Young Clusters Research Articles

Clusters, Clumps, Dust, and Gas (CCDG) in NGC 1614: Benchmarking Cluster Demographics in Extreme Systems

Observations of young star clusters in a variety of galaxies have been used to constrain basic properties related to star formation, such as the fraction of stars found in clusters (Γ) and the shape of the cluster mass function (CMF). However, the results can depend heavily on the reliability of the cluster age-dating process and other assumptions. One of the biggest challenges for successful age-dating lies in breaking the age–reddening degeneracy, where older, dust-free clusters and young, reddened clusters can have similar broadband colors. While this degeneracy affects cluster populations in all galaxies, it is particularly challenging in systems with dusty, extreme star-forming environments. We study the cluster demographics in the luminous infrared galaxy NGC 1614 using Hubble Space Telescope imaging taken in eight optical–near-infrared passbands. For age-dating, we adopt a spectral energy distribution fitting process that limits the maximum allowed reddening by region and includes Hα photometry directly. We find that without these assumptions essentially all clusters in the dust-free UV-bright arm that should have ages ≈50–250 Myr are incorrectly assigned ages younger than 10 Myr. We find that this method greatly reduces the number of clusters in the youngest (τ < 10 Myr) age bin and shows a fairly uniform distribution of massive clusters, the most massive being ≈few × 107 M ⊙. A maximum likelihood fit shows that the CMF is well fitted by a power law with an index of approximately −1.8, with no statistically significant high-mass cutoff. We calculate the fraction of stars born in clusters to be Γ1−10 = 22.4% ± 5.7%. The fraction of stars in clusters decreases quickly over time, with Γ10−100 = 4.5% ± 1.1% and Γ100−400 = 1.7% ± 0.4%, suggesting that clusters dissolve rapidly over the first ∼0.5 Gyr. The decreasing fraction of stars in clusters is consistent with the declining shape observed for the cluster age distribution.

Unveiling gas kinematics and stellar populations in H ii regions inside the low-metallicity dwarf nearby galaxy SDSSJ0859+3923 with MEGARA at the GTC

Abstract In this study, we present Integral Field Unit observations of the galaxy SDSSJ0859+3923, utilizing the MEGARA instrument on the GTC 10.4m telescope. These observations were conducted in two distinct spectral ranges: 4332 – 5222 Å and 6097 – 7345 Å, with a high resolving power (RFWHM ∼6 000), and spatial resolution of 25 pc, considering the galaxy’s distance of 8.5 Mpc. Our observations have identified five H ii regions, whose precise positions were determined using data from the FWFC3-UVIS/HST archive images, where we also detected the associated blue underlying continuum linked to the ionized knots. A detailed kinematic analysis of these regions revealed low velocity dispersion values (around 10 km s−1) in four H ii regions, indicating a lack of significant turbulent events. In the fifth region we observed a peak in velocity dispersion reaching 40 km s−1, which we interpret as the result of hot star winds and/or a recent type-II supernova explosion. We have conducted a comprehensive spectral analysis of the H ii regions, obtaining emission-line fluxes that enabled us to confirm the oxygen abundance (12 + log(O/H) = 7.41 ± 0.15) and, using popstar models, to constrain the age and mass of the ionizing young clusters.

Evolution of the mass-radius relation of expanding very young star clusters

The initial mass–radius relation of embedded star clusters is an essential boundary condition for understanding the evolution of embedded clusters in which stars form to their release into the galactic field via an open star cluster phase. The initial mass–radius relation of embedded clusters deduced by Marks &amp; Kroupa (2012, A&amp;A, 543, A8) is significantly different from the relation suggested by Pfalzner et al. (2016, A&amp;A, 586, A68). Here, we use direct N-body simulations to model the early expansion of embedded clusters after the expulsion of their residual gas. The observationally deduced radii of clusters up to a few million years old, compiled from various sources, are well fitted by N-body models, implying that these observed very young clusters are most likely in an expanding state. We show that the mass–radius relation of Pfalzner et al. (2016) reflects the expansion of embedded clusters following the initial mass–radius relation of Marks &amp; Kroupa (2012). We also suggest that even the embedded clusters in ATLASGAL clumps with HII regions are probably already in expansion. All the clusters collected here from different observations show a mass-radius relation with a similar slope, which may indicate that all clusters were born with a profile resembling that of the Plummer phase-space distribution function.

