Stellar Groups Research Articles

(Re)mind the gap: A hiatus in star formation history unveiled by APOGEE DR17

Context. Analysis of several spectroscopic surveys indicates the presence of a bimodality between the disc stars in the abundance ratio space of [α/Fe] versus [Fe/H]. The two stellar groups are commonly referred to as the high-α and low-α sequences. Some models capable of reproducing such a bimodality invoke the presence of a hiatus in the star formation history in our Galaxy, whereas other models explain the two sequences by means of stellar migration. Aims. Our aim is to show that the existence of the gap in the star formation rate between high-α and low-α is evident in the stars of APOGEE DR17, if one plots [Fe/α] versus [α/H], confirming previous suggestions. We then try to interpret the data by means of detailed chemical models. Methods. We compare the APOGEE DR17 red giant stars with the predictions of a detailed chemical evolution model based on the two-infall paradigm, taking into account also the possible accretion of dwarf satellites. Results. The APOGEE DR17 abundance ratios [Fe/α] versus [α/H] exhibit a sharp increase in [Fe/α] at a nearly constant [α/H] (where α elements considered are Mg, Si, O) during the transition between the two disc phases. This observation strongly supports the hypothesis that a hiatus in star formation occurred during this evolutionary phase. Notably, the most pronounced growth in the [Fe/α] versus [α/H] relation is observed for oxygen, as this element is exclusively synthesised in core-collapse supernovae. The revised version of the two-infall chemical evolution model proposed in this study reproduces the APOGEE DR17 abundance ratios better than before. Particularly noteworthy is the model’s ability to predict the hiatus in the star formation between the two infalls of gas, which form the thick and thin disc, respectively, and thus generate abundance ratios compatible with APOGEE DR17 data. Conclusions. We show that the signature of a hiatus in the star formation is imprinted in the APOGEE DR17 abundance ratios. A chemical model predicting a pause in the star formation of a duration of roughly 3.5 Gyr, and in which the high-α disc starts forming from pre-enriched gas by a previous encounter with a dwarf galaxy, could well explain the observations

An in-depth analysis of the differentially expanding star cluster Stock 18 (Villafranca O-036) using Gaia DR3 and ground-based data

Context. The Villafranca project is combining Gaia data with ground-based surveys to analyze Galactic stellar groups (clusters, associations, or parts thereof) with OB stars. Aims. We want to analyze the poorly studied cluster Stock 18 within the Villafranca project, as it is a very young stellar cluster with a symmetrical and compact H II region around it, Sh 2-170, so it is likely to provide insights into the structure and dynamics of such objects at an early stage of their evolution. Methods. We used Gaia astrometry, photometry, spectrophotometry, and variability data as well as ground-based spectroscopy and imaging to determine the characteristics of Stock 18. We used these data to analyze its core, massive star population, extinction, distance, membership, internal dynamics, density profile, IMF, stellar variability, and Galactic location. Results. Stock 18 is a very young (∼1.0 Ma) cluster located at a distance of 2.91 ± 0.10 kpc and is dominated by the GLS 13 370 system, whose primary (Aa) is an O9 V star. We propose that Stock 18 was in a very compact state (∼0.1 pc) about 1.0 Ma ago and that most massive stars were ejected at that time without significantly affecting the less massive stars as a result of multi-body dynamical interactions. Different age estimates also point toward an age close to 1.0 Ma, indicating that the dynamical interactions took place very shortly after massive star formation. Well-defined expanding stellar clusters have been observed before, but none are as young as this one. If we include all of the stars, the initial mass function is top heavy, but if we discard the ejected ones, it becomes nearly canonical. Therefore, this is another example (in addition to the previous one we found – the Bermuda cluster) of (a) a very young cluster with an already evolved present day mass function (b) that has significantly contributed to the future population of free-floating compact objects. If confirmed in more clusters, the number of such compact objects may be higher in the Milky Way than previously thought. Stock 18 has a variable extinction with an average value of R5495 higher than the canonical one of 3.1. We have discovered a new visual component (Ab) in the GLS 13 370 system. The cluster is above our Galactic mid-plane, likely as a result of the Galactic warp, and it has a distinct motion with respect to its surrounding old population, which is possibly an influence of the Perseus spiral arm.

