Search

Binary stars play a crucial role in our understanding of the formation and evolution of star clusters and their stellar populations. We use Gaia Data Release 3 to homogeneously analyze 78 Galactic open clusters and the unresolved binary systems they host, each composed of two main sequence (MS) stars. We first investigated the structural parameters of these clusters, such as the core radius and the central density, and determined the cluster mass function (MF) and total mass by interpolating the density profile of each cluster. We measured the fraction of binaries with a large mass ratio and the fraction of blue straggler stars (BSSs), and finally investigated possible connections between the populations of binary stars and BSSs with the main parameters of the host cluster. {Remarkably, we find that the MFs of 78 analyzed open clusters follow a similar trend and are well reproduced by two single power-law functions, with a change in slope around masses of 1$M_{\odot}$. The fraction of binary stars ranges from $\sim$15\% to more than $\sim$60\% without significant correlation with the mass and the age of the host cluster. Moreover, we detect hints of a correlation between the total fraction of binary stars and the central density of the host cluster. We compared the fraction of binary stars with that of BSSs, finding that clusters with high and low central density exhibit different trends. The fraction of binaries does not significantly change with the mass of the primary star and the mass ratio. The radial distribution of binary stars depends on cluster age. The binaries of clusters younger than $\sim$800\,Myr typically show a flat radial distribution, with some hints of a double peak. In contrast, the binaries of the remaining clusters are more centrally concentrated than the single stars, which is similar to what is observed in globular clusters., Comment: 27 pages, 27 figures, accepted for publication in Astronomy & Astrophysics

Massive young star clusters are fundamental building blocks of galaxies, and the most abundant reservoirs of newly born stars in the Milky Way. A robust membership assignment is fundamental to study their populations and physical properties. Traditionally, various methods have been used to separate members from the field stars, including the presence of X-ray emission, infrared excess, spectroscopic youth features, and, to a lesser extent, proper motions. With the advent of the Gaia mission and its precision astrometry, we are now able to study complete stellar populations in massive young clusters at kpc distances, over areas much larger than ever before. In this contribution, I will present an application of the Probabilistic Random Forest algorithm to study the stellar population in the Rosette Nebula, harbouring a young (~2 Myr) cluster NGC 2244 located at a distance of ~1.5 kpc. Our new sample of candidate members doubles the previously identified one, allowing us to derive the most complete Initial Mass Function extending down to 0.1-0.2 MSun, study the spatial structure of the region, detect expansion of NGC 2244, and discuss potential scenarios for its formation., {"references":["Bayo et al. 2008, A&A, 492, 277","Bell et al. 2013, MNRAS, 434, 806","Bonilla-Barroso et al. 2022, MNRAS, 511, 4801","Bressan et al. 2012, MNRAS, 427, 127","Kuhn et al. 2014, ApJ, 787, 107","Kuhn et al. 2021, ApJS, 254, 33","Meng et al. 2017, ApJ, 836, 34","Parker et al. 2014, MNRAS, 438, 620","Parker & Wright 2016, MNRAS, 457, 3430","Reis et al. 2019, AJ, 157, 16","Wright & Parker 2019, MNRAS, 489, 2694"]}

GALAH+ is a magnitude-limited survey of high-resolution stellar spectra obtained by the HERMES spectrograph at the Australian Astronomical Observatory. Its third data release provides reduced spectra with new derivations of stellar parameters and abundances of 30 chemical elements for 584 015 dwarfs and giants, 88 per cent of them in the Gaia magnitude range 11 < G < 14. Here, we use these improved values of stellar parameters to build a library of observed spectra which is useful to study variations of individual spectral lines with stellar parameters. This and other improvements are used to derive radial velocities with uncertainties which are generally within 0.1 km s$^{−1}$ or ∼25 per cent smaller than in the previous release. Median differences in radial velocities measured here and by the Gaia DR2 or APOGEE DR16 surveys are smaller than 30 m s$^{−1}$, a larger offset is present only for Gaia measurements of giant stars. We identify 4483 stars with intrinsically variable velocities and 225 stars for which the velocity stays constant over ≥3 visits spanning more than a year. The combination of radial velocities from GALAH+ with distances and sky plane motions from Gaia enables studies of dynamics within streams and clusters. For example, we estimate that the open cluster M67 has a total mass of ∼3300 M$_⊙$ and its outer parts seem to be expanding, though astrometry with a larger time-span than currently available from Gaia eDR3 is needed to judge if the latter result is real.

Gaia DR3 opens a new era of all-sky spectral analysis of stellar populations thanks to the nearly 5.6 million stars observed by the RVS and parametrised by the GSP-spec module. The all-sky Gaia chemical 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 [alpha/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. Several alpha, 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, the Gaia chemical data have the required coverage and precision to unveil galaxy accretion debris and heated disc stars on halo orbits through their [alpha/Fe] ratio, and to allow the study of the chemo-dynamical properties of globular clusters. Gaia DR3 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 (abridged)., Astronomy and Astrophysics (accepted, in press)

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.\ud \ud 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).\ud \ud 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.\ud \ud 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.

Ionized gas probes the influence of massive stars on their environment. The Cygnus X region (d~1.5 kpc) is one of the most massive star forming complexes in our Galaxy, in which the Cyg OB2 association (age of 3-5 Myr and stellar mass $2 \times 10^{4}$ M$_{\odot}$) has a dominant influence. We observe the Cygnus X region at 148 MHz using the Low Frequency Array (LOFAR) and take into account short-spacing information during image deconvolution. Together with data from the Canadian Galactic Plane Survey, we investigate the morphology, distribution, and physical conditions of low-density ionized gas in a $4^{\circ} \times 4^{\circ}$ (100 pc $\times$ 100 pc) region at a resolution of 2' (0.9 pc). The Galactic radio emission in the region analyzed is almost entirely thermal (free-free) at 148 MHz, with emission measures of $10^3 < EM~{\rm[pc~cm^{-6}]} < 10^6$. As filamentary structure is a prominent feature of the emission, we use DisPerSE and FilChap to identify filamentary ridges and characterize their radial ($EM$) profiles. The distribution of radial profiles has a characteristic width of 4.3 pc and a power-law distribution ($β= -1.8 \pm 0.1$) in peak $EM$ down to our completeness limit of 4200 pc cm$^{-6}$. The electron densities of the filamentary structure range from $10 < n_e~{\rm[cm^{-3}]} < 400$ with a median value of 35 cm$^{-3}$, remarkably similar to [N II] surveys of ionized gas. Cyg OB2 may ionize at most two-thirds of the total ionized gas and the ionized gas in filaments. More than half of the filamentary structures are likely photoevaporating surfaces flowing into a surrounding diffuse (~5 cm$^{-3}$) medium. However, this is likely not the case for all ionized gas ridges. A characteristic width in the distribution of ionized gas points to the stellar winds of Cyg OB2 creating a fraction of the ionized filaments through swept-up ionized gas or dissipated turbulence., 19 pages, 14 figures, 1 table; accepted for publication in A&A

A pioneering study showed that the fine structure in the luminosity function (LF) of young star clusters contains information about the evolutionary stage (age) and composition of the stellar population. The notable features include the H-peak, which is the result of the onset of hydrogen burning turning pre-main sequence stars into main sequence stars. The feature moves toward the faint end of the LF, and eventually disappears as the population evolves. Another detectable feature is the Wielen dip, a dip at M_V ~ 7 mag in the LF first identified in 1974 for stars in the solar environment. Later studies also identified this feature in the LF of star clusters. The Wielen dip is caused by the increased importance of H- opacity in a certain range of low-mass stars. We studied the detailed structure in the luminosity function using the data from Gaia DR2 and PARSEC stellar evolution models with the aim to further our understanding of young stellar populations. We analyzed the astrometric properties of stars in the solar neighborhood (< 20 pc) and in various relatively nearby (< 400 pc) young (< 50 Myr) open clusters and OB associations, and compare the features in the luminosity function with those generated by PARSEC models. The Wielen dip is confirmed in the LF of all the populations, including the solar neighborhood, at M_G ~7 mag. The H-peak is present in the LF of the field stars in the solar neighborhood. It likely signals that the population is mixed with a significant number of stars younger than 100 Myr. The H-peak is found in the LF of young open clusters and OB associations, and its location varies with age. Our observations with Gaia DR2 confirm the evolution of the H-peak from 5 Myr up to 47 Myr. The fine structure in the luminosity function in young stellar populations can be used to estimate their age., 16 pages, A&A in press

