only of statistical value.

The Gaia-ESO Survey: Target selection of open cluster stars

Context. The Gaia-ESO Survey (GES) is a public, high-resolution spectroscopic survey, conducted with the multi-object spectrograph Fibre Large Array Multi Element Spectrograph (FLAMES) on the Very Large Telescope (European Southern Observatory, ESO, Cerro Paranal, Chile) from December 2011 to January 2018. Gaia-ESO has targeted all the main stellar components of the Milky Way, including thin and thick disc, bulge, and halo. In particular, a large sample of open clusters has been observed, from very young ones, just out of the embedded phase, to very old ones. Aims. The different kinds of clusters and stars targeted in them are useful to reach the main science goals of the open cluster part of GES, which are the study of the open cluster structure and dynamics, the use of open clusters to constrain and improve stellar evolution models, and the definition of Galactic disc properties (e.g., metallicity distribution). Methods. The Gaia-ESO Survey is organised in 19 working groups (WGs), each one being responsible for a task. We describe here the work of three of them, one in charge of the selection of the targets within each cluster or association (WG4), one responsible for defining the most probable candidate member stars (WG1), and another one in charge of the preparation of the observations (WG6). As the entire GES has been conducted before the second Gaia data release, we could not make use of the Gaia astrometry to define cluster member candidates. We made use of public and private photometry to select the stars to be observed with FLAMES, once brought on a common astrometric system (the one defined by 2MASS). Candidate target selection was based on ground-based proper motions, radial velocities, and X-ray properties when appropriate, for example, and it was mostly used to define the position of the clusters’ evolutionary sequences in the colour-magnitude diagrams. Targets for GIRAFFE were then selected near the sequences in an unbiased way. We used known information on membership, when available, only for the few stars to be observed with UVES. Results. We collected spectra for 62 confirmed clusters in the main observing campaign (and a few more clusters were taken from the ESO archive). Among them are very young clusters, where the main targets are pre-main sequence stars, clusters with very hot and massive stars currently on the main sequence, intermediate-age and old clusters where evolved stars are the main targets. Our strategy of making the selection of targets as inclusive and unbiased as possible and of observing a significant and representative fraction of all possible targets permitted us to collect the largest, most accurate, and most homogeneous spectroscopic data set on open star clusters ever achieved.

Open clusters with proper motions fully separatedfrom the field stars using Gaia DR2

ABSTRACT The study of open star clusters makes us understand a lot about the composition and construction of the Milky Way Galaxy. Thanks to the Gaia DR2 database that helps us to get the genetic members of star clusters using their proper motions and parallaxes, and estimating their physical properties in a very accurate way. This study aims to detect the reasons that make clusters (within 2 kpc) have proper motions completely separated from the background field stars. We studied a large sample of open stellar clusters taken from Dias’s catalogue and drew the vector point diagrams using the astrometric data of Gaia DR2. By visual inspection, we specified 108 objects as separated clusters and 377 ones as melted clusters. We studied their mean parameters in the four galactic quadrants of the Milky Way Galaxy.

Stellar Population Astrophysics (SPA) with TNG

Context. Thanks to the modern understanding of stellar evolution, we can accurately measure the ages of open clusters (OCs). Given their position, they are ideal tracers of the Galactic disc. Gaia data release 2, besides providing precise parallaxes, led to the detection of many new clusters, opening a new era for the study of the Galactic disc. However, detailed information on the chemical abundance for OCs is necessary to accurately date them and to efficiently use them to probe the evolution of the disc. Aims. Mapping and exploring the Milky Way structure is the main aim of the Stellar Population Astrophysics project. Part of this work involves the use of OCs and the derivation of their precise and accurate chemical composition. Here, we aim to analyse a sample of OCs located within about 2 kpc from the Sun, with ages from about 50 Myr to a few gigayears. Methods. We used HARPS-N at the Telescopio Nazionale Galileo and collected very high-resolution spectra (R = 115 000) of 40 red giant/red clump stars in 18 OCs (16 never or scarcely studied plus two comparison clusters). We measured their radial velocities and derived the stellar parameters (Teff, log g, vmicro, and [Fe/H]) based on equivalent width measurement combined with a 1D – LTE atmospherical model. Results. We discuss the relationship between metallicity and Galactocentric distance, adding literature data to our results to enlarge the sample and also taking age into account. We compared the result of observational data with the findings of chemo-dynamical models. These models generally reproduce the metallicity gradient well. However, at young ages we find a large dispersion in metallicity, that is not reproduced by models. Several possible explanations are explored, including uncertainties in the derived metallicity. We confirm the difficulties in determining parameters for young stars (age &lt; 200 Myr), which is attributable to a combination of intrinsic factors (activity, fast rotation, magnetic fields, etc) which atmospheric models cannot easily reproduce and which affect the uncertainty on parameters.

