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Over the past two decades, our understanding of star formation has undergone a major shift, driven by a wealth of data from infrared, submillimeter and radio surveys. The emerging view depicts star formation as a hierarchical process, which predominantly occurs along filamentary structures in the interstellar medium. These structures span a wide range of spatial scales, ultimately leading to the birth of young stars, which distribute in small groups, clusters and OB associations. Given the inherently complex and dynamic nature of star formation, a comprehensive understanding of these processes can only be achieved by examining their end products—namely, the distribution and properties of young stellar populations. In the Gaia era, the nearby OB associations are now characterised with unprecedented detail, allowing for a robust understanding of their formation histories. Nevertheless, to fully grasp the mechanisms of star formation and its typical scale, it is essential to study the much larger associations, which constitute the backbones of spiral arms. The large catalogues of young open clusters that have emerged from Gaia DR3 offer a valuable resource for investigating star formation on larger spatial scales. While the cluster parameters listed in these catalogues are still subject to many uncertainties and systematic errors, ongoing improvements in data analysis and upcoming Gaia releases promise to enhance the accuracy and reliability of these measurements. This review aims to provide a comprehensive summary of recent advancements and a critical assessment of the datasets available. [ABSTRACT FROM AUTHOR]

Since recent years, mass segregation driven by two-body relaxation in star clusters has been proposed to be measured by the so-called dynamical clock, A + , a measure of the area enclosed between the cumulative radial distribution of blue straggler stars and that of a reference population. Since star clusters spend their lifetime immersed in the gravitational potential of their host galaxy, they are also subject to the effects of galactic tides. In this work, I show that the A + index of a star cluster depends on both its internal dynamics in isolation and the effects of galactic tides. Mainly, I focused on the largest sample of open clusters harboring blue straggler stars with robust cluster membership. I found that these open clusters exhibit an overall dispersion of the A + index in diagnostic diagrams, whereas Milky Way globular clusters show a clear linear trend. However, as also experienced by globular clusters, A + values of open clusters show some dependence on their galactocentric distances, in the sense that clusters located closer or farther than ∼ 11 kpc from the Galactic center have larger and smaller A + values, respectively. This different response to two-body relaxation and galactic tides in globular and open clusters, which happen concurrently, can be due to their different masses. More massive clusters can protect their innermost regions from galactic tides more effectively. [ABSTRACT FROM AUTHOR]

Current models of stellar evolution predict that stars more massive than ∼6 M⊙ should have completely depleted all lithium (Li) in their atmospheres by the time when they reach the He core burning phase. Against this, a non-negligible number of red giants with masses ≳6 M⊙ presenting strong Li lines have recently been reported. Motivated by this finding, we have carried out a spectroscopic survey of red supergiants (RSGs) in the Perseus Arm and a selection of young open clusters in the Magellanic Clouds to assess the presence of the Li i 6708Å doublet line. Based on a sample of > 70 objects, close to one third of RSGs in the Perseus Arm display noticeable Li lines, with perhaps a trend towards a lower fraction among more luminous stars. The samples in the Magellanic Clouds are not as large, but hint at a metallicity dependence. Twenty one RSGs in 5 LMC clusters show a very high fraction of Li detection, around 40%. Conversely, 17 RSGs in 5 SMC clusters give only one secure detection. The interpretation of these observational results is not straightforward, but a mechanism for Li production seems most likely. Further characterisation work is ongoing, while theoretical studies into this matter are urgently needed. [ABSTRACT FROM AUTHOR]

In this study, we considered the collective members of Pismis's clusters located at identical distances r(pc) and distributed around a mean absolute magnitude (Mo) following a Gaussian distribution with dispersion. We computed the distances rc(pc) of these clusters by applying a distance equation designed to account for Malmquist bias. Our computed distances were in satisfactory agreement when compared to previous research findings. In the second part of our work, we delved extensively into the Pismis 3 cluster, which is rarely studied, aged, and among the most affluent. We conducted analyses on its dynamical tidal radius, crossing time, relaxation time, and angular velocity. A kinematic analysis of these open clusters was carried out, encompassing aspects such as their apex position using the AD chart method, velocity ellipsoid parameters (VEPs), and the longitude of the vertex revealing a minimal value, as anticipated when pointing exactly at the Galactic Center. Additionally, we explored their ages in relation to evolution, linking them to formation mechanisms, metallicity, and opacity. [ABSTRACT FROM AUTHOR]