Spectroscopy from the final internal data release of the Gaia-ESO Survey (GES) has been combined with Gaia EDR3 to assign membership probabilities to targets observed towards 63 Galactic open clusters and 7 globular clusters. The membership probabilities are based chiefly on maximum likelihood modelling of the 3D kinematics of the targets, separating them into cluster and field populations. From 43211 observed targets, 13985 are identified as highly probable cluster members ($P>0.9$), with an average membership probability of 0.993. The addition of GES radial velocities successfully drives down the fraction of false positives and we achieve better levels of discrimination in most clusters over the use of astrometric data alone, especially those at larger distances. Since the membership selection is almost purely kinematic, the union of this catalogue with GES and Gaia is ideal for investigating the photometric and chemical properties of clusters as a function of stellar mass, age and Galactic position., Accepted version for publication in MNRAS. 16 pages + 38 pages of Appendices

The properties of a planetary system may be influenced by properties of the host star, such as its birth environment or age. Recently, a link has been found between a star's local phase space density in the Galaxy and the presence of a hot Jupiter: hot Jupiters are preferentially found around stars with higher local phase space densities. Using kinematic data for exoplanet hosts from Gaia EDR3, we show that the phase space density is primarily determined by a star's Galactic kinematics -- notably, its orbital velocity around the Galactic centre relative to a circular orbit. Stars on kinematically "colder" orbits have higher local phase space densities than stars on kinematically "hotter" orbits. Once this effect is accounted for, we find no evidence that hot Jupiter hosts are in higher-density regions of phase space than other stars. Younger stars are on average on colder orbits, and older stars are on average on hotter orbits, so we argue that the preference for hot Jupiter hosts to be in high-density regions reflects an age bias: hot Jupiters are more common around younger stars, before their population is reduced by tidal inspiral. Our work gives a hint of the relations that PLATO may uncover when it provides precise and accurate ages for a large number of main-sequence exoplanet hosts., AJM acknowledges funding from the Swedish Research Council (grant 2017-04945), the Swedish National Space Agency (grant 120/19C), and the Fund of the Walter Gyllenberg Foundation of the Royal Physiographic Society in Lund., {"references":["Winter et al (2020). Nature, 586, 528","Collier Cameron & Jardine (2018). MNRAS, 476, 2542","Hamer & Schlaufmann (2019). AJ, 158, 190"]}

In this work we use a series of open clusters and associations observed by the Gaia-ESO Survey (GES) to study the use of lithium abundances (Li I spectral line at 6708 Å) as an age indicator for pre- and main-sequence FGKM late-type stars. Previous studies of open clusters have shown that lithium depletion is not only strongly age dependent, but also shows a complex pattern with several other parameters, such as rotation, chromospheric activity and metallicity. Using the available data from both GES iDR6 and Gaia EDR3, we performed a thorough membership analysis and obtained lists of candidate members for 41 open clusters, ranging in age from 1-3 Myr to 5 Gyr. We then conducted a comparative study that allowed us to quantify the observable lithium dispersion in each cluster and study influence of rotation, activity and metallicity in the lithium dispersion of the selected candidates. All this allows us to calibrate a Li-age relation and create empirical lithium envelopes for several clusters in our sample., {"references":["Gilmore et al 2012 (https://ui.adsabs.harvard.edu/abs/2012Msngr.147...25G/abstract)","Bressan et al 2012 (https://ui.adsabs.harvard.edu/abs/2012MNRAS.427..127B/abstract)","Gaia Collaboration et al 2021 (https://ui.adsabs.harvard.edu/abs/2021A%26A...649A...1G/abstract)","Montes et al 2001 (https://ui.adsabs.harvard.edu/abs/2001A%26A...379..976M/abstract)","Gutiérrez Albarrán et al 2020 (https://ui.adsabs.harvard.edu/abs/2020A%26A...643A..71G/abstract)","Baraffe et al 2015 (https://ui.adsabs.harvard.edu/abs/2015A%26A...577A..42B/abstract)"]}

[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., L.C. acknowledges the support of the postdoc fellowship from the French Centre National d’Etudes Spatiales (CNES). FA acknowledges funding from MICINN (Spain) through the Juan de la Cierva-Incorporacián program under contract IJC2019-04862-I. We also thank undergraduate students Jaume Dolcet and Ignacio García-Soriano for collaborating in preliminary studies for this work. This work was (partially) supported by the Spanish Ministry of Science, Innovation and University (MICIU/FEDER, UE) through grant RTI2018-095076-B-C21, and the Institute of Cosmos Sciences University of Barcelona (ICCUB, Unidad de Excelencia ‘María de Maeztu’) through grant CEX2019-000918-M.

Aims. We aim to demonstrate the scientific potential of the Gaia Early Data Release 3 (EDR3) for the study of different aspects of the Milky Way structure and evolution and we provide, at the same time, a description of several practical aspects of the data and examples of their usage., Methods. We used astrometric positions, proper motions, parallaxes, and photometry from EDR3 to select different populations and components and to calculate the distances and velocities in the direction of the anticentre. In this direction, the Gaia astrometric data alone enable the calculation of the vertical and azimuthal velocities; also, the extinction is relatively low compared to other directions in the Galactic plane. We then explore the disturbances of the current disc, the spatial and kinematical distributions of early accreted versus in situ stars, the structures in the outer parts of the disc, and the orbits of open clusters Berkeley 29 and Saurer 1., Results. With the improved astrometry and photometry of EDR3, we find that: (i) the dynamics of the Galactic disc are very complex with oscillations in the median rotation and vertical velocities as a function of radius, vertical asymmetries, and new correlations, including a bimodality with disc stars with large angular momentum moving vertically upwards from below the plane, and disc stars with slightly lower angular momentum moving preferentially downwards; (ii) we resolve the kinematic substructure (diagonal ridges) in the outer parts of the disc for the first time; (iii) the red sequence that has been associated with the proto-Galactic disc that was present at the time of the merger with Gaia-Enceladus-Sausage is currently radially concentrated up to around 14 kpc, while the blue sequence that has been associated with debris of the satellite extends beyond that; (iv) there are density structures in the outer disc, both above and below the plane, most probably related to Monoceros, the Anticentre Stream, and TriAnd, for which the Gaia data allow an exhaustive selection of candidate member stars and dynamical study; and (v) the open clusters Berkeley 29 and Saurer 1, despite being located at large distances from the Galactic centre, are on nearly circular disc-like orbits., Conclusions. Even with our simple preliminary exploration of the Gaia EDR3, we demonstrate how, once again, these data from the European Space Agency are crucial for our understanding of the different pieces of our Galaxy and their connection to its global structure and history.

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) > 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.

[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, μα*, μδ), 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%., Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. The Gaia mission website is http://www.cosmos.esa.int/gaia. The authors are current or past members of the ESA Gaia mission team and of the Gaia DPAC. This work was partially supported by the MINECO (Spanish Ministry of Economy) through grant ESP2016-80079-C2-1-R and RTI2018-095076-B-C21 (MINECO/FEDER, UE), and MDM-2014-0369 of ICCUB (Unidad de Excelencia “María de Maeztu”). This work has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement H2020-MSCA-COFUND-2016-754433. This work has been partially supported by the Spanish Government (SEV2015-0493), by the Spanish Ministry of Science and Innovation (contract TIN2015-65316-P), by Generalitat de Catalunya (contract 2014-SGR-1051). The research leading to these results has also received funding from the collaboration between Fujitsu and BSC (Script Language Platform). L.C. acknowledges support from “programme national de physique stellaire” (PNPS) and from the “programme national cosmologie et galaxies”.