Constraining the Diffusion Coefficient and Cosmic-Ray Acceleration Efficiency Using Gamma-Ray Emission from the Star-forming Region RCW 38

Stellar winds from massive stars may be significant sources of cosmic rays (CRs). To investigate this connection, we report a detailed study of gamma-ray emission near the young Milky Way star cluster (≈0.5 Myr old) in the star-forming region RCW 38 and compare this emission to its stellar wind properties and diffuse X-ray emission. Using 15 yr of Fermi-LAT data in the 0.2–300 GeV band, we find a significant (σ > 22) detection coincident with the star cluster, producing a total gamma-ray luminosity (extrapolated over 0.1–500 GeV) of L γ =(2.66 ± 0.92) × 1034 erg s−1 adopting a power-law spectral model (Γ = 2.34 ± 0.04). Using an empirical relationship and STARBURST99, we estimate the total wind power to be 8 × 1036 erg s−1, corresponding to a CR acceleration efficiency of η CR ≃ 0.4 for an assumed diffusion coefficient consistent with D = 1028 cm2 s−1. Alternatively, a lower acceleration efficiency of 0.1 can produce this L γ if the diffusion coefficient is smaller, D ≃ 2.5 × 1027 cm2 s−1. Additionally, we analyze Chandra X-ray data from the region and compare the hot-gas pressure to the CR pressure. We find the former is 4 orders of magnitude greater, suggesting that the CR pressure is not dynamically important relative to stellar winds. As RCW 38 is too young for supernovae to have occurred, the high CR acceleration efficiency in RCW 38 demonstrates that stellar winds may be an important source of Galactic CRs.

Unveiling the shape: A multi-wavelength analysis of the galaxy clusters Abell 76 and Abell 1307

We analyse the dynamical state of the galaxy clusters Abell 76 and Abell 1307 from the optical point of view, presenting a coherent scenario that responds to the X-ray emissions observed in these structures. Our study is based on 231 and 164 spectroscopic redshifts, for the clusters A76 and A1307, respectively, obtained mostly with the Telescopio Nazionale Galileo, and complemented with others collected from the SDSS DR16 spectroscopic database and the literature. We find that A76 and A1307 are two galaxy clusters at z = 0.0390 and 0.0815, respectively, with a velocity dispersion of 650 ± 56 km s−1 and 863 ± 85 km s−1, and they show velocity distributions following, in practice, Gaussian profiles. From our dynamical analysis, X-ray studies and SZ-Planck emission, we obtain a mean total mass M500 = 1.7 ± 0.6 ⋅ 1014 M⊙ and 3.5 ± 1.3 ⋅ 1014 M⊙ for A76 and A1307, respectively. Using the SDSS DR16 photometric database, we find that the spatial distribution of likely cluster members in the case of A76 is very anisotropic, while A1307 shows a compact distribution of galaxies, but it is double peaked and elongated in the south-north direction. Using XMM-Newton X-ray data, we compared the surface brightness maps with galaxy distributions and noticed that both distributions are correlated. We reconstructed the total mass profile and velocity anisotropy of both clusters by analysing the full projected phase space, through the MG-MAMPOSSt code. Our study reveals a slight indication of radial orbits for A76, while A1307 seems to prefer more isotropic orbits in the whole cluster range. In summary, A76 represents a typical young cluster, in an early stage of formation, with a very low X-ray surface brightness but a high temperature showing a very anisotropic galaxy distribution. A1307 is however more consolidated and massive showing in-homogeneous galaxy distribution and an asymmetric X-ray emission, which suggest a scenario characterised by recent minor mergers.