As a stellar group forms within its parent molecular cloud, new members first appear in the deep interior. These overcrowded stars continually diffuse outward to the cloud boundary, and even beyond. Observations have so far documented only the interior drift. Those stars that actually leave the cloud form an expanding envelope that I call the 'stellar mantle.' Simple fluid models for the cloud and mantle illustrate their basic structure. The mantle's expansion speed is subsonic with respect to the cloud's dynamical temperature. I describe, in qualitative terms, how the expanding mantle and Galactic tidal radius might together shape the evolution of specific types of stellar groups. The massive stars in OB associations form in clouds that contract before extruding a substantial mantle. In contrast, the more slowly evolving clouds forming open clusters and T associations have extended mantles that encounter a shrinking tidal radius. These clouds are dispersed by internal stellar outflows. If the remaining group of stars is gravitationally bound, it appears as a long-lived open cluster, truncated by the tidal radius. If the group is unbound, it is a late-stage T association that will soon be torn apart by the tidal force. The 'distributed' populations of pre-main sequence stars observed in the outskirts of several star-forming regions are too distant to be stellar mantles. Rather, they could be the remnants of especially low-mass T associations. [ABSTRACT FROM AUTHOR]

In the article, a method is developed for estimating the dynamic masses of the stellar and gas components of cluster-cloud systems under conditions of non-isolation and significant non-stationarity of stellar and gas subsystems in a regular field. A number of estimates of the dynamic masses of the stellar and gas components of the system are made without using the virial theorem for zero and positive values of the total energies of the system as a whole and of its gaseous subsystem. The well-known estimates of the free-fall time for fragments in the force field of a sphere of uniform and inhomogeneous density sphere simulating a system of gas and stars are refined. The estimates of the quantities are supplemented by estimates of the radial velocities of the fragments in such systems. A number of relations between the parameters of the stellar and gas subsystems in the considered models of embedded clusters are obtained. It is shown that instead of one virial coefficient to describe the system, it is necessary to use three coefficients, the formulas for which are given. The relations make it possible to estimate the dynamic masses of non-isolated and non-stationary subsystems from data on the structural-dynamic characteristics of an embedded cluster. It is shown that the ratio , where is the mean square of the velocities of the stellar subsystem in the case of its virial equilibrium, and is the mean square of the critical velocities of the stars in this subsystem. It is shown that embedded clusters with parameters and have maximum values along some curve in the space ( and are the mass and radius of the th subsystem). The embedded clusters with parameters close to this curve are the least susceptible to destruction space relaxation processes. It is shown that, depending on the initial energies and , the masses of a subsystem of stars in an embedded cluster can be much smaller than the virial masses of this subsystem. This result is also of particular interest for estimating the dynamic masses of galaxy clusters. It is noted that an increase in the degree of non-stationarity of the considered models of embedded clusters leads to a decrease in the periods of oscillations of the stellar subsystem. [ABSTRACT FROM AUTHOR]

The aim of this survey is the homogeneous characterization of a large sample of H ii regions with active star formation in order to detect observational trends supporting the two main models of massive star formation. [ABSTRACT FROM AUTHOR]

Very young massive planets are sufficiently luminous by their internal heat of formation to permit detailed studies, including spectroscopy of their atmospheres with large telescopes at sufficient resolution (λ/Δλ ≳ 1000) to identify major constituents to inform models of planet formation and early evolution. We obtained 1–2.4  μ m (YJHK) spectra of the planetary-mass 'b' companion of 2MASS J04372171+2651014, a 1–3 Myr-old M dwarf member of the Taurus star-forming region, and one of the youngest such objects discovered to date. These indicate the presence of CO and possibly H2O and CH4 in the atmosphere, all suggesting a T eff of around 1200 K, characteristic of a L-T transition spectral type and consistent with previous estimates based on its luminosity and age. The absence or attenuation of spectral features at shorter wavelengths suggests the presence of micron-size dust, consistent with the object's red colour. The spectrum of 2M0437b resembles those of the HR 8799 planets, especially the innermost 'b' planet, with the exception of a pronounced flux deficit in the H -band of uncertain origin. [ABSTRACT FROM AUTHOR]