Understanding how stars form is one of the fundamental questions which astronomy aims to answer. Currently, it is well accepted that the majority of stars form in groups and that their predominant mechanism of formation is the core-collapse. However, several mechanisms have been suggested to explain the formation of substellar objects, and their contribution is still under debate. The main goal of this thesis is to determine the initial mass function, the mass distribution of stars at birth time, in different associations and star-forming regions. The mass function constitutes a fundamental observational parameter to constrain stellar and substellar formation theories since different formation mechanisms predict different fraction of stellar and substellar objects. We used the Gaia Data Release 2 catalogue together with ground-based observations from the COSMIC-DANCe project to look for high probability members via a probabilistic model of the distribution of the observable quantities in both the cluster and background populations. We applied this method to the 30 Myr open cluster IC 4665 and the 1 - 10 Myr star-forming region Upper Scorpius (USC) and r Ophiuchi (r Oph). We found very rich populations of substellar objects which largely exceed the numbers predicted by core-collapse models. In USC, where our sensitivity is best, we found a large number of free-floating planets and we suggest that ejection from planetary systems must have a similar contribution than core-collapse in their formation. The age is a fundamental parameter to study the formation and evolution of stars and is essential to accurately convert luminosities to masses. For that, we also presented a strategy to study the dynamical traceback age of young local associations through an orbital traceback analysis. We applied this method to determine the age of the b Pictoris moving group and in the future, we plan to apply it to other regions such as USC. The members we identified with the membership analysis are excellent targets for follow-up studies such as a search for discs, exoplanets, characterisation of brown dwarfs and free-floating planets. I this thesis, we presented a search for discs hosted by members of IC 4665 and we found six excellent candidates to be imaged with ALMA or the JWST. The tools we developed, are ready to be used in other regions such as USC and r Oph, where we expect to find a larger number of disc-host stars., Comprendre comment se forment les étoiles est l’une des questions fondamentales auxquelles l’astronomie entend répondre. Malheureusement, nous ne pouvons pas étudier la formation stellaire en temps réel et différentes méthodes indirectes doivent être utilisées pour faire la lumière sur ce sujet. L’objectif principal de cette thèse est de déterminer la fonction de masse initiale, la distribution de masse des étoiles à leur naissance, dans différentes associations et régions de formation d’étoiles. La fonction de masse est le produit direct de la formation stellaire et constitue donc un paramètre d’observation fondamental pour contraindre les théories de formation stellaire et sous-estellaire. Nous nous sommes concentrés sur l’amas ouvert de 30 Ma IC 4665 et la région de formation d’étoiles de 1 - 10 Ma de Upper Scorpius (USC) et r Ophiuchi (r Oph). Nous avons combiné l’astrométrie et la photométrie de Gaia Data Release 2 avec nos observations au sol pour préparer un catalogue profond et étendu de chaque région. Ensuite, nous avons calculé les probabilités d’appartenance en utilisanttoute l’astrométrie et la photométrie disponibles et identifié les membres à haute probabilité. Nous avons utilisé la liste finale des membres pour estimer la distribution de magnitude, et les fonctions de luminosité et masse de ces associations. Alors que la première a l’avantage d’être indépendante des modèles d’évolution, tandis que les fonctions de luminosité et de masse peuvent être utilisées pour contraindre les mécanismes de formation d’étoiles. L’étude d’IC 4556 nous a permis d’identifier des objets sous-stellaires, sans pour autant pouvoir fournir un recensement complet dans ce domaine de masse. Dans USC et r Oph, nous avons identifié une population très riche d’objets sous-stellaires, significativement plus nombreux que les prédictions des modèles de formation par effondrement de coeurs moléculaires, suggérant que la formation de naines brunes et d’objets de masses planétaires isolés par des phénomènes d’éjection dans des systèmes planétaires a une contribution importante et du même ordre que l’effondrement des coeurs moléculaires à la population finale d’objets dans un amas. L’âge est un paramètre fondamental pour étudier la formation et l’évolution des étoiles pour plusieurs raisons: premièrement puisqu’il établit une échelle de temps sur laquelle placer les observations. Deuxièmement car il est essentiel pour convertir les luminosités en masses, avec l’aide de modèles d’évolution stellaire. Les incertitudes sur l’age de USC et r Oph se traduisant en erreurs importantes dans notre estimation de la fonction de masse, j’ai développé une stratégie d’étude de "l’âge dynamique" au moyen d’une analyse orbitale de traçage des mouvements des membres d’associations jeunes. J’ai ainsi mis au point une stratégie incluant i) les observations et la recherche de données dans les archives publiques, ii) la réduction et l’analyse des spectres échelles obtenus; iii) et l’analyse dynamique, pour déterminer l’âge d’une association. La méthodologie, développée avec l’association b Pictoris (b Pic), est prête à être appliquée à d’autres régions et en particulier à USC et r Oph. Les membres que nous avons identifiés sont par ailleurs d’excellentes cibles pour des études complémentaires telles que la recherche de disques (produit également fondamental de la formation stellaire), d’exoplanètes, de système multiples, mais aussi pour la caractérisation des atmosphères et propriétés physiques des naines brunes et des planètes errantes. [...]

We used Gaia DR2 data in order to estimate parameters of the Pleiades. With the data on stars with m_G, 18 pages, 10 figures, 1 table

Context. Lupus is recognised as one of the closest star-forming regions, but the lack of trigonometric parallaxes in the pre-Gaia era hampered many studies on the kinematic properties of this region and led to incomplete censuses of its stellar population. Aims. We use the second data release of the Gaia space mission combined with published ancillary radial velocity data to revise the census of stars and investigate the 6D structure of the Lupus complex. Methods. We performed a new membership analysis of the Lupus association based on astrometric and photometric data over a field of 160 deg2 around the main molecular clouds of the complex and compared the properties of the various subgroups in this region. Results. We identified 137 high-probability members of the Lupus association of young stars, including 47 stars that had never been reported as members before. Many of the historically known stars associated with the Lupus region identified in previous studies are more likely to be field stars or members of the adjacent Scorpius-Centaurus association. Our new sample of members covers the magnitude and mass range from G 8 to G 18 mag and from 0.03 to 2.4 M-, respectively. We compared the kinematic properties of the stars projected towards the molecular clouds Lupus 1-6 and showed that these subgroups are located at roughly the same distance (about 160 pc) and move with the same spatial velocity. Our age estimates inferred from stellar models show that the Lupus subgroups are coeval (with median ages ranging from about 1 to 3 Myr). The Lupus association appears to be younger than the population of young stars in the Corona-Australis star-forming region recently investigated by our team using a similar methodology. The initial mass function of the Lupus association inferred from the distribution of spectral types shows little variation compared to other star-forming regions. Conclusions. In this paper, we provide an updated sample of cluster members based on Gaia data and construct the most complete picture of the 3D structure and 3D space motion of the Lupus complex., With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (MDM-2017-0737)

Eclipsing binary stars allow derivation of accurate and precise masses and radii. When they reside in star clusters, properties of even higher precision, along with additional information, can be extracted. Asteroseismology of solar-like oscillations offers similar possibilities for single stars. We improve the previously established properties of the Hyades eclipsing binary HD27130 and re-assess the asteroseismic properties of the giant star $\epsilon$ Tau. The physical properties of these members of the Hyades are then used to constrain the helium content and age of the cluster. New multi-colour light curves were combined with multi-epoch radial velocities to yield masses and radii of HD27130. $T_{\rm eff}$ was derived from spectroscopy and photometry, and verified using the Gaia parallax. We estimate the cluster age from re-evaluated asteroseismic properties of $\epsilon$ Tau while using HD27130 to constrain the helium content. The masses and radii, and $T_{\rm eff}$ of HD 27130 were found to be $M=1.0245\pm0.0024 M_{\odot}$, $R=0.9226\pm0.015 R_{\odot}$, $T_{\rm eff}=5650\pm50$ K for the primary, and $M=0.7426\pm0.0016 M_{\odot}$, $R=0.7388\pm0.026 R_{\odot}$, $T_{\rm eff}=4300\pm100$ K for the secondary component. Our re-evaluation of $\epsilon$ Tau suggests that the previous literature estimates are trustworthy, and that the Hipparcos parallax is more reliable than the Gaia DR2 parallax. The helium content of HD27130 and thus of the Hyades is found to be $Y=0.27$ but with significant model dependence. Correlations with the adopted metallicity results in a robust helium enrichment law with $\frac{\Delta Y}{\Delta Z}$ close to 1.2. We estimate the age of the Hyades to be 0.9 $\pm$ 0.1 (stat) $\pm$ 0.1 (sys) Gyr in slight tension with recent age estimates based on the cluster white dwarfs. (abridged), Comment: 14 pages, 8 figures, accepted for publication in Astronomy & Astrophysics

Context: Stellar clusters are benchmarks for theories of star formation and evolution. The high precision parallax data of the Gaia mission allows significant improvements in the distance determination to stellar clusters and its stars. In order to have accurate and precise distance determinations, systematics like the parallax spatial correlations need to be accounted for, especially for stars in small sky regions. Aims: Provide the astrophysical community with a free and open code designed to simultaneously infer cluster parameters (i.e. distance and size) and the distances to its stars using Gaia parallax measurements. It includes cluster oriented prior families and is specifically designed to deal with the Gaia parallax spatial correlations. Methods: A Bayesian hierarchical model is created to allow 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. Conclusions: The combination of prior knowledge with the treatment of Gaia parallax spatial correlations produces accurate (error, Comment: 14 pages, 10 figures, accepted in A&A, for the associated code visit https://github.com/olivares-j/Kalkayotl

International audience; Thanks to Gaia DR2, the science of Open Clusters is revitalised. New memberships are now available for a large number of stars and clusters, fostering the determination of their physical properties and the study of the global properties of the population of Open Clusters in the Galaxy. A biased selection of recent resources and results is presented here.