A 3D view of multiple populations’ kinematics in Galactic globular clusters

We present the first 3D kinematic analysis of multiple stellar populations (MPs) in a representative sample of 16 Galactic globular clusters (GCs). For each GC in the sample, we studied the MP line-of-sight, plane-of-the-sky and 3D rotation, and velocity distribution anisotropy. The differences between first-population (FP) and second-population (SP) kinematic patterns were constrained by means of parameters specifically defined to provide a global measure of the relevant physical quantities and to enable a meaningful comparison among different clusters. Our analysis provides the first observational description of the MP kinematic properties and of the path they follow during their long-term dynamical evolution. In particular, we find evidence of differences between the rotation of MPs along all velocity components with the SP preferentially rotating faster than the FP. The difference between the rotation strength of MPs is anticorrelated with the cluster dynamical age. We also observe that FPs are characterized by isotropic velocity distributions at any dynamical age probed by our sample. On the contrary, the velocity distribution of SP stars is found to be radially anisotropic in dynamically young clusters and isotropic at later evolutionary stages. The comparison with a set of numerical simulations shows that these observational results are consistent with the long-term evolution of clusters forming with an initially more centrally concentrated and more rapidly rotating SP subsystem. We discuss the possible implications these findings have on our understanding of MP formation and early evolution.

Twins in diversity: understanding circumstellar disc evolution in the twin clusters of W5 complex

ABSTRACT Young star-forming regions in massive environments are ideal test beds to study the influence of surroundings on the evolution of discs around low-mass stars. We explore two distant young clusters, IC 1848-East and West located in the massive W5 complex. These clusters are unique due to their similar (distance, age and extinction) yet distinct (stellar density and far-ultraviolet radiation fields) physical properties. We use deep multiband photometry in optical, near-infrared and mid-infrared wavelengths complete down to the substellar limit in at least five bands. We trace the spectral energy distribution of the sources to identify the young pre-main sequence members in the region and derive their physical parameters. The disc fraction for the East and West clusters down to 0.1 M$_\odot$ was found to be $\sim 27\,\pm$ 2 per cent (N$_\mathrm{disc}$ = 184, N$_\mathrm{discless}$ = 492) and $\sim 17\,\pm$ 1 per cent (N$_\mathrm{disc}$ = 173, N$_\mathrm{discless}$ = 814), respectively. While no spatial variation in the disc fraction is observed, these values are lower than those in other nearby young clusters. Investigating the cause of this decrease, we find a correlation with the intense feedback from massive stars throughout the cluster area. We also identified the disc sources undergoing accretion and observed the mass accretion rates to exhibit a positive linear relationship with the stellar host mass and an inverse relationship with stellar age. Our findings suggest that the environment significantly influences the dissipation of discs in both clusters. These distant clusters, characterized by their unique attributes, can serve as templates for future studies in outer galaxy regions, offering insights into the influence of feedback mechanisms on star and planetary formation.

Witnessing an extreme, highly efficient galaxy formation mode with resolved Lyman-α and Lyman-continuum emission