An empirical isochrone archive for nearby open clusters

Context. The ages of star clusters and co-moving stellar groups contain essential information about the Milky Way. Their special properties and placement throughout the galactic disk make them excellent tracers of galactic structure and key components to unlocking its star formation history. Yet, even though the importance of stellar population ages has been widely recognized, their determination remains a challenging task often associated with highly model-dependent and uncertain results. Aims. We propose a new approach to this long-standing problem, which relies on empirical isochrones of known clusters extracted from high-quality observational data. These purely observation-based data products open up the possibility of relative age determination, free of stellar evolution model assumptions. Methods. For the derivation of the empirical isochrones, we used a combination of the statistical analysis tool principal component analysis for preprocessing and the supervised machine learning method support vector regression for curve extraction. To improve the statistical reliability of our result, we defined the empirical isochrone of a color-magnitude diagram (CMD) of a cluster as the median calculated from a set of nboot = 1000 curves derived from bootstrapped data. The algorithm requires no physical priors, is computationally fast, and can easily be generalized over a large range of CMD combinations and evolutionary stages of clusters. Results. We provide empirical isochrones in all Gaia DR2 and DR3 color combinations for 83 nearby clusters (d < 500 pc), which cover an estimated age range of 7 Myr to 3 Gyr. In doing so, we pave the way for a relative comparison between individual stellar populations based on an age-scaling ladder of empirical isochrones of known clusters. Furthermore, due to the exceptional precision of the available observational data, we report accurate lower main sequence empirical isochrones for many clusters in our sample, which are of special interest as this region is known to be especially complex to model. We validate our method and results by comparing the extracted empirical isochrones to cluster ages in the literature. We also investigate the added information that empirical isochrones covering the lower main sequence can provide on case studies of the IC 4665 cluster and the Meingast 1 stream. Conclusions. The archive of empirical isochrones offers a novel approach to validating age estimates and can be used as an age-scaling ladder or age brackets for new populations and serve as calibration data for further constraining stellar evolution models.

The chemical diversity of the metal-poor Milky Way

ABSTRACT We present a detailed study of the chemical diversity of the metal-poor Milky Way using data from the GALAH DR3 survey. Considering 17 chemical abundances relative to iron ([X/Fe]) for 9923 stars, we employ principal component analysis (PCA) and extreme deconvolution (XD) to identify 10 distinct stellar groups. This approach, free from chemical or dynamical cuts, reveals known populations, including the accreted halo, thick disc, thin disc, and in situ halo. The thick disc is characterized by multiple substructures, suggesting it comprises stars formed in diverse environments. Our findings highlight the limited discriminatory power of magnesium in separating accreted and disc stars. Elements such as Ba, Al, Cu, and Sc are critical in distinguishing disc from accreted stars, while Ba, Y, Eu, and Zn differentiate disc and accreted stars from the in situ halo. This study demonstrates the potential power of combining a latent space representation of the data (PCA) with a clustering algorithm (XD) in Galactic archaeology, in providing new insights into the Galaxy’s assembly and evolutionary history.

Kinematic study of the orion complex: Analysing the young stellar clusters from big and small structures

Abstract In this work, we analysed young stellar clusters with spatial and kinematic coherence in the Orion star-forming complex. For this study, we selected a sample of pre-main sequence candidates using parallaxes, proper motions and positions on the colour-magnitude diagram. After applying a hierarchical clustering algorithm in the 5D parameter space provided by Gaia DR3, we divided the recovered clusters into two regimes: Big Structures and Small Structures, defined by the number of detected stars per cluster. In the first regime, we found 13 stellar groups distributed along the declination axis in the regions where there is a high density of stars. In the second regime, we recovered 34 clusters classified into two types: 14 as small groups completely independent from the larger structures, including four candidates of new clusters, and 12 classified as sub-structures embedded within five larger clusters. Additionally, radial velocity data from APOGEE-2 and GALAH DR3 was included to study the phase space in some regions of the Orion complex. From the Big Structure regime, we found evidence of a general expansion in the Orion OB1 association over a common centre, giving a clue about the dynamical effects the region is undergoing. Likewise, in the Small Structure regime, the projected kinematics shows the ballistic expansion in the λ Orionis association and the detection of likely events of clusters’ close encounters in the OB1 association.