International audience; Galactic Open Clusters (OCs) are crucial to investigate the formation and evolution of the Galactic disc. The Open Clusters Chemical Abundance from Spanish Observatory survey (OCCASO) aims to provide high precision radial velocity (typical uncertainties between 100 to 200 m/s) and to determine abundances for more than 20 chemical species in around 30 Northern OCs (Casamiquela et al, 2016). We use high resolution (R > 65,000) high signal-to-noise (˜70) spectra of at least 6 Red Clump stars for each cluster. In this work we combined Gaia DR2 mean parallaxes and proper motions, and OCCASO radial velocities to obtain 3D spatial velocities and determine the orbits of the clusters. We also study Galactic trends with Galactocentric radius and age of Fe peak and alpha elements for the 18 OCs currently observed in the OCCASO survey, and we compare them with chemo-dynamical models of the Milky Way.

In the present study, we have used Deep CCD images of the extremely young open star cluster NGC 6611, up to a limiting magnitude of V ∼ 22.86 mag in V, R and I passbands. The resulting color-magnitude V; (V–I) diagram as well as their radial density profiles has been determined. Using 2MASS data, we confirmed the consistency between the 2MASS photometry, by fitting isochrones, the extinction E(V–I) = 0.530 ± 0.04 mag, E(J–H) = 0.31 ± 0.02, from the color magnitude diagram the cluster distance =2.2 ± 0.21 kpc and age = 3.6 Myr, based on the fitting of theoretical stellar isochrones of solar metallicity Z = 0.019. The distance modulus of the cluster is estimated at 12.3. The radial stellar density profiles and the cluster center have been determined by two methods. The core and cluster radii are determined from the radial stellar density profiles. Only about 40% of the cluster members are present in the core region. The cluster luminosity function has been calculated. The mass function slope of the entire cluster is ∼−0.67 ± 0.12. The effects of mass segregation, most probably due to dynamical evolution, have been observed in the cluster.

On the basis of the authors’ compiled catalog containing spectroscopic abundances of 14 chemical elements for 90 open star clusters of the Galaxy, we show that in the young clusters not only barium but also three other studied elements of slow neutron-capture, Y, La, and Ce, reveal higher relative abundances than those in the field stars, with differences beyond the error limits. We also find that, at high metallicity ([Fe/H]> −0.1), the relative abundances of the r-process element Eu in the clusters with eccentric, high Z max orbits are lower, and the relative abundances of primary α-elements O and Mg are higher, than the average values for the field giants. At the same time, at lower metallicity, the [Eu/Fe] ratios in the clusters are, on average, the same as in the field, but with a much larger dispersion, and the [O, Mg/Fe] ratios are lower than in the field giants. Taking into account that both α-elements and r-elements are ejected into the interstellar medium as a result of Type II supernova explosions, and that their yields are dependent on the pre-supernova mass, these properties naturally fit into the assumption that the clusters of different metallicity with eccentric, high Z max orbits are formed by interaction of two types of high-velocity clouds with the interstellar medium of the Galactic disk. On the one hand, these are low-metallicity high-velocity clouds formed from the “primary” gas; on the other hand, there are more metal-rich intermediate-velocity clouds generated in the Galactic “fountain” process.

A good fraction of very high energy (VHE; > 100 GeV) sources do not have confirmed counterparts at any of the other branches of electromagnetic spectrum. In the extra-galactic sky, most of the VHE sources with counter-parts are blazars, a sub-class of active galactic nuclei. However, VHE sources with identified counterparts in our Galaxy do have a wide range of categories, ranging such as supernovae remnants, colliding wind binaries, pulsar wind nebulae, micro-quasars, etc. The VHE emission and radio emission due to synchrotron process do occur from the same population of relativistic electrons in several type of sources, so the radio band is the first band to search for possible counterparts to unidentified VHE sources. In addition, the radio spectral index also gives important constraints on compactness and energetics of the electron population. In this work we present a two-frequency radio survey, at frequencies 325 MHz and 610 MHz, of the Cygnus OB2 region with the Giant Metrewave Radio Telescope (GMRT) in order to address several astrophysical problems. The survey covers a wide area (~ 5 square degrees) at sub-mJy sensitivity which helps to discover faint radio sources to several objects for the first time. Also this is the first deep wide area radio survey of the region below 1.4 GHz, and the low radio frequency will preferentially pickup more non-thermal radio sources as compared to high radio frequencies. Here, we present the early results from this survey.

Asteroseismology offers the possibility of probing stellar interiors and testing evolutionary and seismic models. Precise photometry and spectroscopy obtained during multi-site campaigns on young open clusters allows discovering rich samples of pulsating stars and using them in a simultaneous seismic modelling called ensemble asteroseismology. The aim of this study is to obtain the age of the open cluster NGC 6910 by means of ensemble asteroseismology of the early-type pulsating members, to derive their stellar parameters, and to classify the excited modes. We used time-series analysis, performed photometric and spectroscopic mode identification, and calculated grids of evolutionary and seismic models to apply the procedure of ensemble asteroseismology for nine pulsating members of NGC 6910. With two iterations of the procedure of ensemble asteroseismology applied to nine pulsating stars we derived an age of 10.6$^{+0.9}_{-0.8}$ Myr for NGC 6910. Of the nine pulsating stars examined in the paper, eight are $\beta$ Cep stars, including three that are hybrid $\beta$ Cep and slowly pulsating B-type (SPB) pulsators, and one is an SPB star. Interestingly, the least massive $\beta$ Cep star, NGC 6910-38, has a mass of about 5.6 M$_\odot$. The present theory does not predict unstable $p$ modes in B-type stars with such a low mass. The $g$ modes with relatively high frequencies ($>3.5$ d$^{-1}$), observed in three members of the cluster, are also stable according to seismic modelling. Both findings pose a challenge for theoretical calculations and prompt a revision of the opacities. The procedure of ensemble asteroseismology was found to be successful for NGC 6910 and $\chi$ Per on the basis of pulsating B-type stars and can therefore be applied to other young open clusters that are rich in such stars., Comment: 22 pages, 24 figures. Caution: arXiv abstract is shorter than in the article

The publication of the Gaia Data Release 2 (Gaia DR2) opens a new era in Astronomy. It includes precise astrometric data (positions, proper motions and parallaxes) for more than $1.3$ billion sources, mostly stars. To analyse such a vast amount of new data, the use of data mining techniques and machine learning algorithms are mandatory. The search for Open Clusters, groups of stars that were born and move together, located in the disk, is a great example for the application of these techniques. Our aim is to develop a method to automatically explore the data space, requiring minimal manual intervention. We explore the performance of a density based clustering algorithm, DBSCAN, to find clusters in the data together with a supervised learning method such as an Artificial Neural Network (ANN) to automatically distinguish between real Open Clusters and statistical clusters. The development and implementation of this method to a $5$-Dimensional space ($l$, $b$, $\varpi$, $\mu_{\alpha^*}$, $\mu_\delta$) to the Tycho-Gaia Astrometric Solution (TGAS) data, and a posterior validation using Gaia DR2 data, lead to the proposal of a set of new nearby Open Clusters. We have developed a method to find OCs in astrometric data, designed to be applied to the full Gaia DR2 archive., Comment: 18 pages, accepted by Astronomy and Astrophysics (A&A) the 11th June, 2018

The Lyapunov, Lagrange and Poincaré criteria are tested for orbits of 461 open clusters in an axisymmetric potential. Lyapunov exponents and Poincaré sections are computed, and all of the trajectories are found to be stable according to the Lagrange and Poincaré stability criteria. At the same time, some trajectories appear to exhibit minor instability according to the Lyapunov stability criterion.