The improved astrometry precision of Gaia -eDR3 allows us to perform a detailed study of the Upper Scorpius OB association and revisit its spatial, kinematic, and age substructure. We achieve this by combining clustering techniques and complementing with age estimations based on Gaia photometry. Our census retrieves 3661 candidate members for Upper Scorpius with contamination ∼9 per cent. We also extract an astrometrically clean sample of 3004 sources with contamination ∼6 per cent. We show that Upper Scorpius can be divided into at least three main kinematic groups. We systematically investigate and characterize the Upper Scorpius' internal structure, revealing that at least ∼34 per cent of its stellar populations are contained in seven spatial substructures, with well defined boundaries, kinematics, and relative ages with suggested names: π Scorpii (20 |$^{\pm 2}_{\pm 1}$|  Myr), α Scorpii (14 |$^{\pm 2}_{\pm 1}$|  Myr), δ Scorpii (9 |$^{\pm 2}_{\pm 1}$|  Myr), β Scorpii (8 |$^{\pm 1}_{\pm 1}$|  Myr), ω Scorpii (8 |$^{\pm 1}_{\pm 1}$|  Myr), ν Scorpii (7 |$^{\pm 1}_{\pm 1}$|  Myr), after their brightest member, and the well known ρ Ophiuchi (4 |$^{\pm 1}_{\pm 1}$|  Myr). We find a clear correlation in (1) density-age, providing an empirical expansion law to be tested in other associations, and (2) tangential velocity-age, providing constrains on the dynamics of these substructures and the position of potential past triggering events. We estimate the time at which four potential supernovae events occurred in Upper Scorpius. Based on these findings, we tie together previous work on the region and suggest a star formation history with unprecedented temporal resolution. [ABSTRACT FROM AUTHOR]

We report the discovery of a resolved (0.9 arcsec) substellar companion to a member of the 1–5 Myr Taurus star-forming region. The host star (2M0437) is a single mid-M type (T eff ≈ 3100 K) dwarf with a position, space motion, and colour–magnitude that support Taurus membership, and possible affiliation with a ∼2.5-Myr-old subgroup. A comparison with stellar models suggests a 2–5 Myr age and a mass of 0.15–0.18M⊙. Although K2 detected quasi-periodic dimming from close-in circumstellar dust, the star lacks detectable excess infrared emission from a circumstellar disc and its H α emission is not commensurate with accretion. Astrometry based on 3 yr of AO imaging shows that the companion (2M0437b) is comoving, while photometry of two other sources at larger separation indicates that they are likely heavily reddened background stars. A comparison of the luminosity of 2M0437b with models suggests a mass of 3–5 MJUP, well below the deuterium burning limit, and an effective temperature of 1400–1500 K, characteristic of a late L spectral type. The H − K colour is redder than the typical L dwarf, but comparable to other directly detected young planets, e.g. those around HR 8799. The discovery of a super-Jupiter around a very young, very low-mass star challenges models of planet formation by either core accretion (which requires time) or disc instability (which requires mass). We also detected a second, comoving, widely separated (75 arcsec) object that appears to be a heavily extincted star. This is certainly a fellow member of this Taurus subgroup and statistically likely to be a bound companion. [ABSTRACT FROM AUTHOR]

Current models of stellar evolution predict that stars more massive than ∼6 M⊙ should have completely depleted all lithium (Li) in their atmospheres by the time when they reach the He core burning phase. Against this, a non-negligible number of red giants with masses ≳6 M⊙ presenting strong Li lines have recently been reported. Motivated by this finding, we have carried out a spectroscopic survey of red supergiants (RSGs) in the Perseus Arm and a selection of young open clusters in the Magellanic Clouds to assess the presence of the Li i 6708Å doublet line. Based on a sample of > 70 objects, close to one third of RSGs in the Perseus Arm display noticeable Li lines, with perhaps a trend towards a lower fraction among more luminous stars. The samples in the Magellanic Clouds are not as large, but hint at a metallicity dependence. Twenty one RSGs in 5 LMC clusters show a very high fraction of Li detection, around 40%. Conversely, 17 RSGs in 5 SMC clusters give only one secure detection. The interpretation of these observational results is not straightforward, but a mechanism for Li production seems most likely. Further characterisation work is ongoing, while theoretical studies into this matter are urgently needed. [ABSTRACT FROM AUTHOR]

