Context. Open clusters are convenient probes of the structure and history of the Galactic disk. They are also fundamental to stellar evolution studies. The second Gaia data release contains precise ...astrometry at the submilliarcsecond level and homogeneous photometry at the mmag level, that can be used to characterise a large number of clusters over the entire sky. Aims. In this study we aim to establish a list of members and derive mean parameters, in particular distances, for as many clusters as possible, making use of Gaia data alone. Methods. We compiled a list of thousands of known or putative clusters from the literature. We then applied an unsupervised membership assignment code, UPMASK, to the Gaia DR2 data contained within the fields of those clusters. Results. We obtained a list of members and cluster parameters for 1229 clusters. As expected, the youngest clusters are seen to be tightly distributed near the Galactic plane and to trace the spiral arms of the Milky Way, while older objects are more uniformly distributed, deviate further from the plane, and tend to be located at larger Galactocentric distances. Thanks to the quality of Gaia DR2 astrometry, the fully homogeneous parameters derived in this study are the most precise to date. Furthermore, we report on the serendipitous discovery of 60 new open clusters in the fields analysed during this study.
Context.
The large astrometric and photometric survey performed by the
Gaia
mission allows for a panoptic view of the Galactic disc and its stellar cluster population. Hundreds of stellar clusters ...were only discovered after the latest
Gaia
data release (DR2) and have yet to be characterised.
Aims.
Here we make use of the deep and homogeneous
Gaia
photometry down to
G
= 18 to estimate the distance, age, and interstellar reddening for about 2000 stellar clusters identified with
Gaia
DR2 astrometry. We use these objects to study the structure and evolution of the Galactic disc.
Methods.
We relied on a set of objects with well-determined parameters in the literature to train an artificial neural network to estimate parameters from the
Gaia
photometry of cluster members and their mean parallax.
Results.
We obtain reliable parameters for 1867 clusters. Our catalogue confirms the relative lack of old stellar clusters in the inner disc (with a few notable exceptions). We also quantify and discuss the variation of scale height with cluster age, and we detect the Galactic warp in the distribution of old clusters.
Conclusions.
This work results in a large and homogeneous cluster catalogue, allowing one to trace the structure of the disc out to distances of ∼4 kpc. However, the present sample is still unable to trace the outer spiral arm of the Milky Way, which indicates that the outer disc cluster census might still be incomplete.
Context. The Gaia Second Data Release provides precise astrometry and photometry for more than 1.3 billion sources. This catalog opens a new era concerning the characterization of open clusters and ...test stellar models, paving the way for better understanding of the disk properties. Aims. The aim of the paper is to improve the knowledge of cluster parameters, using only the unprecedented quality of the Gaia photometry and astrometry. Methods. We have made use of the membership determination based on the precise Gaia astrometry and photometry. We applied an automated Bayesian tool, BASE-9, to fit stellar isochrones on the observed G, GBP, GRP magnitudes of the high probability member stars. Results. We derive parameters such as age, distance modulus, and extinction for a sample of 269 open clusters, selecting only low reddening objects and discarding very young clusters, for which techniques other than isochrone-fitting are more suitable for estimating ages.
Context.
The iron abundance Fe/H in the atmosphere of FGK-type stars is crucial in stellar and galactic physics. The number of stars with a measured value of Fe/H is increasing considerably thanks to ...spectroscopic surveys. However, different methodologies, inputs, and assumptions used in spectral analyses lead to different precisions in Fe/H and possibly to systematic differences, which need to be evaluated. It is essential to understand the characteristics of each survey to fully exploit their potential, in particular if the surveys are combined to probe a larger galactic volume and to improve statistics.
Aims.
The purpose of this study is to compare Fe/H determinations from the largest spectroscopic surveys to other catalogues taken as reference. Offsets and dispersions of the residuals are examined, as are their trends with other parameters. The investigated surveys are the latest public releases of APOGEE, GALAH, RAVE, LAMOST, SEGUE, and the
Gaia
-ESO Survey.
Methods.
We use reference samples that provide independent determinations of Fe/H, which are compared to those from the surveys for common stars. The distribution of the residuals is assessed through simple statistics that measure the offset between two catalogues and the dispersion representative of the precision of both catalogues. When relevant, linear fits are performed. A large sample of FGK-type stars with Fe/H based on high-resolution, high-signal-to-noise spectroscopy was built from the PASTEL catalogue to provide a reference sample. We also use FGK members in open and globular clusters to assess the internal consistency of Fe/H of each survey. The agreement of median Fe/H values for clusters observed by different surveys is discussed.