Through the modeling of near-infrared photometry of star-plus disk systems with the codes bedisk/beray, we successfully describe the Wide-Field Infrared Survey Explorer (WISE) photometric characteristics of Be stars in five young open clusters, NGC 663, NGC 869, NGC 884, NGC 3766, and NGC 4755, broadly studied in the literature. WISE photometry allows previously known Be stars to be detected and to find new Be candidates which could be confirmed spectroscopically. The location of Be stars in the WISE color-magnitude diagram, separates them in two groups; active (Be stars hosting a developed circumstellar disk) and quiescent objects (Be stars in a diskless phase), and this way, we can explore how often stars are observed in these different stages. The variability observed in most active variable Be stars is compatible with a disk dissipation phase. We find that 50% of Be stars in the studied open clusters are in an active phase. We can interpret this as Be stars having a developed circumstellar disk one-half of the time. The location of Be stars with a developed disk in the color-magnitude diagram require mass loss rates in agreement with values recently reported in the literature. For these objects, we expect to have a tight relation between the equivalent width of the Hα line and the mass of the disk, if the inclination is known. Also, near-infrared photometry of Be stars in stellar clusters has the potential of being useful to test whether there is a preferential viewing angle., Instituto de Astrofísica de La Plata

Context. Stellar physical properties of star clusters are poorly known and the cluster parameters are often very uncertain. Aims. Our goals are to perform a spectrophotometric study of the B star population in open clusters to derive accurate stellar parameters, search for the presence of circumstellar envelopes, and discuss the characteristics of these stars. Methods. The BCD spectrophotometric system is a powerful method to obtain stellar fundamental parameters from direct measurements of the Balmer discontinuity. To this end, we wrote the interactive code MIDE3700. The BCD parameters can also be used to infer the main properties of open clusters: distance modulus, color excess, and age. Furthermore, we inspected the Balmer discontinuity to provide evidence for the presence of circumstellar disks and identify Be star candidates. We used an additional set of high-resolution spectra in the H region to confirm the Be nature of these stars. Results. We provide Te, log g, Mv, Mbol, and spectral types for a sample of 68 stars in the field of the open clusters NGC6087, NGC6250, NGC6383, and NGC6530, as well as the cluster distances, ages, and reddening. Then, based on a sample of 230 B stars in the direction of the 11 open clusters studied along this series of three papers, we report 6 new Be stars, 4 blue straggler candidates, and 15 B-type stars (called Bdd) with a double Balmer discontinuity, which indicates the presence of circumstellar envelopes. We discuss the distribution of the fraction of B, Be, and Bdd star cluster members per spectral subtype. The majority of the Be stars are dwarfs and present a maximum at the spectral type B2-B4 in young and intermediate-age open clusters (, Facultad de Ciencias Astronómicas y Geofísicas, Instituto de Astrofísica de La Plata

The origin and evolution of stellar populations is one of the greatest challenges in modern astrophysics. It is known that the majority of the stars has its origin in stellar clusters (Carpenter 2000; Porras et al. 2003; Lada & Lada 2003). However, only less than one tenth of these clusters remains bounded after the first few hundred million years (Lada & Lada 2003). Ergo, the understanding of the origin and evolution of stars demands meticulous analyses of stellar clusters in these crucial ages.The project Dynamical Analysis of Nearby Clusters (DANCe, Bouy et al. 2013), from which the present work is part of, provides the scientific framework for the analysis of Nearby Young Clusters (NYC) in the solar neighbourhood (< 500 pc). The DANCe carefully designed observations of the well known Pleiades cluster provide the perfect case study for the development and testing of statistical tools aiming at the analysis of the early phases of cluster evolution.The statistical tool developed here is a probabilistic intelligent system that performs Bayesian inference for the parameters governing the probability density functions (PDFs) of the cluster population (PDFCP). It has been benchmarked with the Pleiades photometric and astrometric data of the DANCe survey. As any Bayesian framework, it requires the setting up of priors. To avoid the subjectivity of these, the intelligent system establish them using the Bayesian Hierarchical Model (BHM) approach. In it, the parameters of prior distributions, which are also inferred from the data, are drawn from other distributions in a hierarchical way.In this BHM intelligent system, the true values of the PDFCP are specified by stochastic and deterministic relations representing the state of knowledge of the NYC. To perform the parametric inference, the likelihood of the data, given these true values, accounts for the properties of the data set, especially its heteroscedasticity and missing value objects. By properly accounting for these properties, the intelligent system: i) Increases the size of the data set, with respect to previous studies working exclusively on fully observed objects, and ii) Avoids biases associated to fully observed data sets, and restrictions to low-uncertainty objects (sigma-clipping procedures).The BHM returns the posterior PDFs of the parameters in the PDFCPs, particularly of the spatial, proper motions and luminosity distributions. In the BHM each object in the data set contributes to the PDFs of the parameters proportionally to its likelihood. Thus, the PDFCPs are free of biases resulting from typical high membership probability selections (sampling bias).As a by-product, the BHM also gives the PDFs of the cluster membership probability for each object in the data set. These PDFs together with an optimal probability classification threshold, which is obtained from synthetic data sets, allow the classification of objects into cluster and field populations. This by-product classifier shows excellent results when applied on synthetic data sets (with an area under the ROC curve of 0.99). From the analysis of synthetic data sets, the expected value of the contamination rate for the PDFCPs is 5.8 ± 0.2%.The following are the most important astrophysical results of the BHM applied tothe Pleiades cluster. First, used as a classifier, it finds ∼ 200 new candidate members, representing 10% new discoveries. Nevertheless, it shows outstanding agreement (99.6% of the 105 objects in the data set) with previous results from the literature. Second, the derived present day system mass distribution (PDSMD) is in general agreement with the previous results of Bouy et al. (2015).Thus, by better modelling the data set and eliminating unnecessary restrictions to it, the new intelligent system, developed and tested in the present work, represents the state of the art for the statistical analysis of NYC populations., Il semble maintenant établi que la majorité des étoiles se forment dans des amas (Carpenter 2000; Porras et al. 2003; Lada & Lada 2003). Comprendre l'origine et l'évolution des populations stellaires est donc l'un des plus grands défis de l'astrophysique moderne. Malheureusement, moins d'un dixième de ces amas restent gravitationellement liés au delà de quelques centaines de millions d'années (Lada & Lada 2003). L’étude des amas stellaires doit donc se faire avant leur dissolution dans la galaxie.Le projet Dynamical Analysis of Nearby Clusters (DANCe, Bouy et al. 2013), dont le travail fait partie, fournit le cadre scientifique pour l'analyse des amas proches et jeunes (NYC) dans le voisinage solaire. Les observations de l'amas ouvert des Pléiades par le projet DANCe offrent une opportunité parfaite pour le développement d'outils statistiques visant à analyser les premières phases de l'évolution des amas.L'outil statistique développé ici est un système intelligent probabiliste qui effectue une inférence bayésienne des paramètres régissant les fonctions de densité de probabilité (PDF) de la population de l'amas (PDFCP). Il a été testé avec les données photométriques et astrométriques des Pléiades du relevé DANCe. Pour éviter la subjectivité de ces choix des priors, le système intelligent les établit en utilisant l'approche hiérarchique bayésienne (BHM). Dans ce cas, les paramètres de ces distributions, qui sont également déduits des données, proviennent d'autres distributions de manière hiérarchique.Dans ce système intelligent BHM, les vraies valeurs du PDFCP sont spécifiées par des relations stochastiques et déterministes représentatives de notre connaissance des paramètres physiques de l'amas. Pour effectuer l'inférence paramétrique, la vraisemblance (compte tenu de ces valeurs réelles), tient en compte des propriétés de l'ensemble de données, en particulier son hétéroscédasticité et des objects avec des valeurs manquantes.Le BHM obtient les PDF postérieures des paramètres dans les PDFCP, en particulier celles des distributions spatiales, de mouvements propres et de luminosité, qui sont les objectifs scientifiques finaux du projet DANCe. Dans le BHM, chaque étoile du catalogue contribue aux PDF des paramètres de l'amas proportionnellement à sa probabilité d'appartenance. Ainsi, les PDFCP sont exempts de biais d'échantillonnage résultant de sélections tronquées au-dessus d'un seuil de probabilité défini plus ou moins arbitrairement.Comme produit additionnel, le BHM fournit également les PDF de la probabilité d'appartenance à l'amas pour chaque étoile du catalogue d'entrée, qui permettent d'identifier les membres probables de l'amas, et les contaminants probables du champ. La méthode a été testée avec succès sur des ensembles de données synthétiques (avec une aire sous la courbe ROC de 0,99), ce qui a permis d'estimer un taux de contamination pour les PDFCP de seulement 5,8 %.Ces nouvelles méthodes permettent d'obtenir et/ou de confirmer des résultats importants sur les propriétés astrophysiques de l'amas des Pléiades. Tout d'abord, le BHM a découvert 200 nouveaux candidats membres, qui représentent 10% de la population totale de l'amas. Les résultats sont en excellent accord (99,6% des 100 000 objets dans l'ensemble de données) avec les résultats précédents trouvés dans la littérature, ce qui fournit une validation externe importante de la méthode. Enfin, la distribution de masse des systèmes actuelle (PDSMD) est en général en bon accord avec les résultats précédents de Bouy et al. 2015, mais présente l'avantage inestimable d'avoir des incertitudes beaucoup plus robustes que celles des méthodes précédentes.Ainsi, en améliorant la modélisation de l'ensemble de données et en éliminant les restrictions inutiles ou les hypothèses simplificatrices, le nouveau système intelligent, développé et testé dans le présent travail, représente l'état de l'art pour l'analyse statistique des populations de NYC.