J1316+2614 at z = 3.613 is the UV-brightest (MUV = −24.7) and strongest Lyman continuum-emitting (fescLyC ≈ 90%) star-forming galaxy known; it also shows signatures of inflowing gas from its blue-dominated Lyα profile. We present high-resolution imaging with the Hubble Space Telescope (HST) and the Very Large Telescope (VLT) of the LyC, Lyα, rest-UV, and optical emission of J1316+2614. Detailed analysis of the LyC and UV light distributions reveals compact yet resolved profiles, with LyC and UV morphologies showing identical half-light radii of reff ≃ 220 pc. The continuum-subtracted Lyα emission, obtained with the HST ramp-filter FR551N, reveals an extended filamentary structure of ≃6.0 kpc oriented south to north with only residual flux within the stellar core, suggesting a Lyα ‘hole’. Our spectral energy distribution analysis shows that J1316+2614 is characterised by a young (5.7 ± 1.0 Myr), nearly un-obscured stellar population with a high star-formation rate (SFR = 898 ± 181 M⊙ yr−1) and a stellar mass of M⋆young = (4.8 ± 0.3) × 109 M⊙. Additionally, the spectral energy distribution analysis supports the absence of an underlying old stellar population (M⋆old ≤ 2.8 × 109 M⊙, 3σ). J1316+2614 presents remarkably high SFR and stellar mass surface densities of log(Σ SFR[M⊙ yr−1 kpc−2]) = 3.47 ± 0.11 and log(ΣM⋆[M⊙pc−2]) = 4.20 ± 0.06, respectively, which are among the highest observed in star-forming galaxies and are more typically observed in local young massive star clusters and globular clusters. Our findings indicate that J1316+2614 is a powerful, young, and compact starburst that is leaking a significant amount of LyC photons due to a lack of gas and dust within the starburst. We explored the conditions for gas expulsion using a simple energetic balance and find that, given the strong binding force in J1316+2614, a high star-formation efficiency (ϵSF ≥ 0.7) is necessary to explain the removal of gas and its exposed nature. Our results thus suggest a close link between high ϵSF and high fescLyC. This high efficiency can also naturally explain the remarkably high SFR, UV luminosity, and efficient mass growth of J1316+2614, which acquired at least 62% of its mass in the last 6 Myr. J1316+2614 may exemplify an intense, feedback-free starburst with a high ϵSF, similar to those proposed for UV-bright galaxies at high redshifts.

Stripped Helium Star and Compact Object Binaries in Coeval Populations: Predictions Based on Detailed Binary Evolution Models

Massive stars mainly form in close binaries, where their mutual interactions can profoundly alter their evolutionary paths. Evolved binaries consisting of a massive OB-type main-sequence star with a stripped helium star or a compact companion represent a crucial stage in the evolution toward double compact objects, whose mergers are (potentially) detectable via gravitational waves. The recent detection of X-ray-quiet OB+black hole binaries and OB+stripped helium star binaries has set the stage for discovering more of these systems in the near future. In this work, based on 3670 detailed binary-evolution models and using empirical distributions of initial binary parameters, we compute the expected population of such evolved massive binaries in coeval stellar populations, including stars in star clusters and in galaxies with starburst activities, for ages up to 100 Myr. Our results are vividly illustrated in an animation that shows the evolution of these binaries in the color–magnitude diagram over time. We find that the number of OB+black hole binaries peaks around 10 Myr, and OB+neutron star binaries are most abundant at approximately 20 Myr. Both black holes and neutron stars can potentially be found in populations with ages up to 90 Myr. Additionally, we analyze the properties of such binaries at specific ages. We find that OB+helium stars and OB+black hole binaries are likely to be identifiable as single-lined spectroscopic binaries. Our research serves as a guide for future observational efforts to discover such binaries in young star clusters and starburst environments.

The Radcliffe wave as traced by young open clusters. Stellar parameters, activity indicators, and abundances of solar-type members of eight young clusters

The Radcliffe wave has only recently been recognised as a approx 3\,kpc long coherent gas structure encompassing most of the star-forming regions in the solar vicinity. Since its discovery, it has been mainly studied from the perspective of dynamics, but a detailed chemical study is necessary to understand its nature and the composition of the natal clouds that gave rise to it. For this paper we used some of the connected young open clusters (age lesssim 100 Myr) as tracers of the molecular clouds. We performed high-resolution spectroscopy with GIARPS at the TNG of 53 stars that are bona fide members of seven clusters located at different positions along the Radcliffe wave. We provide radial velocities and atmospheric parameters for all of them. For a subsample consisting of 41 FGK stars, we also studied the chromospheric activity and the content of Li, from which we inferred the age of the parent clusters. These values agree with the evolutionary ages reported in the literature. For these FGK stars, we determined the chemical abundances for 25 species. Pleiades, ASCC\,16, and NGC\,7058 exhibit a solar metallicity while Melotte\,20, ASCC\,19, NGC\,2232, and Roslund\,6 show a slightly subsolar value (approx \,$-$0.1\,dex). On average, the clusters show a chemical composition compatible with that of the Sun, especially for alpha - and Fe-peak elements. Neutron-capture elements, on the other hand, present a slight overabundance of about 0.2\,dex, especially barium. Finally, considering also ASCC\,123, which was studied by our group in a previous research project, we inferred a correlation between the chemical composition and the age or position of the clusters along the wave, demonstrating their physical connection within an inhomogeneous mixing scenario.