The Gaia-ESO Survey: Galactic evolution of lithium from iDR6

Context. After more than 50 years, astronomical research still struggles to reconstruct the history of lithium enrichment in the Galaxy and to establish the relative importance of the various 7Li sources in enriching the interstellar medium (ISM) with this fragile element. Aims. To better trace the evolution of lithium in the Milky Way discs, we exploit the unique characteristics of a sample of open clusters (OCs) and field stars for which high-precision 7Li abundances and stellar parameters are homogeneously derived by the Gaia-ESO Survey (GES). Methods. We derive possibly un-depleted 7Li abundances for 26 OCs and star forming regions with ages from young (∼3 Myr) to old (∼4.5 Gyr), spanning a large range of galactocentric distances, 5 &lt; RGC/kpc &lt; 15, which allows us to reconstruct the local late Galactic evolution of lithium as well as its current abundance gradient along the disc. Field stars are added to look further back in time and to constrain 7Li evolution in other Galactic components. The data are then compared to theoretical tracks from chemical evolution models that implement different 7Li forges. Results. Thanks to the homogeneity of the GES analysis, we can combine the maximum average 7Li abundances derived for the clusters with 7Li measurements in field stars. We find that the upper envelope of the 7Li abundances measured in field stars of nearly solar metallicities (−0.3 &lt; [Fe/H]/dex &lt; +0.3) traces very well the level of lithium enrichment attained by the ISM as inferred from observations of cluster stars in the same metallicity range. We confirm previous findings that the abundance of 7Li in the solar neighbourhood does not decrease at super-solar metallicity. The comparison of the data with the chemical evolution model predictions favours a scenario in which the majority of the 7Li abundance in meteorites comes from novae. Current data also seem to suggest that the nova rate flattens out at later times. This requirement might have implications for the masses of the white dwarf nova progenitors and deserves further investigation. Neutrino-induced reactions taking place in core-collapse supernovae also produce some fresh lithium. This likely makes a negligible contribution to the meteoritic abundance, but could be responsible for a mild increase in the 7Li abundance in the ISM of low-metallicity systems that would counterbalance the astration processes.

Abundance–age relations with red clump stars in open clusters

Context. Precise chemical abundances coupled with reliable ages are key ingredients to understanding the chemical history of our Galaxy. Open clusters (OCs) are useful for this purpose because they provide ages with good precision. Aims. The aim of this work is to investigate the relation between different chemical abundance ratios and age traced by red clump (RC) stars in OCs. Methods. We analyzed a large sample of 209 reliable members in 47 OCs with available high-resolution spectroscopy. We applied a differential line-by-line analysis, performing a comprehensive chemical study of 25 chemical species. This sample is among the largest samples of OCs homogeneously characterized in terms of atmospheric parameters, detailed chemistry, and age. Results. In our metallicity range (−0.2 &lt; [M/H] &lt; +0.2) we find that while most Fe-peak and α elements show a flat dependence on age, the s-process elements show a decreasing trend with increasing age with a remarkable knee at 1 Gyr. For Ba, Ce, Y, Mo, and Zr, we find a plateau at young ages (&lt; 1 Gyr). We investigate the relations between all possible combinations among the computed chemical species and age. We find 19 combinations with significant slopes, including [Y/Mg] and [Y/Al]. The ratio [Ba/α] shows the most significant correlation. Conclusions. We find that the [Y/Mg] relation found in the literature using solar twins is compatible with the one found here in the solar neighborhood. The age–abundance relations in clusters at large distances(d &gt; 1 kpc) show larger scatter than those in clusters in the solar neighborhood, particularly in the outer disk. We conclude that, in addition to pure nucleosynthetic arguments, the complexity of the chemical space introduced by the Galactic dynamics must be taken into account in order to understand these relations, especially outside of the local bubble.