In this paper we analyse the evolutionary status of three open clusters: NGC 1817, NGC 2141, and Berkeley 81. They are all of intermediate age, two are located in the Galactic anti-centre direction while the third one is located in the Galactic centre direction. All of them were observed with LBC@LBT using the Bessel B, V, and I filters. The cluster parameters have been obtained using the synthetic colour-magnitude diagram (CMD) method, i.e. the direct comparison of the observational CMDs with a library of synthetic CMDs generated with different evolutionary sets (Padova, FRANEC, and FST). This analysis shows that NGC 1817 has subsolar metallicity, age between 0.8 and 1.2 Gyr, reddening E(B-V) in the range 0.21 and 0.34, and distance modulus (m-M)_0 of about 10.9; NGC 2141 is older, with age in the range 1.25 and 1.9 Gyr, E(B-V) between 0.36 and 0.45, (m-M)_0 between 11.95 and 12.21, and subsolar metallicity; Berkeley~81 has metallicity about solar, with age between 0.75 and 1.0 Gyr, has reddening E(B-V)~0.90 and distance modulus (m-M)_0~12.4. Exploiting the large field of view of the instrument we derive the structure parameters for NGC 2141 and Be 81 by fitting a King profile to the estimated density profile. Combining this information with the synthetic CMD technique we estimate a lower limit for the cluster total mass for these two systems., 20 pages, 24 figures

Due to numerous new discoveries of open star clusters in the last two decades, astronomers need an easy-to-use resource to get visual information on the relative position of clusters in the sky. Therefore we propose a new atlas of open star clusters. It is based on a table compiled from the largest modern cluster catalogues. The atlas shows the positions and sizes of 3291 clusters and associations, and consists of two parts. The first contains 108 maps of 12 by 12 degrees with an overlapping of 2 degrees in three strips along the Galactic equator. The second one is an online web application, which shows a square field of an arbitrary size, either in equatorial coordinates or in galactic coordinates by request. The atlas is proposed for the sampling of clusters and cluster stars for further investigation. Another use is the identification of clusters among overdensities in stellar density maps or among stellar groups in images of the sky. © 2017 A. F. Seleznev et al. Acknowledgment: The work of Anton F. Seleznev was partly supported by the Ministry of Education and Science (the basic part of the State assignment, RK no. AAAA-A17-117030310283-7). The work of Anton F. Seleznev and of Eka-terina Avvakumova was supported also by the Act no. 211 of the Government of the Russian Federation, agreement no. 02.A03.21.0006. Authors are very grateful to Tatiana Selezneva for the graphic design of the web application.

Only a small number of galactic open clusters survives for longer than few hundred million years. Longer lifetimes are routinely explained in term of larger initial masses, particularly quiet orbits, and off-plane birth-places. We derive in this work the actual mass of NGC 4337, one of the few open clusters in the Milky Way inner disk that managed to survive for about 1.5 Gyr. We derive its mass in two different ways. First, we exploit an unpublished photometric data set in the UBVI passbands to estimate -using star counts- the cluster luminosity profile, and luminosity and mass function, and hence its actual mass both from the luminosity profile and from the mass function.This data-set is also used to infer crucial cluster parameters, as the cluster half-mass radius and distance. Second, we make use of a large survey of cluster star radial velocities to derive dynamical estimates for the cluster mass. Under the assumption of virial equilibrium and neglecting the external gravitational field leads to values for the mass significantly larger than those obtained by mean of observed density distribution or with the mass function but still marginally compatible with the inferred values of the invisible mass in form of both low mass stars or remnants of high mass stars in the cluster. Finally, we derive the cluster initial mass by computing the mass loss experienced by the cluster during its lifetime, and adopting the various estimates of the actual mass., Facultad de Ciencias Astronómicas y Geofísicas, Instituto de Astrofísica de La Plata

We present the results of a survey to detect low-mass companions of UMa group members, carried out in 2003-2006 with NACO at the ESO VLT. While many extra-solar planets and planetary candidates have been found in close orbits around stars by the radial velocity and the transit method, direct detections at wider orbits are rare. The Ursa Major (UMa) group, a young stellar association at an age of about 200-600 Myr and an average distance of 25 pc, has not yet been addressed as a whole although its members represent a very interesting sample to search for and characterize sub-stellar companions by direct imaging. Our goal was to find or to provide detection limits on wide sub-stellar companions around nearby UMa group members using high-resolution imaging. We searched for faint companions around 20 UMa group members within 30 pc. The primaries were placed below a semi-transparent coronagraph, a rather rarely used mode of NACO, to increase the dynamic range of the images. In most cases, second epoch images of companion candidates were taken to check whether they share common proper motion with the primary. Our coronagraphic images rule out sub-stellar companions around the stars of the sample. A dynamical range of typically 13-15 mag in the Ks band was achieved at separations beyond 3" from the star. Candidates as faint as Ks ~ 20 were securely identified and measured. The survey is most sensitive between separations of 100 and 200 au but only on average because of the very different target distance. Field coverage reaches about 650 au for the most distant targets. Most of the 200 candidates are visible in two epochs. All of those were rejected being distant background objects., accepted for publication in A&A; 25 pages, 15 figures, 7 tables

We present V - and I-passband photometry of massive stars in the Cyg OB1 and Cyg OB2 associations, based on about 80 observing nights spanning 300 days in the 2011 season. The variability of 22 supergiants and 48 OB-stars with luminosity classes III-V is analyzed. We report two new variable OB stars and 15 variable supergiants of which four are new discoveries. The light variations of Schulte 12 are interpreted as microvariability. We also present light curves of the red supergiants BC Cyg and BI Cyg which exhibit brightness drop of more than 0.4 mag during the season.