The seven open star clusters FSR 0717, FSR 1698, Loden 2313, Ruprecht 49, Majaess 61, Mayer 2, and Teutsch 48, which are located near the Milky Way's Galactic plane are studied in this article using the Gaia DR3 dataset. The estimated numbers of potential cluster members are 102, 362, 285, 197, 95, 116, 160, and 397, correspondingly. The calculated radius values are 3.10 ± 0.20, 3.20 ± 0.30, 5.10 ± 0.10, 5.30 ± 0.20, 4.50 ± 0.40, 3.50 ± 0.20, and 5.20 ± 0.20 arcmin, respectively. The clusters' log ages of 8.80 ± 0.15, 7.80 ± 0.05, 6.95 ± 0.05, 8.00 ± 0.10, 8.70 ± 0.05, 7.95 ± 0.10, and 8.95 ± 0.05 yr are determined using solar metallicity isochrones. The distances of 3176.76 ± 17.30, 2122.76 ± 14.35, 1157.07 ± 15.20, 3924.10 ± 20.10, 2513.21 ± 9.04, 3364.34 ± 8.01, and 3505.36 ± 7.93 pc that were obtained by the isochrone's best fitting are comparable to the distances determined by inverting median parallaxes. The slopes of the mass function are matched the Salpeter mean value. Total masses are determined to be 128.55 ± 11, 548.30 ± 8, 254.57 ± 7, 194.09 ± 6, 92.32 ± 15, 191.50 ± 12, and 455.62 ± 10 M ⊙ , respectively. • Introduction anddData analysis. • Spatial structure: clusters' sizes, centers, and richness. • Color-magnitude diagram (age and distance). • Luminosity and mass function. • The relaxation time and conclusions. [ABSTRACT FROM AUTHOR]

The aim of this survey is the homogeneous characterization of a large sample of H ii regions with active star formation in order to detect observational trends supporting the two main models of massive star formation. [ABSTRACT FROM AUTHOR]

Abstract—Several parameters of the Pleiades cluster were estimated. We used Gaia DR2 data on the coordinates, proper motions, and radial velocities of stars in regions with radius d° =2.5° and size 60° × 60° around the cluster center. Based on data on stars with magnitudes mG ≤ 18m, we constructed a map and profile of the density, luminosity and mass functions of the cluster, determined the cluster radius, 10.9° ± 0.3° (26.3 ± 0.7 pc), and the radius of its core, 2.62° (6.24 pc), and obtained estimates for the number of stars in the cluster, 1542 ± 121, and their mass, 855 ± 104M⊙; numbers of stars in the core of the cluster, 1097 ± 77, and their mass 665 ± 71M⊙. Distribution of stars with mG< 16m at distances rs from the cluster center in three-dimensional space of rs< 1 pc and at rs ∼ 1.4–5 pc contains radial density waves. Based on the data on stars with mG< 16m, we determined the average rotation velocity of the core of the cluster vc = 0.56 ± 0.07 km s−1 at distances d in the sky plane d ≤ 4.6 pc from its center. The rotation is "prograde", the angle between the projection of the axis of rotation of the cluster core onto the sky plane and the direction to the North Pole of the Galaxy is ϕ = 18.8° ± 4.4°, the angle between the axis of rotation of the cluster core and the sky plane is ϑ = 43.2° ± 4.9°, the rotation velocity of the cluster core at a distance of d ≃ 5.5pc from its center is close to zero: vc = 0.1 ± 0.3 km s−1. According to the data on stars with mG< 17m, the velocity of the "retrograde" rotation of the cluster at a distance of d ≃ 7.1 pc from its center is vc = 0.48 ± 0.20 km s−1, the angle ϕ = 37.8° ± 26.4°. The dependences of moduli of the tangential and radial components of the velocity field of the stars of the cluster core in the sky plane on the distance d to the center of the cluster contain a number of periodic oscillations. The dispersions of the velocities of the stars in the cluster core σv increase on average with an increase in rs, which, like the radial density waves and the waves of oscillations of the velocity field in the sky plane, indicates the nonstationarity of the cluster in the field of regular forces. The Jeans wavelength in the cluster core decreases, and the velocity dispersion of the stars in the core under the Jeans instability increases after taking into account the influence of the external field of the Galaxy on the cluster. The region of gravitational instability in the Pleiades cluster is located in the interval rs = 2.2–5.7 pc and contains 39.4–60.5% of the total number of stars in the considered samples of cluster stars. Estimates of the Pleiades dynamic mass and tidal radius are obtained. [ABSTRACT FROM AUTHOR]

We present deep CCD photometry (V ~ 21 mag) in BVR Johnson-Cousin filters supported with 2MASS JHKs data set for the open star cluster NGC 7790. The cluster's parameters such as reddening, distance, metallicity and age etc... are derived using two color diagrams in optical and near-infrared bands. These parameters are EB-V = 0.512 ± 0.006, EV-R = 0.382 ± 0.005, EJ-H = 0.337 ± 0.001 and EJ-Ks = 0.505 ± 0.001 mag, the distance modulus (m - M)0 = 12.41 ± 0.001 mag, which gives a distance of 3.035 kpc and the metallicity Z = 0.008. The age obtained by fitting the observed CMDs with Padova isochrones is 63.95 Myr. From the analysis of 2MASS data, the limiting radius of the cluster is found to be 9.7 ± 0.83 arcmin, it reflects that the cluster is a compact cluster. The dynamical relaxation time of NGC 7790 is also determined and found to be 0.17 of its age, which means that the cluster is dynamically relaxed. Moreover, luminosity and mass functions are also investigated. [ABSTRACT FROM AUTHOR]