Discovering Subsolar Metallicity Brown Dwarf Candidates in the Small Magellanic Cloud

We present the discovery of the first rich population of brown dwarf candidates (cBD) at subsolar metallicity, observed by JWST outside the Milky Way (MW) in the young SMC star cluster NGC 602. Located in the Small Magellanic Cloud (SMC) “wing,” in a very low-density environment (1.3 cm−3) and at subsolar metallicity, NGC 602 is very young, with an age of 2–3 Myr. The low stellar density in this star cluster together with JWST NIRCam images in eight filters allowed us to individually resolve and derive accurate photometric measurements for 64 candidate BDs with masses ranging from 0.05 to 0.08 M ⊙ or 50 to 84 M Jup, according to brown dwarf (BD) evolutionary models. This is the first detection of a young BD population outside the MW. Their spatial distribution indicates that they appear colocated with the pre-main-sequence stars. Although further detailed work is required to quantitatively derive the initial mass function and confirm the true nature of the cBD, this discovery is particularly relevant in the effort to refine our understanding of the subsolar mass function at very low metallicities and young ages.

Detection of extended gamma-ray emission in the vicinity of Cl Danks 1 and 2

Abstract We report the detection of high-energy gamma-ray emission towards the G305 star-forming region. Using almost 15 years of observation data from Fermi Large Area Telescope, we detected an extended gamma-ray source in this region with a significance of ∼13σ. The gamma-ray radiation reveals a clear pion-bump feature and can be fitted with the power law parent proton spectrum with an index of −2.5. The total cosmic ray (CR) proton energy in the gamma-ray production region is estimated to be the order of $10^{49}\ \rm erg$. We further derived the CR radial distribution from both the gamma-ray emission and gas distribution and found it roughly obeys the 1/r type profile, though a constant profile is not ruled out. This is consistent with other similar systems and expected from the continuous injection of CRs by the central powerful young massive star cluster Danks 1 or Danks 2 in this region. Together with former detections of similar gamma-ray structures, such as Cygnus cocoon, Westerlund 1, Westerlund 2, NGC 3603, and W40, the detection supports the hypothesis that young massive star clusters are CR accelerators.

A deep investigation of the poorly studied open cluster King 18 using CCD VRI, Gaia DR3 and 2MASS

In this paper, we re-estimate the astrometric and photometric parameters of the young open star cluster King 18 based on Gaia Data Release 3 (DR3), Two Micron All-sky Survey (2MASS) and VRI CCD observations using the f/4.9 Newtonian focus of 74-inch telescope at Kottamia Astronomical Observatory (KAO) in Egypt. King 18 is a poorly studied open star cluster, for which new results are found in the current study. In order to estimate the membership and determine all the astrophysical parameters of the cluster, we have used data from Gaia DR3 and KAO. The center, cluster radius, radial density distribution, color-magnitude diagrams, distance, age, and reddening of King 18 are calculated. Also, the luminosity and mass functions, the total mass and the relaxation time of the cluster are estimated. The slope value of the mass function (α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha $$\\end{document}) of King 18 is found to be 2.27± 0.17, which is comparable with Salpeter value. Our estimates for the average cluster age and the relaxation time are 224 ± 6.3 and 28.92 Myrs, respectively. This indicates that King 18 is dynamically stable and a relaxed cluster. The cluster distance modulus from Gaia, 2Mass and VRI observations has been determined to be 12.380 ± 1.320, 12.320 ± 0.107 and 12.280 ± 0.290 mag respectively, which corresponds to distances of 2992.26, 2910.72 and 2857.59 pc, respectively. These results are in good agreement within the error. Moreover the color excesses E(V–I), E(J–Ks\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {K}_s$$\\end{document}) and E(GBP\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {G}_{{BP}}$$\\end{document}–GRP\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {G}_{{RP}}$$\\end{document}) are 0.850 ± 0.087, 0.380 ± 0.091 and 0.980 ± 0.130 respectively. Finally, the proper motions (μα\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu _{\\alpha }$$\\end{document}cosδ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\delta $$\\end{document}, μδ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu _{\\delta }$$\\end{document}), and parallaxes (ϖ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\varpi $$\\end{document}) are -2.603±0.018\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-2.603 \\pm 0.018$$\\end{document}, -2.106±0.013\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-2.106 \\pm 0.013$$\\end{document} and 0.324 ± 0.040, respectively.