The Gaia ultracool dwarf sample–IV. GTC/OSIRIS optical spectra of Gaia late-M and L dwarfs

ABSTRACT As part of our comprehensive, ongoing characterization of the low-mass end of the main sequence in the Solar neighbourhood, we used the OSIRIS instrument at the 10.4 m Gran Telescopio Canarias to acquire low- and mid-resolution (R${\approx }$300 and R${\approx }$2500) optical spectroscopy of 53 late-M and L ultracool dwarfs. Most of these objects are known but poorly investigated and lacking complete kinematics. We measured spectral indices, determined spectral types (six of which are new) and inferred effective temperature and surface gravity from BT-Settl synthetic spectra fits for all objects. We were able to measure radial velocities via line centre fitting and cross correlation for 46 objects, 29 of which lacked previous radial velocity measurements. Using these radial velocities in combination with the latest Gaia DR3 data, we also calculated Galactocentric space velocities. From their kinematics, we identified two candidates outside of the thin disc and four in young stellar kinematic groups. Two further ultracool dwarfs are apparently young field objects: 2MASSW J1246467+402715 (L4β), which has a potential, weak lithium absorption line, and G 196–3B (L3β), which was already known as young due to its well-studied primary companion.

A stochastic and analytical model of hierarchical fragmentation

Context. Molecular clouds are the most important incubators of young stars clustered in various stellar structures whose spatial extension can vary from a few AU to several thousand AU. Although the reality of these stellar systems has been established, the physical origin of their multiplicity remains an open question. Aims. Our aim was to characterise these stellar groups at the onset of their formation by quantifying both the number of stars they contain and their mass using a hierarchical fragmentation model of the natal molecular cloud. Methods. We developed a stochastic and predictive model that reconciles the continuous multi-scale structure of a fragmenting molecular cloud with the discrete nature of the stars that are the products of this fragmentation. In this model a gas structure is defined as a multi-scale object associated with a subregion of a cloud. Such a structure undergoes quasi-static subfragmentation until star formation. This model was implemented within a gravo-turbulent fragmentation framework to analytically follow the fragmentation properties along spatial scales using an isothermal and adiabatic equations of state (EOSs). Results. We highlighted three fragmentation modes depending on the amount of fragments produced by a collapsing gas structure, namely a hierarchical mode, a monolithic mode, and a mass dispersal mode. Using an adiabatic EOS we determined a characteristic spatial scale where further fragmentation is prevented, around a few tens of AU. We show that fragmentation is a self-regulated process as fragments tend to become marginally unstable following a M ∝ R Bonnor–Ebert-like mass-size profile. Supersonic turbulent fragmentation structures the cloud down to R ≈ 0.1 pc, and gradually turns into a less productive Jeans-type fragmentation under subsonic conditions so hierarchical fragmentation is a scale dependant process. Conclusions. Our work suggests that pre-stellar objects resulting from gas fragmentation, have to progressively increase their accretion rate in order to form stars. A hierarchical fragmentation scenario is compatible with both the multiplicity of stellar systems identified in Taurus and the multi-scale structure extracted within NGC 2264 molecular cloud. This work suggests that hierarchical fragmentation is one of the main mechanisms explaining the presence of primordial structures of stellar clusters in molecular clouds.

Dynamical traceback age of the Octans young stellar association

Context. Octans is one of the most distant (d ∼ 150 pc) young stellar associations of the solar neighbourhood, and it has not yet been sufficiently explored. Its age is still poorly constrained in the literature and requires further investigation. Aims. We take advantage of the state-of-the-art astrometry delivered by the third data release of the Gaia space mission combined with radial velocity measurements obtained from high-resolution spectroscopy to compute the 3D positions and 3D spatial velocities of the stars and derive the dynamical traceback age of the association. Methods. We created a clean sample of cluster members by removing potential outliers from our initial list of candidate members. We then performed an extensive traceback analysis using different subsamples of stars, different metrics to define the size of the association, and different models for the Galactic potential to integrate the stellar orbits in the past. Results. We derive a dynamical age of $ 34^{+2}_{-2} $ Myr that is independent from stellar models and represents the most precise age estimate currently available for the Octans association. After correcting the radial velocity of the stars for the effect of gravitational redshift, we obtain a dynamical age of $ 33^{+3}_{-1} $ Myr, which is in very good agreement with our first solution. This shows that the effect of gravitational redshift is small for such a distant young stellar association. Our result is also consistent with the less accurate age estimates obtained in previous studies from lithium depletion (30–40 Myr) and isochrones (20–30 Myr). By integrating the stellar orbits in time, we show that the members of Octans and Octans-Near had different locations in the past, which indicates that the two associations are unrelated despite the close proximity in the sky. Conclusions. This is the first reliable and precise dynamical age result for the Octans young stellar association. Our results confirm that it is possible to derive precise dynamical ages via the traceback method for ∼30 Myr old stellar clusters at about ∼150 pc with the same precision level that has been achieved in other studies for young stellar groups within 50 pc of the Sun. This represents one more step towards constructing a self-consistent age scale based on the 3D space motion of the stars in the young stellar clusters of the solar neighbourhood.