The Distribution of Open Clusters in the Galaxy

In this work we explore the new catalog of galactic open clusters that became available recently, containing 1,750 clusters that have been re-analyzed using the Gaia DR2 catalog to determine the stellar memberships. We used the young open clusters as tracers of spiral arms and determined the spiral pattern rotation speed of the Galaxy and the corotation radius, the strongest Galactic resonance. The sample of open clusters used here is increased by dozens of objects with respect to our previous works. In addition, the distances and ages values are better determined, using improvements to isochrone fitting and including an updated extinction polynomial for the Gaia DR2 photometric band-passes, and the Galactic abundance gradient as a prior for metallicity. In addition to the better age determinations, the catalog contains better positions in the Galactic plane and better proper motions. This allow us to discuss not only the present space distribution of the clusters, but also the space distribution of the clusters's birthplaces, obtained by integration of the orbits for a time equal to their age. The value of the rotation velocity of the arms (28.5 ± 1.0 km s−1 kpc−1) implies that the corotation radius (Rc) is close to the solar Galactic orbit (Rc/R0 = 1.01±0.08), which is supported by other observational evidence discussed in this text. A simulation is presented, illustrating the motion of the clusters in the reference frame of corotation. We also present general statistics of the catalog of clusters, like spatial distribution, distribution relative to height from the Galactic plane, and distribution of ages and metallicity. An important feature of the space distribution, the corotation gap in the gas distribution and its consequences for the young clusters, is discussed.

3D Morphology of Open Clusters in the Solar Neighborhood with Gaia EDR 3: Its Relation to Cluster Dynamics

Abstract We analyze the 3D morphology and kinematics of 13 open clusters (OCs) located within 500 pc of the Sun, using Gaia EDR 3 and kinematic data from the literature. Members of OCs are identified using the unsupervised machine-learning method StarGO, using five parameters (X, Y, Z, ). The OC sample covers an age range of 25 Myr to 2.65 Gyr. We correct the asymmetric distance distribution that is due to parallax error using Bayesian inversion. The uncertainty in the corrected distance for a cluster at 500 pc is 3.0–6.3 pc, depending on the intrinsic spatial distribution of its members. We determine the 3D morphology of the OCs in our sample and fit the spatial distribution of stars within the tidal radius in each cluster with an ellipsoid model. The shapes of the OCs are well described with oblate spheroids (NGC 2547, NGC 2516, NGC 2451A, NGC 2451B, and NGC 2232), prolate spheroids (IC 2602, IC 4665, NGC 2422, Blanco 1, and Coma Berenices), or triaxial ellipsoids (IC 2391, NGC 6633, and NGC 6774). The semimajor axis of the fitted ellipsoid is parallel to the Galactic plane for most clusters. Elongated filament-like substructures are detected in three young clusters (NGC 2232, NGC 2547, and NGC 2451B), while tidal-tail-like substructures (tidal tails) are found in older clusters (NGC 2516, NGC 6633, NGC 6774, Blanco 1, and Coma Berenices). Most clusters may be supervirial and expanding. N-body models of rapid gas expulsion with a star formation efficiency of ≈1/3 are consistent with clusters more massive than 250 M ⊙, while clusters less massive than 250 M ⊙ tend to agree with adiabatic gas expulsion models. Only five OCs (NGC 2422, NGC 6633, NGC 6774, Blanco 1, and Coma Berenices) show clear signs of mass segregation.

Binary-driven stellar rotation evolution at the main-sequence turn-off in star clusters

ABSTRACT The impact of stellar rotation on the morphology of star cluster colour–magnitude diagrams is widely acknowledged. However, the physics driving the distribution of the equatorial rotation velocities of main-sequence turn-off stars is as yet poorly understood. Using Gaia Data Release 2 photometry and new Southern African Large Telescope medium-resolution spectroscopy, we analyse the intermediate-age ($\sim 1\text{-}{\rm Gyr}$-old) Galactic open clusters NGC 3960, NGC 6134, and IC 4756 and develop a novel method to derive their stellar rotation distributions based on SYCLIST stellar rotation models. Combined with literature data for the open clusters NGC 5822 and NGC 2818, we find a tight correlation between the number ratio of slow rotators and the clusters’ binary fractions. The blue-main-sequence stars in at least two of our clusters are more centrally concentrated than their red-main-sequence counterparts. The origin of the equatorial stellar rotation distribution and its evolution remains as yet unidentified. However, the observed correlation in our open cluster sample suggests a binary-driven formation mechanism.