We study the motions inside 28 OB associations with the use of Gaia DR2 proper motions. The average velocity dispersion calculated for 28 OB associations including more than 20 stars with Gaia DR2 proper motion is σ v  = 4.5 km s−1. The median virial and stellar masses of OB associations are M vir = 8.9 × 105 and M st = 8.1 × 103 M⊙, respectively. The median star-formation efficiency in parent giant molecular clouds appears to be ϵ = 1.2 per cent. Gaia DR2 proper motions confirm the expansion in the Per OB1, Car OB1, and Sgr OB1 associations found earlier with Gaia DR1 data. We also detect the expansion in Gem OB1, Ori OB1, and Sco OB1 associations, which became possible for the first time now when analysed with Gaia DR2 proper motions. The analysis of the distribution of OB stars in the Per OB1 association shows the presence of a shell-like structure with the radius of 40 pc. Probably, the expansion of the Per OB1 association started with the velocity greater than the present-day expansion velocity equal to 5.0 ± 1.7 km s−1. [ABSTRACT FROM AUTHOR]

The poorly studied Berkeley 8 (Be 8) open cluster is analyzed from CCD UBV RI photometric data taken with the 0.90 m telescope at the Sierra Nevada Observatory. The Z = +0.008 PARSEC isochrone gave us a reddening of E(B-V) = 0.69 ± 0.03, a distance of 3410 ± 300 pc, and an age of 2.8 ± 0.2 Gyr. Its median Gaia DR2 distance, d = 3676 ± 810 pc, is in good agreement with our photometric distances, 3410--3620 pc within the uncertainties. The kinematic parameters of five likely members of Be 8 with the circular orbits, ecc = [0.23, 0.30], reflect the properties of the Galactic thin disc, which is also consistent with what is expected of its metal content ([M/H] = --0.27). Be 8 with R > 9 kpc (co-rotation gap at 9 kpc) may have been originating from a different galactic radius or different star formation region. [ABSTRACT FROM AUTHOR]

Star clusters stand at the intersection of much of modern astrophysics: the ISM, gravitational dynamics, stellar evolution, and cosmology. Here, we review observations and theoretical models for the formation, evolution, and eventual disruption of star clusters. Current literature suggests a picture of this life cycle including the following several phases: ▪ Clusters form in hierarchically structured, accreting molecular clouds that convert gas into stars at a low rate per dynamical time until feedback disperses the gas. ▪ The densest parts of the hierarchy resist gas removal long enough to reach high star-formation efficiency, becoming dynamically relaxed and well mixed. These remain bound after gas removal. ▪ In the first ∼100 Myr after gas removal, clusters disperse moderately fast, through a combination of mass loss and tidal shocks by dense molecular structures in the star-forming environment. ▪ After ∼100 Myr, clusters lose mass via two-body relaxation and shocks by giant molecular clouds, processes that preferentially affect low-mass clusters and cause a turnover in the cluster mass function to appear on ∼1–10-Gyr timescales. ▪ Even after dispersal, some clusters remain coherent and thus detectable in chemical or action space for multiple galactic orbits. In the next decade, a new generation of space– and adaptive optics–assisted ground-based telescopes will enable us to test and refine this picture. [ABSTRACT FROM AUTHOR]

Aims. Within a 400 pc sphere around the Sun, we search for Praesepe's tidal tails in the Gaia Data Release 2 (DR2) data set. Methods. We used a modified convergent-point method to find stars with space velocities close to the space velocity of the Praesepe cluster. Results. We find a clear indication for the existence of Praesepe's tidal tails, both extending up to 165 pc from the centre of the cluster. A total of 1393 stars populate the cluster and its tails, giving a total mass of 794 M⊙. We determined a tidal radius of 10.77 pc for the cluster and a tidal mass of 483 M⊙. The corresponding half-mass radius is 4.8 pc. We also found a clear indication for mass segregation in the cluster. The tidal tails outside 2 tidal radii are populated by 389 stars. The total contamination of our sample by field stars lies between 50 and 100 stars or 3.6–7.2%. We used an astrometrically and photometrically clean subsample of Gaia DR2, which makes our Praesepe sample incomplete beyond MG ≈ 12.0 mag; this corresponds to about 0.25 M⊙. A comparison with an N-body model of the cluster and its tails shows remarkably good coincidence. Three new white dwarfs are found in the tails. [ABSTRACT FROM AUTHOR]