Results.
All the surveys overestimate the low metallicities, and some of them also underestimate the high metallicities. They perform well in the most populated intermediate metallicity range whatever the resolution. In most cases, the typical precision that we deduce from the comparisons is in good agreement with the uncertainties quoted in the catalogues. Some exceptions to this general behaviour are discussed.
Context.
The possibility of identifying co-natal stars that have dispersed into the Galactic disc based on chemistry alone is called strong chemical tagging. It has been debated for a long time ...whether this is indeed feasible; it holds the promise of reconstructing the detailed star formation history of a large fraction of stars in the Galactic disc.
Aims.
We investigate the feasibility of strong chemical tagging using known member stars of open clusters.
Methods.
We analysed the largest sample of cluster members that have been homogeneously characterised with high-resolution differential abundances for 16 different elements. We also investigated the possibility of finding the known clusters in the APOGEE DR16 red clump sample with 18 chemical species. For both purposes, we used a clustering algorithm and an unsupervised dimensionality reduction technique to blindly search for groups of stars in chemical space.
Results.
Even if the internal coherence of the stellar abundances in the same cluster is high, typically 0.03 dex, the overlap in the chemical signatures of the clusters is large. In the sample with the highest precision and no field stars, we only recover 9 out of the 31 analysed clusters at a 40% threshold of homogeneity and precision. This ratio slightly increases when we only use clusters with 7 or more members. In the APOGEE sample, field stars are present along with four populated clusters. In this case, only one of the open clusters was moderately recovered.
Conclusions.
In our best-case scenario, more than 70% of the groups of stars are in fact statistical groups that contain stars belonging to different real clusters. This indicates that the chances of recovering the majority of birth clusters dissolved in the field are slim, even with the most advanced clustering techniques. We show that different stellar birth sites can have overlapping chemical signatures, even when high-resolution abundances of many different nucleosynthesis channels are used. This is substantial evidence against the possibility of strong chemical tagging. However, we can hope to recover some particular birth clusters that stand out at the edges of the chemical distribution.
Context.
Open clusters are key targets for studies of Galaxy structure and evolution, and stellar physics. Since the
Gaia
data release 2 (DR2), the discovery of undetected clusters has shown that ...previous surveys were incomplete.
Aims.
Our aim is to exploit the Big Data capabilities of machine learning to detect new open clusters in
Gaia
DR2, and to complete the open cluster sample to enable further studies of the Galactic disc.
Methods.
We use a machine-learning based methodology to systematically search the Galactic disc for overdensities in the astrometric space and identify the open clusters using photometric information. First, we used an unsupervised clustering algorithm, DBSCAN, to blindly search for these overdensities in
Gaia
DR2 (
l
,
b
,
ϖ
,
μ
α
*
,
μ
δ
), and then we used a deep learning artificial neural network trained on colour–magnitude diagrams to identify isochrone patterns in these overdensities, and to confirm them as open clusters.
Results.
We find 582 new open clusters distributed along the Galactic disc in the region |
b
| < 20°. We detect substructure in complex regions, and identify the tidal tails of a disrupting cluster
UBC 274
of ∼3 Gyr located at ∼2 kpc.
Conclusions.
Adapting the mentioned methodology to a Big Data environment allows us to target the search using the physical properties of open clusters instead of being driven by computational limitations. This blind search for open clusters in the Galactic disc increases the number of known open clusters by 45%.
Context.
The high-precision parallax data of the
Gaia
mission allows for significant improvements in the distance determination to stellar clusters and their stars. In order to obtain accurate and ...precise distance determinations, systematics such as parallax spatial correlations need to be accounted for, especially with regard to stars in small sky regions.
Aims.
Our aim is to provide the astrophysical community with a free and open code designed to simultaneously infer cluster parameters (i.e., distance and size) and distances to the cluster stars using
Gaia
parallax measurements. The code includes cluster-oriented prior families and it is specifically designed to deal with the
Gaia
parallax spatial correlations.
Methods.
A Bayesian hierarchical model is created to allow for the inference of both the cluster parameters and distances to its stars.
Results.