Bu tezde NGC 752, NGC 6940 ve Hyades açık yıldız kümelerindeki kırmızı dev yıldızlarının yüzey kimyasal bollukları, yüksek çözünürlüklü (R ≈ 60 000) ve sinyal/gürültü (S/G ≥ 120) oranlı tayflar kullanılarak elde edilmiştir. Kırmızı dev yıldızların küme üyelikleri dikine hız ölçümleri ile doğrulanmıştır. Kümelerin yaş ve dönme noktası kütlesi değerleri renk-parlaklık diyagramlarına uygulanan eş yaş eğrileri ile tespit edilmiştir. Model atmosfer parametreleri: etkin sıcaklık, yüzeyçekimleri, metal bolluğu, mikrotürbülans hızı (Teff, log g, [Fe/H], ξt) nötral ve iyonize demir ve titanyum çizgilerinin eşdeğer genişlikleri kullanılarak belirlenmiştir. NGC 752, NGC 6940 ve Hyades kümelerinin metal bollukları sırasıyla [Fe/H] = −0.02 ± 0.05, [Fe/H] = 0.04 ± 0.02 ve [Fe/H] = 0.12 ± 0.04 olarak hesaplanmıştır. Eşdeğer genişlik ya da sentetik tayf analizi yöntemleri kullanılarak hafif tek-atom numaralı (Na, Al), α (Mg, Si, Ca), Fe-grubu (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn), ve nötronyakalama (Y, La, Nd, Eu) elementleri için kimyasal bolluk analizi gerçekleştirilmiştir. LiCNO element bollukları ve 12C/13C izotop oranları da elde edilmiştir. Lityum bolluğu Li I 6707 Å rezonans çizgisinden, Oksijen bolluğu 6300 Å’da bulunan yasaklanmış [O I] çizgisinden hesaplanmıştır. Karbon bollukları 4300 Å yöresindeki CH G-bandı ile 5165 Å ve 5635 Å’de bulunan C2 Swan bantlarından; Nitrojen bollukları ve 12C/13C izotop oranları ise 7995–8040 Å aralığında yer alan 12,13CN molekül bantlarından elde edilmiştir. LiCNO bollukları, 12C/13C izotop oranları ve fotometrik veriler birlikte değerlendirilerek kırmızı dev yıldızlarının evrim durumları tahmin edilmiş ve sonuçlar yıldız evrim kuramları ile birlikte tartışılmıştır., In this thesis, surface chemical abundances of some selected red giant members of the NGC 752, NGC 6940 and Hyades open clusters were analyzed by using the high resolution (R ≈ 60 000) and high signal to noise ratio (S/N ≥ 120) spectra. Cluster memberhips were confirmed by measuring the stellar radial velocities. By applying isochrones to the colour-magnitude diagrams, age and turn-off mass parameters of the clusters were determined. Model atmosphere parameters (Teff, log g, [Fe/H] and ξt) were found via equivalent widths of the Fe I, Fe II, Ti I, and Ti II lines. Metallicities of the open clusters NGC 752, NGC 6940 and Hyades were derived as [Fe/H]= −0.02 ± 0.05, [Fe/H] = 0.04 ± 0.02 ve [Fe/H] = 0.12 ± 0.04, respectively. Abundances of the light odd-Z (Na, Al), α (Mg, Si, Ca), Fe-group (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn), and n-capture (Y, La, Nd, Eu) elements were calculated by using the equivalent width or synthetic spectrum analysis methods. The abundances of LiCNO and 12C/13C ratio were also determined. Lithium abundances were derived from the Li I 6707 Å resonance doublet and oxygen abundances were analyzed from the forbidden [O I] line at 6300 Å. Carbon abundances were determined using the CH G-band feature located near 4300 Å and the C2 Swan bandheads at 5165 and 5635 Å. Nitrogen abundances and 12C/13C were derived from 12,13CN molecular bands in the 7995-8040 Å region. By using the LiCNO abundances and, 12C/13C ratios along with the photometric data, evolutionary status of the red giants was estimated and the results were discussed in terms of stellar evolution theories.

The discovery of 2012VP113 initiated the debate on the origin of the Sedna family of planetesimals in orbit around the Sun. Sednitos roam the outer regions of the Solar System between the Egeworth--Kuiper belt and the Oort cloud, in extraordinary wide (a>150au) orbits with a large perihelion distance of q>30au compared to the Earth's (a=1au and eccentricity e=(1-q/a) ~ 0.0167 or q=1au). This population is composed of a dozen objects, which we consider a family because they have similar perihelion distance and inclination with respect to the ecliptic i=10--30deg. They also have similar argument of perihelion omega=340+/-55deg. There is no ready explanation for their origin. Here we show that these orbital parameters are typical for a captured population from the planetesimal disk of another star.Assuming the orbital elements of Sednitos have not changed since they acquired their orbits, we reconstruct the encounter that led to their capture. We conclude that they might have been captured in a near miss with a 1.8MSun star that impacted the Sun at ~340au at an inclination with respect to the ecliptic of 17--34deg with a relative velocity at infinity of ~4.3km/s. We predict that the Sednitos-region is populated by 930 planetesimals and the inner Oort cloud acquired ~440 planetesimals through the same encounter., Accepted for publication in MNRAS; 7 pages, 2 figures, 1 table; extended discussion, corrected typos and references

We present a new integral-field spectroscopic dataset of the central part of the Orion Nebula (M 42), observed with the MUSE instrument at the ESO VLT. We reduced the data with the public MUSE pipeline. The output products are two FITS cubes with a spatial size of ~5.9'x4.9' (corresponding to ~0.76 pc x 0.63 pc) and a contiguous wavelength coverage of 4595...9366 Angstrom, spatially sampled at 0.2". We provide two versions with a sampling of 1.25 Angstrom and 0.85 Angstrom in dispersion direction. Together with variance cubes these files have a size of 75 and 110 GiB on disk. They represent one of the largest integral field mosaics to date in terms of information content. We make them available for use in the community. To validate this dataset, we compare world coordinates, reconstructed magnitudes, velocities, and absolute and relative emission line fluxes to the literature and find excellent agreement. We derive a two-dimensional map of extinction and present de-reddened flux maps of several individual emission lines and of diagnostic line ratios. We estimate physical properties of the Orion Nebula, using the emission line ratios [N II] and [S III] (for the electron temperature $T_e$) and [S II] and [Cl III] (for the electron density $N_e$), and show two-dimensional images of the velocity measured from several bright emission lines., Comment: Resubmitted to A&A after incorporating referee comments; access to full dataset via http://muse-vlt.eu/science/data-releases

Wavelength measurements in stellar spectra cannot readily be interpretedas true stellar motion on the sub-km s-1 accuracy level dueto the presence of many other effects, such as gravitational redshiftand stellar convection, which also produce line shifts. Following arecommendation by the IAU, the result of an accurate spectroscopicradial-velocity observation should therefore be given as the``barycentric radial-velocity measure'', i.e. the absolute spectralshift as measured by an observer at zero gravitational potential locatedat the solar-system barycentre. Standard procedures for reducingaccurate radial-velocity observations should be reviewed to take intoaccount this recommendation. We describe a procedure to determineaccurate barycentric radial-velocity measures of bright stars, based ondigital cross-correlation of spectra obtained with the ELODIEspectrometer (Observatoire de Haute-Provence) with a synthetic templateof Fe I lines. The absolute zero point of the radial-velocity measuresis linked to the wavelength scale of the Kurucz (1984) Solar Flux Atlasvia ELODIE observations of the Moon. Results are given for the Sun and42 stars, most of them members of the Hyades and Ursa Major clusters.The median internal standard error is 27 m s-1. The externalerror is estimated at around 120 m s-1, mainly reflecting theuncertainty in the wavelength scale of the Solar Flux Atlas. For the Sunwe find a radial-velocity measure of +257+/- 11 m s-1referring to the full-disk spectrum of the selected Fe I lines. Based onobservations made at Observatoire de Haute-Provence

Fundamental parameters characterizing the end-state of intermediate-mass stars may be constrained by discovering planetary nebulae (PNe) in open clusters (OCs). Cluster membership may be exploited to establish the distance, luminosity, age, and physical size for PNe, and the intrinsic luminosity and mass of its central star. Four potential PN-OC associations were investigated, to assess the cluster membership for the PNe. Radial velocities were measured from intermediate-resolution optical spectra, complemented with previous estimates in the literature. When the radial velocity study supported the PN/OC association, we analyzed if other parameters (e.g., age, distance, reddening, central star brightness) were consistent with this conclusion. Our measurements imply that the PNe VBe3 and HeFa1 are not members of the OCs NGC5999 and NGC6067, respectively, and likely belong to the background bulge population. Conversely, consistent radial velocities indicate that NGC2452/NGC2453 could be associated, but our results are not conclusive and additional observations are warranted. Finally, we demonstrate that all the available information point to He2-86 being a young, highly internally obscured PN member of NGC4463. New near-infrared photometry acquired via the Vista Variables in the Via Lactea ESO public survey was used in tandem with existing UBV photometry to measure the distance, reddening, and age of NGC4463, finding d=1.55+-0.10 kpc, E(B-V)=0.41+-0.02, and tau=65+-10 Myr, respectively. The same values should be adopted for the PN if the proposed cluster membership will be confirmed., Accepted for publication in A&A

"From uvby − β photometry of the open cluster NGC 2343 (20 stars) we were able to determine membership of the stars to the cluster and fix its age and reddening."

"Using the historical database of the Washington Double Star Catalog (WDS) a nd astrometric data obtained by applying the Diffracto-Astrometry (DiAs) technique to Hubble Space Telescope Wide Field Planetary Camera 2 (HST/WFPC2) public images, the relative kinematics of the Orion Trapezium ( OT) stellar components is obtained with a precision of ∼ 1 km s − 1 ."