Two waves of massive stars running away from the young cluster R136.

Massive stars are predominantly born in stellar associations or clusters1. Their radiation fields, stellar winds and supernovae strongly impact their local environment. In the first few million years of a cluster's life, massive stars are dynamically ejected and run away from the cluster at high speed2. However, the production rate of dynamically ejected runaways is poorly constrained. Here we report on a sample of 55 massive runaway stars ejected from the young cluster R136 in the Large Magellanic Cloud. An astrometric analysis of Gaia data3-5 reveals two channels of dynamically ejected runaways. The first channel ejects massive stars in all directions and is consistent with dynamical interactions during and after the birth of R136. The second channel launches stars in a preferred direction and may be related to a cluster interaction. We found that 23-33% of the most luminous stars initially born in R136 are runaways. Model predictions2,6,7 have significantly underestimated the dynamical escape fraction of massive stars. Consequently, their role in shaping and heating the interstellar and galactic media and their role in driving galactic outflows are far more important than previously thought8,9.

The spectroscopic binary fraction of the young stellar cluster M17

Context. Significant progress has been made toward understanding the formation of massive (M &gt; 8 M⊙) binaries in close orbits (with periods of less than a month). Some of the observational studies leading to this progress are the detection of a very low velocity dispersion among the massive stars in the young region M17 and the measurement of a positive trend of velocity dispersion with age in Galactic clusters. The velocity dispersion observed in M17 could be explained either by the lack of binaries among the stars in this region, which implies the highly unlikely scenario of a different formation mechanism for M17 than for other Galactic regions, or by larger binary separations than typically observed, but with a binary fraction similar to other young Galactic clusters. The latter implies that, over time, the binary components migrate toward each other. This is in agreement with the finding that the radial velocity dispersion of young Galactic clusters correlates positively with their age. Aims. We aim to determine the origin of the strikingly low velocity dispersion by determining the observed and intrinsic binary fraction of massive stars in M17 through multi-epoch spectroscopy. Methods. We performed a multi-epoch spectroscopic survey consisting of three epochs separated by days and months, respectively. We complemented this survey with existing data covering timescales of years. We determined the radial velocity of each star at each epoch by fitting the stellar absorption profiles. The velocity shifts between epochs were used to determine whether a close companion is present. Results. We determined an observed binary fraction of 27% and an intrinsic binary fraction of 87%, consistent with that of other Galactic clusters. We conclude that the low velocity dispersion is due to a large separation among the young massive binaries in M17. Our result is in agreement with a migration scenario in which massive stars are born in binaries or higher-order systems at large separation and harden within the first million years of evolution. Such an inward migration may either be driven by interaction with a remnant accretion disk or with other young stellar objects present in the system, or by dynamical interactions within the cluster. Our results imply that possibly both dynamical interactions and binary evolution are key processes in the formation of gravitational wave sources.