Using synthetic data that mimics
Gaia
parallax uncertainties and spatial correlations, we observe that our cluster-oriented prior families result in distance estimates with smaller errors than those obtained with an exponentially decreasing space density prior. In addition, the treatment of the parallax spatial correlations minimizes errors in the estimated cluster size and stellar distances, and avoids the underestimation of uncertainties. Although neglecting the parallax spatial correlations has no impact on the accuracy of cluster distance determinations, it underestimates the uncertainties and may result in measurements that are incompatible with the true value (i.e., falling beyond the 2
σ
uncertainties).
Conclusions.
The combination of prior knowledge with the treatment of
Gaia
parallax spatial correlations produces accurate (error < 10%) and trustworthy estimates (i.e., true values contained within the 2
σ
uncertainties) of cluster distances for clusters up to ∼5 kpc, along with cluster sizes for clusters up to ∼1 kpc.
Context.
The distribution of member stars in the surroundings of an open cluster (OC) can shed light on the process of its formation, evolution, and dissolution. The analysis of structural parameters ...of OCs as a function of their age and position in the Galaxy constrains theoretical models of cluster evolution. The
Gaia
catalog is very appropriate for finding members of OCs at large distance from their centers.
Aims.
We revisit the membership lists of OCs from the solar vicinity, in particular, by extending these membership lists to the peripheral areas through
Gaia
EDR3. We then take advantage of these new member lists to study the morphological properties and the mass segregation levels of the clusters.
Methods.
We used the clustering algorithm HDBSCAN on
Gaia
parallaxes and proper motions to systematically search for members up to 50 pc from the cluster centers. We fit a King’s function on the radial density profile of these clusters and a Gaussian mixture model (GMM) on their two-dimensional member distribution to study their shape. We also evaluated the degree of mass segregation of the clusters and the correlations of these parameters with the age and Galactic position of the clusters.
Results.
Our method performs well on 389 clusters out of the 467 clusters we selected, including several recently discovered clusters that were poorly studied until now. We report the detection of vast coronae around almost all the clusters and report the detection of 71 OCs with tidal tails. This multiplies the number of these structures that are identified by more than four. The size of the cores is smaller for old clusters than for young ones on average. Moreover, the overall size of the clusters seems to increase slightly with age, but the fraction of stars in the halo seems to decrease. As expected, the mass segregation is more pronounced in the oldest clusters, but no clear trend with age is evident.
Conclusions.
OCs are more extended than previously expected, regardless of their age. The decrease in the proportion of stars populating the clusters halos highlights the different cluster evaporation processes and the short timescales they need to affect the clusters. Reported parameters such as cluster sizes or mass segregation levels all depend on cluster ages, but cannot be described as single functions of time.
Context.
Open clusters (OCs) trace the evolution of the Galactic disc with great accuracy.
Gaia
and large ground-based spectroscopic surveys make it possible to determine their properties and study ...their kinematics with unprecedented precision.
Aims.
We study the kinematical behaviour of the OC population over time. We take advantage of the latest age determinations of OCs to investigate the correlations of the 6D phase-space coordinates and orbital properties with age. The phase-space distribution, age-velocity relation, and action distribution are compared to those of field stars. We also investigate the rotation curve of the Milky Way traced by OCs, and we compare it to that of other observational or theoretical studies.
Methods.
We gathered nearly 30 000 radial velocity (RV) measurements of OC members from both
Gaia
-RVS data and ground-based surveys and catalogues. We computed the weighted mean RV, Galactic velocities, and orbital parameters of 1382 OCs. We investigated their distributions as a function of age and by comparison to field stars.
Results.
We provide the largest RV catalogue available for OCs, half of it based on at least three members. Compared to field stars, we note that OCs are not on exactly the same arches in the radial-azimuthal velocity plane, while they seem to follow the same diagonal ridges in the Galactic radial distribution of azimuthal velocities. Velocity ellipsoids in different age bins all show a clear anisotropy. The heating rate of the OC population is similar to that of field stars for the radial and azimuthal components, but it is significantly lower for the vertical component. The rotation curve drawn by our sample of clusters shows several dips that match the wiggles derived from nonaxisymmetric models of the Galaxy. From the computation of orbits, we obtain a clear dependence of the maximum height and eccentricity on age. Finally, the orbital characteristics of the sample of clusters as shown by the action variables follow the distribution of field stars. The additional age information of the clusters indicates some (weak) age dependence of the known moving groups.
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 < M/H < +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 (< 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
> 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.
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