Search for age pattern across spiral arms of the Milky Way

The age pattern across spiral arms is one of the key observational features utilised to study the dynamic nature of the Galaxy’s spiral structure. With the most updated samples of high-mass star formation region (HMSFR) masers, O stars and open clusters, we investigated their distributions and kinematic properties in the vicinity of the Sun. We found that the Sagittarius-Carina Arm traced by HMSFRs, O stars (≲ 10 Myr) and young open clusters (<30 Myr) seem to deviate gradually towards the Galactic Anticenter (GAC) direction. The Local Arm traced by HMSFRs, O stars, young clusters and also medium-young clusters (30–100 Myr) are inclined to gradually deviate toward the Galactic Center (GC) direction. The properties for the Local Arm are supported by a simplified simulation of cluster motions in the Galaxy. Indications of systematic motions in the circular and radial velocities are noticed for the old open clusters (>200 Myr). These results are consistent with the idea that star formation can be triggered by spiral shocks of density waves, and indicate that the corotation radius of the Galaxy is located between the Sagittarius-Carina Arm and the Local Arm, close to the Solar circle.

Galactic chemical evolution and chemical tagging with open clusters

The article presents the consolidated results drawn from the chemical composition studies of Reddy et al. (2012, 2013, 2015, 2016) and Reddy & Lambert (2019), who through the high-dispersion echelle spectra ( $$R = 60000$$ ) of red giant members in a large sample of Galactic open clusters (OCs), derived stellar parameters and chemical abundances for 24 elements by either line equivalent widths or synthetic spectrum analyses. The focus of this article is on the issues with radial-metallicity distribution and the potential chemical tags offered by OCs. Results of these studies confirm the lack of an age–metallicity relation for OCs but argue that such a lack of trend for OCs arise from the limited coverage in metallicity compared to that of field stars which span a wide range in metallicity and age. Results demonstrate that the sample of clusters constituting a steep radial metallicity gradient of slope −0.052 ± 0.011 dex kpc $$^{-1}$$ at R $$_\mathrm{gc}<$$ 12 kpc are younger than 1.5 Gyr and located close to the Galactic midplane ( $$|z|<\,$$ 0.5 kpc). Whereas the clusters describing a shallow slope of −0.015 ± 0.007 dex kpc $$^{-1}$$ at R $$_\mathrm{gc}>$$ 12 kpc are relatively old with a striking spread in age and height above the midplane (0.5 $$\,<|z|<\,$$ 2.5 kpc). Results of these studies reveal that OCs and field stars yield consistent radial metallicity gradients if the comparison is limited to samples drawn from the similar vertical heights. The computation of Galactic orbits reveals that the outer disk OCs were actually born inward of 12 kpc but the orbital eccentricity has taken them to present locations very far from their birthplaces. Published results for OCs show that the abundances of the heavy elements La, Ce, Nd and Sm but not so obviously Y and Eu vary from one cluster to another across a sample all having about the solar metallicity. For La, Ce, Nd and Sm the amplitudes of the variations at solar metallicity scale approximately with the main s-process contribution to solar system material. Consideration of published abundances of field stars suggest that such a spread in heavy element abundances is present for the thin and thick disk stars of different metallicity. This result provides an opportunity to chemically tag stars by their heavy elements and to reconstruct dissolved open clusters from the field star population.

The Gaia-ESO Survey: Galactic evolution of lithium at high metallicity

Context. Reconstructing the Galactic evolution of lithium (Li) is the main tool used to constrain the source(s) of Li enrichment in the Galaxy. Recent results have suggested a decline in Li at supersolar metallicities, which may indicate reduced production. Aims. We exploit the unique characteristics of the Gaia-ESO Survey open star cluster sample to further investigate this issue and to better constrain the evolution of Li at high metallicity. Methods. We trace the upper envelope of Li abundance versus metallicity evolution using 18 clusters and considering members that should not have suffered any Li depletion. Results. At variance with previous claims, we do not find any evidence of a Li decrease at high metallicity. The most metal-rich clusters in the sample ([Fe/H] = ∼0.3) actually show the highest Li abundances, with A(Li) &gt; 3.4. Our results clearly show that previous findings, which were based on field stars, were affected by selection effects. The metal-rich population in the solar neighbourhood is composed of relatively old and cool stars that have already undergone some Li depletion; hence, their measured Li does not represent the initial interstellar medium abundance, but a lower limit to it.