Massive star cluster formation

The mode of star formation that results in the formation of globular clusters and young massive clusters is difficult to constrain through observations. We present models of massive star cluster formation using the TORCH framework, which uses the Astrophysical MUltipurpose Software Environment (AMUSE) to couple distinct multi-physics codes that handle star formation, stellar evolution and dynamics, radiative transfer, and magnetohydrodynamics. We upgraded TORCH by implementing the N-body code PETAR, thereby enabling TORCH to handle massive clusters forming from 106 M⊙ clouds with ≥105 individual stars. We present results from TORCH simulations of star clusters forming from 104, 105, and 106 M⊙ turbulent spherical gas clouds (named M4, M5, M6) of radius R = 11.7 pc. We find that star formation is highly efficient and becomes more so at a higher cloud mass and surface density. For M4, M5, and M6 with initial surface densities 2.325 × 101,2,3 M⊙ pc−2, after a free-fall time of tff = 6.7,2.1,0.67 Myr, we find that ∼30%, 40%, and 60% of the cloud mass has formed into stars, respectively. The end of simulation-integrated star formation efficiencies for M4, M5, and M6 are ϵ⋆ = M⋆/Mcloud = 36%, 65%, and 85%. Observations of nearby clusters similar in mass and size to M4 have instantaneous star formation efficiencies of ϵinst ≤ 30%, which is slightly lower than the integrated star formation efficiency of M4. The M5 and M6 models represent a different regime of cluster formation that is more appropriate for the conditions in starburst galaxies and gas-rich galaxies at high redshift, and that leads to a significantly higher efficiency of star formation. We argue that young massive clusters build up through short efficient bursts of star formation in regions that are sufficiently dense (Σ ≥ 102 M⊙ pc−2) and massive (Mcloud ≥ 105 M⊙). In such environments, stellar feedback from winds and radiation is not strong enough to counteract the gravity from gas and stars until a majority of the gas has formed into stars.

GOALS-JWST: Constraining the Emergence Timescale for Massive Star Clusters in NGC 3256

We present the results of a James Webb Space Telescope NIRCam and NIRSpec investigation into the young massive star cluster (YMC) population of NGC 3256, the most cluster-rich luminous infrared galaxy in the Great Observatories All Sky LIRG Survey. We detect 3061 compact YMC candidates with a signal-to-noise ratio ≥3 at F150W, F200W, and F335M. Based on yggdrasil stellar population models, we identify 116/3061 sources with F150W – F200W > 0.47 and F200W – F355M > −1.37 colors, suggesting that they are young (t ≤ 5 Myr), dusty (A V = 5−15), and massive (M ⊙ > 105). This increases the sample of dust-enshrouded YMCs detected in this system by an order of magnitude relative to previous Hubble Space Telescope studies. With NIRSpec integral field unit pointings centered on the northern and southern nucleus, we extract the Paα and 3.3 μm polycyclic aromatic hydrocarbon (PAH) equivalent widths for eight bright and isolated YMCs. Variations in both the F200W – F335M color and 3.3 μm PAH emission with the Paα line strength suggest a rapid dust clearing (<3−4 Myr) for the emerging YMCs in the nuclei of NGC 3256. Finally, with both the age and dust emission accurately measured, we use yggdrasil to derive the color excess (E(B − V)) for all eight YMCs. We demonstrate that YMCs with strong 3.3 μm PAH emission (F200W – F335M > 0) correspond to sources with E(B − V) > 3, which are typically missed in UV-optical studies. This underscores the importance of deep near-infrared imaging for finding and characterizing these very young and dust-embedded sources.