The Evolution of Stellar X-Ray Activity and Angular Momentum as Seen by eROSITA, TESS, and Gaia

We have assembled a sample of ∼8200 stars with spectral types F5V–M5V, all having directly measured X-ray luminosities from eROSITA and rotation periods from TESS and having empirically estimated ages via their membership in stellar clusters and groups identified in Gaia astrometry (ages 3–500 Myr). This is the largest such study sample yet assembled for the purpose of empirically constraining the evolution of rotationally driven stellar X-ray activity. We observe rotation–age–activity correlations that are qualitatively as expected: stars of a given spectral type spin down with age, and they become less X-ray active as they do so. We provide simple functional representations of these empirical relationships that predict X-ray luminosity from basic observables to within 0.3 dex. Interestingly, we find that the rotation–activity relationship is far simpler and more monotonic in form when expressed in terms of stellar angular momentum instead of rotation period. We discuss how this finding may relate to the long-established idea that rotation–activity relationships are mediated by stellar structure (e.g., convective turnover time, surface area). Finally, we provide an empirical relation that predicts stellar angular momentum from basic observables, without requiring a direct measurement of stellar rotation, to within 0.5 dex.

Outward migration in Nascent stellar groups

ABSTRACT 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.

Unveiling two expanding stellar groups formed through violent relaxation in the lagoon nebula cluster

ABSTRACT The current kinematic state of young stellar clusters can give clues on their actual dynamical state and origin. In this contribution, we use Gaia DR3 data of the Lagoon Nebula Cluster (LNC) to show that the cluster is composed of two expanding groups, likely formed from different molecular cloud clumps. We find no evidence of massive stars having larger velocity dispersion than low-mass stars or being spatially segregated across the LNC, as a whole, or within the Primary group. However, the Secondary group, with 1/5th of the stars, exhibits intriguing features. On the one hand, it shows a bipolar nature, with an aspect ratio of ∼3:1. In addition, the massive stars in this group exhibit larger velocity dispersion than the low-mass stars, although they are not concentrated towards the centre of the group. This suggests that this group may have undergone dynamical relaxation, first, and some explosive event afterward. However, further observations and numerical work have to be performed to confirm this hypothesis. The results of this work suggest that, although stellar clusters may form by the global and hierarchical collapse of their parent clump, still some dynamical relaxation may take place.

Identification of birth places of high-velocity stars: CepOB2 association

We have searched high-velocity stars (runaway, walk-away, pulsars, HMXBs and LMXBs), which could have, most probably, originated in the complex CepOB2 association, i.e. in the cores (small clusters or multiple stellar systems) of it. With the trace-back motion study of them we found at least two pairs consisting of a runaway star and pulsar, which were within stellar groups in the Cep OB2 association ∼ 1.5-5.5 Myr ago.

The Age Distribution of Stellar Orbit Space Clumps

The orbit distribution of young stars in the Galactic disk is highly structured, from well-defined clusters to streams of stars that may be widely dispersed across the sky, but are compact in orbital action-angle space. The age distribution of such groups can constrain the timescales over which conatal groups of stars disperse into the “field.” Gaia data have proven powerful in identifying such groups in action-angle space, but the resulting member samples are often too small and have too narrow a color–magnitude diagram (CMD) coverage to allow robust age determinations. Here, we develop and illustrate a new approach that can estimate robust stellar population ages for such groups of stars. This first entails projecting the predetermined action-angle distribution into the 5D space of positions, parallaxes, and proper motions, where much larger samples of likely members can be identified over a much wider range of the CMD. It then entails isochrone fitting that accounts for: (a) widely varying distances and reddenings; (b) outliers and binaries; (c) sparsely populated main-sequence turnoffs, by incorporating the age information of the low-mass main sequence; and (d) the possible presence of an intrinsic age spread in the stellar population. When we apply this approach to 92 nearby stellar groups identified in 6D orbit space, we find that they are predominantly young (≲1 Gyr), mono-age populations. Many groups are established (known) localized clusters with possible tidal tails, while others tend to be widely dispersed and manifestly unbound. This new age-dating tool offers a stringent approach to understanding on which orbits stars form in the solar neighborhood and how quickly they disperse into the field.