"The study of the binary and multiple star population in open clusters represents an important field of research in stellar astrophysics. However, the H-R diagram is afected if these stars are not taken into account, since the color dispersion among the stars in the CMD along the main sequence is due in pa rt to a large population of unresolved binary stars. But this problem cannot be solved by classical photomet ry because this technique does not have enough spatial resolution. One solution to this problem can be based on s peckle interferometry, a technique that allows high-precision astrometric measurements for binary star s: angular separation and position angle."

We employed the ESO Max Planck Institute (MPI) wide-field camera (Baade et al.) and obtained deep images in the VIC pass-bands in the region of the IC 2944/2948 complex (l ~ 294.°8; b ~ -1.°6), and complemented them with literature and archival data. We used this material to derive the photometric, spectroscopic and kinematic properties of the brightest (V < 16) stars in the region. The VI deep photometry on the other end, helped us to unravel the lower main sequence of a few, possibly physical, star groups in the area. Our analysis confirmed previous suggestions that the extinction towards this line of sight follows the normal law (RV = 3.1). We could recognize B-type stars spread in distance from a few hundred pc to at least 2 kpc. We found two young groups (age ~ 3 Myr) located, respectively, at about 2.3 and 3.2 kpc from the Sun. They are characterized by a significant variable extinction (E(B - V) ranging from 0.28 to 0.45 mag), and host a significant pre-main-sequence population. We computed the initial mass functions for these groups and obtained slopes G{cyrillic} from -0.94 to -1.02 (eG{cyrillic} = 0.3) in a scale where the classical Salpeter law is -1.35. We estimated the total mass of bothmain stellar groups in ~ 1100 and ~ 500M⊙, respectively. Our kinematic analysis indicated that both groups of stars deviate from the standard rotation curve of the Milky Way, in line with literature results for this specific Galactic direction. Finally, along the same line of sight, we identified a third group of early-type stars located at ~8 kpc from the Sun. This group might be located in the far side of the Sagittarius-Carina spiral arm., Facultad de Ciencias Astronómicas y Geofísicas, Instituto de Astrofísica de La Plata

We present a kinematic analysis of three triple stellar systems belonging to two open clusters: CPD-60{\deg}961 and HD66137 in NGC2516, and HD315031 in NGC6530. All three systems are hierarchical triples with a close binary bound to a third body in a wider orbit, whose presence is detected through velocity variations of the close binary barycentre. Orbital parameters are derived from radial velocity curves. Absolute parameters for all stars are estimated assuming cluster membership. Some dynamical and evolutionary aspects of these systems are discussed, particularly the possible influence of Kozai cycles. The two systems of NGC2516 have similar orbital configurations with inner periods of 11.23 d and 8.70 d and outer periods of 9.79 yr and 9.24 yr. We report also radial velocity measurements of the components of the visual binary CPD-60{\deg}944 in NGC2516. Including results from previous works, this cluster would harbor 5 hierarchical triples. The young system HD315031 has an inner binary with a period of 1.37 d and a very eccentric (e=0.85) outer orbit with a period of 483 d. Possible dynamical evolutionary scenarios are discussed. Long-term radial velocity monitoring is highlighted as strategy for the detection of subsystems with intermediate separations, which are hard to cover with normal spectroscopic studies or visual techniques., Comment: 9 pages, 5 figures. Accepted by Monthly Notices of the Royal Astronomical Society

Context. An OB association is a gravitationally unbound group of stars. The identification of the members of the association is an important aspect of its study. There are few astrometric membership studies for OB associations, but the combination of these studies with astrophysical techniques becomes a very powerful tool for this type of research. Aims. We perform new spectroscopic and astrometric studies of the members of the association Centaurus OB1 (Cen OB1) and the open cluster NGC 4755. Methods. To carry out this study we used new spectroscopic optical observations obtained at CASLEO, and proper motion and photometric data from the Tycho-2 Catalogue and other databases available via the Internet. Results. With this investigation we found 52 members of Cen OB1 (l = 303º.7, b = 0º.4) and four members of NGC 4755 (l = 303º.2, b = 2º.5), an open cluster integrated into the Cen OB1 stellar association. The distance, heliocentric radial velocity, proper motion, and age of Cen OB1 are 2.6 ± 0.5 kpc, ~-20 km s-1, μαcosδ = -4.78 ± 0.10 mas yr-1, μδ = -0.93 ± 0.10 mas yr-1, and ~20 × 106 years old, respectively. The same parameters found for NGC 4755 are 1.9 ± 0.5 kpc, -18 ± 2 km s-1, μαcosδ = -4.14 ± 0.19 mas yr-1, μδ = -1.09 ± 0.12 mas yr-1, and ~15 × 106 years old. The pulsars PSR J1253-5820 and PSR J1254-6150 could have been born as the result of supernovae explosions that occurred in the past by massive members of Cen OB1. We have seen Infrared Astronomical Satellite (IRAS) sources near Cen OB1, that show that star formation is still occurring. The age gradient observed between the different members of Cen OB1 makes it likely that the stellar association and IRAS sources represent different stages of a sequential formation process., Facultad de Ciencias Astronómicas y Geofísicas, Instituto Argentino de Radioastronomía, Instituto de Astrofísica de La Plata

Theories on the origin of magnetic fields in massive stars remain poorly developed, because the properties of their magnetic field as function of stellar parameters could not yet be investigated. To investigate whether magnetic fields in massive stars are ubiquitous or appear only in stars with a specific spectral classification, certain ages, or in a special environment, we acquired 67 new spectropolarimetric observations for 30 massive stars. Among the observed sample, roughly one third of the stars are probable members of clusters at different ages, whereas the remaining stars are field stars not known to belong to any cluster or association. Spectropolarimetric observations were obtained during four different nights using the low-resolution spectropolarimetric mode of FORS2 (FOcal Reducer low dispersion Spectrograph) mounted on the 8-m Antu telescope of the VLT. Furthermore, we present a number of follow-up observations carried out with the high-resolution spectropolarimeters SOFIN mounted at the Nordic Optical Telescope (NOT) and HARPS mounted at the ESO 3.6m between 2008 and 2011. To assess the membership in open clusters and associations, we used astrometric catalogues with the highest quality kinematic and photometric data currently available. The presence of a magnetic field is confirmed in nine stars previously observed with FORS1/2: HD36879, HD47839, CPD-282561, CPD-472963, HD93843, HD148937, HD149757, HD328856, and HD164794. New magnetic field detections at a significance level of at least 3sigma were achieved in five stars: HD92206c, HD93521, HD93632, CPD-468221, and HD157857. Among the stars with a detected magnetic field, five stars belong to open clusters with high membership probability. According to previous kinematic studies, five magnetic O-type stars in our sample are candidate runaway stars., Comment: 14 pages, nine figures, four tables, accepted for publication in A&A

Context. The Hercules-Lyra association, a purported nearby young moving group, contains a few tens of zero age main sequence stars of spectral types F to M. The existence and the properties of the Her-Lyr association are controversial and have been discussed in the literature. Aims. The present work reassesses the properties and the member list of the Her-Lyr association based on kinematics and age indicators. Many objects form multiple systems or have low-mass companions and so we need to properly account for multiplicity. Methods. We use our own new imaging observations and archival data to identify multiple systems. The colors and magnitudes of kinematic candidates are compared to isochrones. We derive further information on the age based on Li depletion, rotation, and coronal and chromospheric activity. A set of canonical members is identified to infer mean properties. Membership criteria are derived from the mean properties and used to discard non-members. Results. The candidates selected from the literature belong to 35 stellar systems, 42.9% of which are multiple. Four multiple systems (V538 Aur, DX Leo, V382 Ser, and HH Leo) are confirmed in this work by common proper motion. An orbital solution is presented for the binary system which forms a hierarchical triple with HH Leo. Indeed, a group of candidates displays signatures of youth. Seven canonical members are identified The distribution of Li equivalent widths of canonical Her-Lyr members is spread widely and is similar to that of the Pleiades and the UMa group. Gyrochronology gives an age of 257 ± 46 Myr which is roughly in between the ages of the Pleiades and the Ursa Major group. The measures of chromospheric and coronal activity support the young age. Four membership criteria are presented based on kinematics, lithium equivalent width, chromospheric activity, and gyrochronological age. In total, eleven stars are identified as certain members including co-moving objects plus additional 23 possible members while 14 candidates are doubtful or can be rejected. A comparison to the mass function, however, indicates the presence of a large number of additional low-mass members, which remain unidentified. peerReviewed