Massive star cluster formation

Two main mechanisms have classically been proposed for the formation of runaway stars. In the binary supernova scenario (BSS), a massive star in a binary explodes as a supernova, ejecting its companion. In the dynamical ejection scenario, a star is ejected during a strong dynamical encounter between multiple stars. We propose a third mechanism for the formation of runaway stars: the subcluster ejection scenario (SCES), where a subset of stars from an infalling subcluster is ejected out of the cluster via a tidal interaction with the contracting gravitational potential of the assembling cluster. We demonstrate the SCES in a star-by-star simulation of the formation of a young massive cluster from a 106 M⊙ gas cloud using the TORCH framework. This star cluster forms hierarchically through a sequence of subcluster mergers determined by the initial turbulent, spherical conditions of the gas. We find that these mergers drive the formation of runaway stars in our model. Late-forming subclusters fall into the central potential, where they are tidally disrupted, forming tidal tails of runaway stars that are distributed highly anisotropically. Runaways formed in the same SCES have similar ages, velocities, and ejection directions. Surveying observations, we identify several SCES candidate groups with anisotropic ejection directions. The SCES is capable of producing runaway binaries: two wide dynamical binaries in infalling subclusters were tightened through ejection. This allows for another velocity kick via subsequent via a subsequent BSS ejection. An SCES-BSS ejection is a possible avenue for the creation of hypervelocity stars unbound to the Galaxy. The SCES occurs when subcluster formation is resolved. We expect nonspherical initial gas distributions to increase the number of calculated runaway stars, bringing it closer to observed values. The observation of groups of runaway stars formed via the SCES can thus reveal the assembly history of their natal clusters.

A binary supernova OB-runaway candidate inside Berkeley 97

OB-runaway stars ejected by the binary supernova mechanism can be found near young open star clusters. In this paper, we present an OB-runaway candidate as a pre-SN binary companion to the progenitor of the pulsar PSR J2238$+$5903 inside the young open star cluster Berkeley 97. We tried to find a kinematic outlier based on Gaia DR3 proper motions and parallaxes to be the pre-supernova binary companion to the progenitor of the pulsar. We took the spectra of two bright early B-type stars of the cluster, determined their effective temperature and surface gravity, and updated the parameters of the cluster. Through isochrone fitting of the color-magnitude diagram of the star cluster, we identified the members and determined the stellar parameters of the runaway star. Two bright members of the cluster, HD 240015 and HD 240016, are massive stars with spectral types of B0.5II and B1.5II and effective temperatures of $T_ eff K and $T_ eff K, respectively, as well as surface gravities of $ g cm/s^2 We find that Berkeley 97 is a star cluster with an age of $ age yr )=7.1$, an uncertainty of $&lt;0.1$ dex, and an interstellar extinction of $A_ V mag. The runaway star has an effective temperature of $T_ eff K with a surface gravity of $ g cm/s^2 (B8V type star). By tracing back the proper motion of the runaway star, the explosion center was found for different possible pulsar ages of 10, 20, and 26.6 kyr. The pulsar moving out from the 20 kyr position must have a space velocity of $ km $, which is consistent with the general pulsar velocity distribution. This supports the idea that the pulsar originated from the cluster as a result of a binary supernova. Despite its young age, $ kyr, the supernova remnant is not visible.

The Clearing Timescale for Infrared-selected Star Clusters in M83 with HST

We present an analysis of Hubble Space Telescope data from Wide Field Camera 3 (WFC3)/Ultraviolet Imaging Spectrometer, WFC3/IR, and the Advanced Camera for Surveys, investigating the young stellar cluster (YSC) population in the face-on spiral galaxy M83. Within the field of view of the IR pointings, we identify 454 sources with compact F814W continuum and Paβ line emission with a S/N ≥ 3 as possible YSC candidates embedded in dust. We refine this selection to 97 candidates based on their spectral energy distributions, multiwavelength morphology, and photometric uncertainties. For sources that are detected in all bands and have mass >102.8 M ⊙ (53 sources), we find that by 2 Myr 75% of IR-selected star clusters have an A V ≤ 1 and that by 3 Myr the fraction rises to ∼82%. This evidence of early clearing implies that presupernova (pre-SN) feedback from massive stars is responsible for clearing the majority of the natal gas and dust that surround IR-selected star clusters in M83. Further, this result is consistent with previous estimates based on WFC3 observations and adds to the growing body of literature suggesting pre-SN feedback to be crucial for YSC emergence in normal star-forming galaxies. Finally, we find a weak correlation between the YSC concentration index and age over the first 10 Myr, which matches previous studies and indicates little or no change in the size of YSCs in M83 during their early evolution.