The features and make up of the population of X-ray sources in Galactic star clusters reflect the properties of the underlying stellar environment. Cluster age, mass, stellar encounter rate, binary frequency, metallicity, and maybe other properties as well, determine to what extent we can expect a contribution to the cluster X-ray emission from low-mass X-ray binaries, millisecond pulsars, cataclysmic variables, and magnetically active binaries. Sensitive X-ray observations with XMM-Newton and certainly Chandra have yielded new insights into the nature of individual sources and the effects of dynamical encounters. They have also provided a new perspective on the collective X-ray properties of clusters, in which the X-ray emissivities of globular clusters and old open clusters can be compared to each other and to those of other environments. I will review our current understanding of cluster X-ray sources, focusing on star clusters older than about 1 Gyr, illustrated with recent results. [ABSTRACT FROM AUTHOR]

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 Zmax 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 Zmax 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 list of the Castor stream members is compiled based on the data from various authors. The membership probabilities for some stars are revised based on the individual apex, multiplicity, observational errors, and peculiarity. The apex of the Castor moving group is determined using the apex diagram method. The parameters of the Castor and Ursa Major streams are compared and the positions of the two streams on the apex diagram are found to differ by 225°, implying that the two groups move in almost opposite directions. Stars of both moving groups are intermixed in space, the Castor stream occupies a smaller volume than the UMa stream and is located inside it. Our results can be useful for understanding the morphology of the Galactic disk in the Sun’s vicinity.

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.

Using Gaia DR2 data, combined with OmegaCAM ground-based optical photometry from the Accretion Disc with OmegaCAM survey, and detailed radial velocity measurements from ESO- Gaia, we analyse in detail a 10 × 5 deg region around the Wolf–Rayet Star γ2 Vel, including the previously known clusters Gamma Vel and NGC2547. Using clustering analysis that considers positions, proper motions, and parallax, we discover six clusters or associations – four of which appear new. Analysis of the colour–magnitude diagram for these clusters shows that four of them formed coevally from the same molecular clouds 10 Myr ago, while NGC 2547 formed together with a newly discovered cluster 30 Myr ago. This study shows the incredible wealth of data provided by Gaia for the study of young stellar clusters. [ABSTRACT FROM AUTHOR]

Aiming to improve the knowledge of the Galactic structure in the second Galactic quadrant, we selected 10 young open clusters from a sample of more than 200 covered by the SDSS. Our study was based on a photometric data set provided by SDSS and APASS in the visible and by 2MASS and WISE in the infrared. We also carried out spectroscopic observations with GMOS/GEMINI and made use of spectroscopic information available in the literature. The analysis was performed with a suite of tools developed by us to systematically determine the main parameters of the studied clusters, and to identify possible pre-main-sequence populations. We obtained novel parameters and found that the studied clusters are scattered at distances ranging from 1.7 to 8.8 kpc, covering three spiral arms. In particular, we located two of the clusters over the 'Outer Arm' and other two probably belong to the more distant 'New Arm'. Hence, it was possible to get a better picture of the structure of the outer part of the Galaxy. We also obtained traces of coeval star formation processes in all of the selected clusters with a pre-main-sequence population and identified young stellar object candidates in one of them. Regarding the most massive members of the clusters, we discovered 14 B-type stars, eight of them earlier than B3 and another one was identified as a Be star. [ABSTRACT FROM AUTHOR]

As part of the accretion discs in Hα with OmegaCAM (ADHOC) survey, we imaged a region of 12×8 square degrees around the Orion Nebula Cluster in r, i and Hα. Thanks to the high-quality photometry obtained, we discovered three well-separated pre-main sequences in the color-magnitude diagram. The populations are all concentrated towards the cluster’s center. Although several explanations can be invoked to explain these sequences, we are left with two competitive but intriguing scenarios: a population of unresolved binaries with an exotic mass ratio distribution, or three populations with different ages. Independent high-resolution spectroscopy supports the presence of discrete episodes of star formation, each separated by about a million years. The stars from the two putative youngest populations rotate faster than the older ones, in agreement with the evolution of stellar rotation observed in pre-main sequence stars younger than 4 Myr in several star forming regions. Whatever the final explanation, our results prompt a revised look at the formation mode and early evolution of stars in clusters. [ABSTRACT FROM AUTHOR]