Dynamical substructures of local metal-poor halo

ABSTRACT Based on 4098 very metal poor (VMP) stars with six-dimensional phase-space and chemical information from Gaia Data Release 3 (DR3) and Large sky Area Multi-Object fiber Spectroscopic Telescope DR9 as tracers, we apply an unsupervised machine learning algorithm, shared nearest neighbour, to identify stellar groups in the action–energy (J–E) space. We detect seven previously known mergers in local samples, including Helmi Stream, Gaia-Sausage/Enceladus (GSE), metal-weak thick disc (MWTD), Pontus, Wukong, Thamnos, and I’itoi + Sequoia + Arjuna. According to energy, we further divide GSE and Wukong into smaller parts to explore the orbital characteristics of individual fragments. Similarly, the division of Thamnos is based on action. It can be found that the apocentric distances of GSE parts of high and medium energy levels are located at $29.5\pm 3.6$ and $13.0\pm 2.7\ {\rm kpc}$, respectively, which suggests that GSE could account for breaks in the density profile of the Galactic halo at both ≈30 and 15–18 kpc. The VMP stars of MWTD move along prograde orbits with larger eccentricities than those of its more metal-rich stars, which indicates that the VMP part of MWTD may be formed by accreting with dwarf galaxies. Finally, we summarize all substructures discovered in our local VMP samples. Our results provide a reference for the formation and evolution of the inner halo of the Milky Way.

Stellar populations in STARFORGE: the origin and evolution of star clusters and associations

ABSTRACT Most stars form in highly clustered environments within molecular clouds, but eventually disperse into the distributed stellar field population. Exactly how the stellar distribution evolves from the embedded stage into gas-free associations and (bound) clusters is poorly understood. We investigate the long-term evolution of stars formed in the starforge simulation suite – a set of radiation-magnetohydrodynamic simulations of star-forming turbulent clouds that include all key stellar feedback processes inherent to star formation. We use nbody6++gpu to follow the evolution of the young stellar systems after gas removal. We use HDBSCAN to define stellar groups and analyse the stellar kinematics to identify the true bound star clusters. The conditions modeled by the simulations, i.e. global cloud surface densities below 0.15 g cm−2, star formation efficiencies below 15 per cent, and gas expulsion time-scales shorter than a free fall time, primarily produce expanding stellar associations and small clusters. The largest star clusters, which have ∼1000 bound members, form in the densest and lowest velocity dispersion clouds, representing ∼32 and 39 per cent of the stars in the simulations, respectively. The cloud’s early dynamical state plays a significant role in setting the classical star formation efficiency versus bound fraction relation. All stellar groups follow a narrow mass-velocity dispersion power-law relation at 10 Myr with a power-law index of 0.21. This correlation result in a distinct mass–size relationship for bound clusters. We also provide valuable constraints on the gas dispersal time-scale during the star formation process and analyse the implications for the formation of bound systems.

Jupiter-like planets might be common in a low-density environment

Radial velocity surveys suggest that the Solar System may be unusual and that Jupiter-like planets have a frequency < 20% around solar-type stars. However, they may be much more common in one of the closest associations in the solar neighbourhood. Young moving stellar groups are the best targets for direct imaging of exoplanets and four massive Jupiter-like planets have been already discovered in the nearby young β Pic Moving Group (BPMG) via high-contrast imaging, and four others were suggested via high precision astrometry by the European Space Agency’s Gaia satellite. Here we analyze 30 stars in BPMG and show that 20 of them might potentially host a Jupiter-like planet as their orbits would be stable. Considering incompleteness in observations, our results suggest that Jupiter-like planets may be more common than previously found. The next Gaia data release will likely confirm our prediction.

The complex stellar system M 22: constraining the chemical enrichment from AGB stars using magnesium isotope ratios