We determine the masses and radii of central regions of open star cluster (OCL) models with small or zero entropy production and estimate the masses of oscillation generation regions in clustermodels based on the data of the phase-space coordinates of stars. The radii of such regions are close to the core radii of the OCL models. We develop a new method for estimating the total OCL masses based on the cluster core mass, the cluster and cluster core radii, and radial distribution of stars. This method yields estimates of dynamical masses of Pleiades, Praesepe, and M67, which agree well with the estimates of the total masses of the corresponding clusters based on proper motions and spectroscopic data for cluster stars.We construct the spectra and dispersion curves of the oscillations of the field of azimuthal velocities v in OCL models. Weak, low-amplitude unstable oscillations of v develop in cluster models near the cluster core boundary, and weak damped oscillations of v often develop at frequencies close to the frequencies of more powerful oscillations, which may reduce the non-stationarity degree in OCL models. We determine the number and parameters of such oscillations near the cores boundaries of cluster models. Such oscillations points to the possible role that gradient instability near the core of cluster models plays in the decrease of the mass of the oscillation generation regions and production of entropy in the cores of OCL models with massive extended cores. [ABSTRACT FROM AUTHOR]

We investigate the dynamical evolution of 40 open clusters ( OCs) by means of their astrophysical parameters derived from field-decontaminated 2MASS photometry. We find a bifurcation in the planes' core radius versus age and cluster radius versus age, in which part of the clusters appear to expand with time probably due to the presence of stellar black holes while others seem to shrink due to dynamical relaxation. Mass functions ( MFs) are built for three-fourth of the sample (31 OCs), which are used to search for indications of mass segregation and external dynamical processes by means of relations between astrophysical, structural, and evolutionary parameters. We detect a flattening of MF slopes occurring at the evolutionary parameters τcore ≤ 32 and τoverall ≤ 30, respectively. Within the uncertainties involved, the overall MF slopes of 14 out of 31 OCs with moverall > 500 M⊙ are consistent with Kroupa's initial mass function, implying little, or no dynamical evolution for these clusters. The remaining 17 OCs with MF slopes departing from that of Kroupa show mild/large-scale mass segregation due to dynamical evolution. [ABSTRACT FROM AUTHOR]

Aims. This work analyses the spatial distribution of stars in Taurus with a specific focus on multiple stars and wide pairs in order to derive new constraints on star formation and early dynamical evolution scenarios. Methods. We collected the multiplicity data of stars in Taurus to build an up-to-date stellar/multiplicity catalog. We first present a general study of nearest-neighbor statistics on spatial random distribution, comparing its analytical distribution and moments to those obtained from Monte Carlo samplings.We introduce the one-point correlation function to complement the pair correlation function and define the spatial regimes departing from randomness in Taurus. We then perform a set of statistical studies to characterize the binary regime that prevails in Taurus. Results. The function in Taurus has a scale-free trend with a similar exponent as the correlation function at small scale. It extends almost 3 decades up to □ 60 kAU showing a potential extended wide binary regime. This was hidden in the correlation function due to the clustering pattern blending. Distinguishing two stellar populations, single stars versus multiple systems (separation □ 1 kAU), within Class II/III stars observed at high angular resolution, we highlight a major spatial neighborhood di erence between the two populations using nearest-neighbor statistics. The multiple systems are three times more likely to have a distant companion within 10 kAU when compared to single stars. We show that this is due to the presence of most probable physical ultra-wide pairs (UWPs, defined as such from their mutual nearest neighbor property), that are themselves generally composed of multiple systems containing up to five stars altogether. More generally, our work highlights; 1) a new large population of candidate UWPs in Taurus within the range 1-60 kAU in Taurus and 2) the major local structural role they play up to 60 kAU. There are three di erent types of UWPs; either composed of two tight and comparatively massive stars (MM), by one single and one multiple (SM), or by two distant low-mass singles (SS) stars. These UWPs are biased towards high multiplicity and higher-stellar-mass components at shorter separations. The multiplicity fraction per ultra-wide pair with separation less than 10 kAU may be as high as 83:5 ± 19:6%. Conclusions. We suggest that these young pre-main sequence UWPs may be pristine imprints of their spatial configuration at birth resulting from a cascade fragmentation scenario of the natal molecular core. They could be the older counterparts, at least for those separated by less than 10 kAU, to the □ 0.5 Myr prestellar cores/Class 0 multiple objects observed at radio/millimeter wavelengths. [ABSTRACT FROM AUTHOR]

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.

In the field of galaxies images, the relative coordinate positions of each star with respect to all the other stars are adapted. Therefore the membership of star cluster will be adapted by two basic criterions, one for geometric membership and other for physical (photometric) membership. So in this paper, we presented a new method for the determination of open cluster membership based on K-means clustering algorithm. This algorithm allows us to efficiently discriminate the cluster membership from the field stars. To validate the method we applied it on NGC 188 and NGC 2266, membership stars in these clusters have been obtained. The color-magnitude diagram of the membership stars is significantly clearer and shows a well-defined main sequence and a red giant branch in NGC 188, which allows us to better constrain the cluster members and estimate their physical parameters. The membership probabilities have been calculated and compared to those obtained by the other methods. The results show that the K-means clustering algorithm can effectively select probable member stars in space without any assumption about the spatial distribution of stars in cluster or field. The similarity of our results is in a good agreement with results derived by previous works. [ABSTRACT FROM AUTHOR]

The discovery of 2012 VP113 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 > 150 au) orbits with a large perihelion distance of q > 30 au compared to the Earth's (a = 1 au and eccentricity e = (1 - q/a) ≃ 0.0167 or q ≃ 1 au). 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°-30°. They also have similar argument of perihelion = 340° ± 55°. There is no ready explanation for their origin. Here we show that these orbital parameters are typical for a captured population from the planetesimal disc of another star. Assuming that 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 1.8 M⊙ star that impacted the Sun at ≃ 340 au at an inclination with respect to the ecliptic of 17°-34° with a relative velocity at infinity of ~4.3 km s-1. We predict that the Sednitos region is populated by 930 planetesimals and the inner Oort Cloud acquired ~440 planetesimals through the same encounter. [ABSTRACT FROM AUTHOR]

We use Hubble Space Telescope (HST) to reach the end of the white dwarf (WD) cooling sequence (CS) in the solar-metallicity open cluster NGC 6819. Our photometry and completeness tests show a sharp drop in the number of WDs along the CS at magnitudes fainter than mF606W = 26.050 ± 0.075. This implies an age of 2.25 ± 0.20 Gyr, consistent with the age of 2.25 ± 0.30 Gyr obtained from fits to the main-sequence turn-off. The use of different WD cooling models and initial-final-mass relations have a minor impact the WD age estimate, at the level of ~0.1 Gyr. As an important by-product of this investigation we also release, in electronic format, both the catalogue of all the detected sources and the atlases of the region (in two filters). Indeed, this patch of sky studied by HST (of size ~70 arcmin²) is entirely within the main Kepler-mission field, so the high-resolution images and deep catalogues will be particularly useful. [ABSTRACT FROM AUTHOR]

We employed field star decontaminated Two Micron All Sky Survey photometry to study four nearby optical embedded clusters -- Collinder 34, NGC 3293, NGC 3766 and NGC 6231 -- obtaining deep colour-magnitude diagrams and stellar radial density profiles. We found what seem to be pre-main sequences detached in different amounts from main sequences in these diagrams. The structural analysis of each cluster revealed different radial distributions for these two sequences. We argued that the detached evolutionary sequences in our sample may be evidence of sequential star formation. Finally, we compared the sample cluster parameters with those of other young clusters in the literature and point out evidence that NGC 3766 and NGC 6231 might be evolving to OB associations. [ABSTRACT FROM AUTHOR]

With an aim to examine whether the surface oxygen composition suffers any appreciable change due to evolution-induced mixing of nuclear-processed material in the envelope of red giants, abundance determinations for O/Fe/Ni based on the synthetic spectrum- fitting method were performed by using the moderate-dispersion spectra in the 7770-7792 Å region (comprising O I 7771-5, Fe I 7780, and Ni I 7788 lines) for 16 stars of the old open cluster M67 in various evolutionary stages from the turn-off point through the red giant branch. We could not find any meaningful difference in the oxygen abundances between the non-giant group (Teff > 5000 K) and the red-giant group (Teff < 5000 K), which are almost consistent with each other on average (despite that both have rather large dispersions of a few tenths dex caused by insufficient data quality), though only one giant star (S 1054) appears to show an exceptionally low O abundance and thus needs a more detailed study. This result may suggest that oxygen content in the stellar envelope is hardly affected (or any changes are insignificant) by the mixing of H-burning products in the red giant phase, as far as M67 stars of low mass (~1.3M⊙) are concerned, which is consistent with the prediction from the conventional stellar evolution theory of first dredge-up. [ABSTRACT FROM AUTHOR]