ABSTRACT The complex star cluster M 22 (NGC 6656) provides a unique opportunity for studying the slow neutron capture (s-)process nucleosynthesis at low metallicity due to its two stellar groups with distinct iron-peak and neutron capture element abundances. Previous studies attribute these abundance differences to pollution from $3-6 \ \rm {M}_{\odot }$ asymptotic giant branch (AGB) stars which produce significant quantities of the neutron-rich Mg isotopes 25Mg and 26Mg. We report the first-ever measurements of Mg isotopic abundance ratios at $\rm {[Fe/H]} \ \sim -2$ in a globular cluster-like system using very high-resolution and signal-to-noise spectra (R = 110 000, S/N = 300 per pixel at 514 nm) from the VLT/UVES spectrograph for six stars; three in each s-process group. Despite the presence of star-to-star variations in 24Mg, 25Mg, and 26Mg, we find no correlation with heavy element abundances, implying that the nucleosynthetic source of s-process enrichment must not influence Mg isotope ratios. Instead, a key result of this work is that we identify correlations between 26Mg/24Mg and some light elements. Using a custom suite of AGB nucleosynthesis yields tailored to the metallicity of M 22, we find that low mass ($\sim 1 \rm {-} 3 \ \rm {M}_{\odot }$) AGB stars are capable of reproducing the observed s-process abundances of M 22 and that the absence of any difference in Mg isotope ratios between the two s-process groups precludes AGBs with masses above $\sim 3 \ \rm {M}_{\odot }$. This places tighter constraints on possible formation scenarios and suggests an age difference of at least $\sim 280 \rm {-} 480 \ \rm {Myr}$ between the two populations which is independent of isochrone fitting.

The spectroscopic orbit of HD 168112 A,B in NGC 6604: another massive binary target for interferometry

ABSTRACT We present the first spectroscopic orbit of the O-type double-lined star HD 168112 A,B. We analyse 101 high-resolution optical spectra identifying the absorption lines of both components. The orbital solution presents a relatively long period, P = 513.52 ± 0.01 d, and a high eccentricity, e = 0.743 ± 0.005. The binary system consists of two very similar stars of minimum masses of ∼25 M⊙, effective temperatures of ∼40 000 K, and surface gravities of ∼3.7 dex. The system has a minimum semimajor axis a sin i ∼ 1000 R⊙. We confirm that the A and B visual components identified via interferometry do correspond to the spectroscopic ones. We also analyse the underlying stellar groups using Gaia DR3 data and ground-based spectroscopy as part of the Villafranca project, determining that NGC 6604 is at a distance of $1942^{+38}_{-36}$ pc and giving spectral classifications for 23 massive stellar systems in Villafranca O-035 and the surrounding Ser OB2 association, for which we provide the most complete census of massive stars to date.

Binary Star Evolution in Different Environments: Filamentary, Fractal, Halo, and Tidal Tail Clusters

Using membership of 85 open clusters from previous studies based on Gaia Data Release 3 data, we identify binary candidates in the color–magnitude diagram for systems with mass ratio q > 0.4. The binary fraction is corrected for incompleteness at different distances due to the Gaia angular resolution limit. We find a decreasing binary fraction with increasing cluster age, with substantial scatter. For clusters with a total mass >200 M ⊙, the binary fraction is independent of cluster mass. The binary fraction depends strongly on stellar density. Among the four types of cluster environments, the lowest-density filamentary and fractal stellar groups have the highest mean binary fraction: 23.6% and 23.2%, respectively. The mean binary fraction in tidal tail clusters is 20.8% and is lowest in the densest halo-type clusters: 14.8%. We find clear evidence of early disruptions of binary stars in the cluster sample. The radial binary fraction depends strongly on the clustercentric distance across all four types of environments, with the smallest binary fraction within the half-mass radius r h and increasing toward a few r h. Only hints of mass segregation are found in the target clusters. The observed amounts of mass segregation are not significant enough to generate a global effect inside the target clusters. We evaluate the bias of unresolved binary systems (assuming a primary mass of 1 M ⊙) in 1D tangential velocity, which is 0.1–1 km s−1. Further studies are required to characterize the internal star cluster kinematics using Gaia proper motions.

Physical parameters of stellar population in star formation regions of galaxies

ABSTRACT We present the results of a study of young unresolved stellar groupings (clusters, OB associations, and their complexes) associated with H ii regions, based on the coupling of spectroscopic, photometric and H α spectrophotometric observations of star formation regions. Along with our own observations, we use a part of the spectroscopic and H α data from the literature and open data bases. The study is based on the catalogue of 1510 star formation regions with ages ∼10–20 Myr in 19 spiral galaxies, compiled by us earlier. We study the morphology of stellar groupings and their relation with the associated H α emission region. Extinctions, gas chemical abundances, and sizes of star formation regions are measured. Using numerical simple stellar population models computed for metallicities fixed from observations to intrinsic colours of the studied star formation regions, we estimated ages and masses of stellar population of 400 young stellar groupings. Different relations between observational and physical parameters of the young stellar population in star formation regions are discussed.