Young Clusters Research Articles

Candidate Intermediate-mass Black Hole Discovered in an Extremely Young Low-metallicity Cluster in the Tadpole Galaxy KUG 1138+327

Abstract We explore what unusual products a starburst of about 6% solar metallicity and a mean estimated age of ∼5 × 105 yr can produce in KUG 1138+327 at a distance of 24.5 Mpc. Chandra X-ray observations show a dominant point-like source with an average 0.3–10 keV luminosity of 1040.3 erg s−1 and variability by a factor of ∼2 over months. This extreme ultraluminous X-ray source (ULX) is apparently associated with the young central cluster. A multicolor disk modeling of the X-ray spectrum of the source suggests a standard accretion around a black hole. It also has a morphologically elongated nonthermal radio continuum counterpart on the scale of ∼200 pc, probably the longest detected from such a source. The radio, optical, and X-ray findings suggest that it could well be an intermediate-mass black hole undergoing sub-Eddington accretion from a massive star companion. Accounting for the presence of the ULX and the prominent emission lines HeII λ4658 and [Ar IV]λ4711 while lacking Wolf–Rayet spectral features, we estimate the true age of the starburst to be about 2–4 Myr. Only with such a moderate age can the starburst host this extraordinary ULX, probably triggered by a recent influx of extremely low-metallicity gas. This study demonstrates the potential of multiwavelength studies of low-metallicity starbursts to provide insights into what may commonly occur in high-redshift galaxies.

Identification of a Turnover in the Initial Mass Function of a Young Stellar Cluster Down to 0.5 MJ

Abstract A successful theory of star formation should predict the number of objects as a function of their mass produced through star-forming events. Previous studies in star-forming regions and the solar neighborhood have identified a mass function increasing from the hydrogen-burning limit down to about 10 M J. Theory predicts a limit to the fragmentation process, providing a natural turnover in the mass function down to the opacity limit of turbulent fragmentation, thought to be near 1–10 M J. Programs to date have not been sensitive enough to probe the hypothesized opacity limit of fragmentation. We present the first identification of a turnover in the initial mass function below 12 M J within NGC 2024, a young star-forming region. With JWST/NIRCam deep exposures across 0.7–5 μm, we identified several free-floating objects down to roughly 3 M J with sensitivity to 0.5 M J. We present evidence for a double power-law model increasing from about 60 M J to roughly 12 M J, consistent with previous studies, followed by a decrease down to 0.5 M J. Our results support the predictions of star and brown dwarf formation theory, identifying the theoretical turnover in the mass function and suggesting the fundamental limit of turbulent fragmentation to be near 3 M J.

A very young τ-Herculid meteor cluster observed during a 2022 shower outburst

To date, only a few meteor clusters have been instrumentally recorded. This means that every new detection is an important contribution to the understanding of these phenomena, which are thought to be evidence of the meteoroid fragmentation in the Solar System. On May 31 2022 at 6:48:55 UT, a cluster consisting of 52 meteors was detected within 8.5 seconds during a predicted outburst of the τ-Herculid meteor shower. The aim of this paper is to reconstruct the atmospheric trajectories of the meteors and use the collected information to deduce the origin of the cluster. The meteors were recorded by two video cameras during an airborne campaign. Due to only the single station observation, their trajectories were estimated under the assumption that they belonged to the meteor shower. The mutual positions of the fragments, together with their photometric masses, were used to model the processes leading to the formation of the cluster. The physical properties of the cluster meteors are very similar to the properties of the τ-Herculids. This finding confirms the assumption of the shower membership used for the computation of atmospheric trajectories. This is the third cluster that we have studied in detail, but the first one in which we do not see the mass separation of the particles. The cluster is probably less than 2.5 days old, which is too short for such a complete mass separation. Such an age would imply disintegration due to thermal stress. However, we cannot rule out an age of only a few hours, which would allow for other fragmentation mechanisms.

Spatial distribution and clustering properties of the young stellar populations in the Carina Nebula complex and Car OB1

Aims. We use Gaia DR3 astrometry and photometry to analyze the spatial distribution of the young stellar populations and stellar clusters and to search for new OB star candidates in the Carina Nebula complex and the full extent (∼5°, corresponding to ∼200 pc) of the Car OB1 association. Methods. We first performed a new census of high-mass stars in Car OB1 and compiled a comprehensive catalog of 517 stars with known spectral types (128 O-type, WR, and supergiant stars, and 389 B-type stars) that have Gaia DR3 parallaxes consistent with membership in the association. We applied the clustering algorithm DBSCAN on the Gaia DR3 data of the region to find stellar clusters, determine their distances and kinematics, and estimate ages. We also used Gaia astrometry and the additional astrophysical_parameters table to perform a spatially unbiased search for further high-mass members of Car OB1 over the full area of the association. Results. Our DBSCAN analysis finds 15 stellar clusters and groups in Car OB1, four of which were not known before. Most clusters (80%) show signs of expansion or contraction, four of them with a ≥2σ significance. We find a global expansion of the Car OB1 association with a velocity of vout = 5.25 ± 0.02 km s−1. A kinematic traceback of the high-mass stars shows that the spatial extent of the association was at a minimum 3–4 Myr ago. Using astrophysical parameters by Gaia DR3, we identified 15 new O-type and 589 new B-type star candidates in Car OB1. The majority (≳54%) of the high-mass stars constitute a non-clustered distributed stellar population. Based on our sample of high-mass stars, we estimate a total stellar population of at least ∼8 × 104 stars in Car OB1. Conclusions. Our study is the first systematic astrometric analysis that covers the full spatial extent of the Car OB1 association, and it therefore substantially increases the knowledge of the distributed stellar population and spatial evolution of the entire association. Our results suggest suggests Car OB1 to be the most massive known star-forming complex in our Galaxy.

The first CCD photometric study of the member eclipsing binary ZTF J060425.73+365000.1 in the newly discovered young open cluster UBC 68

The first CCD photometric study of the member eclipsing binary ZTF J060425.73+365000.1 in the newly discovered young open cluster UBC 68

Formation of free-floating planetary mass objects via circumstellar disk encounters.

The origin of planetary mass objects (PMOs) wandering in young star clusters remains enigmatic, especially when they come in pairs. They could represent the lowest-mass object formed via molecular cloud collapse or high-mass planets ejected from their host stars. However, neither theory fully accounts for their abundance and multiplicity. Here, we show via hydrodynamic simulations that free-floating PMOs have a unique formation channel via the fragmentation of tidal bridges between encountering circumstellar disks. This process can be highly productive in dense clusters like Trapezium forming metal-poor PMOs with disks. Free-floating multiple PMOs also naturally emerge when neighboring PMOs are caught by their mutual gravity. PMOs may thus form a distinct population that is fundamentally different from stars and planets.

Using Detailed Single-star and Binary-evolution Models to Probe the Large Observed Luminosity Spread of Red Supergiants in Young Open Star Clusters

Abstract Red supergiants (RSGs) represent a late evolutionary stage of massive stars. Recent observations reveal that the observed luminosity range of RSGs in young open clusters is wider than expected from single-star evolution models. Binary evolution effects have been suggested as a possible explanation. Here, we analyze 3670 detailed binary-evolution models, as well as corresponding single-star models, to probe the contribution of binary mass transfer and binary mergers to the luminosity distribution of RSGs in star clusters with ages up to 100 Myr. We confirm that the expected luminosity range of RSGs in a coeval population can span a factor of 10, as a consequence of mergers between two main-sequence stars, which reproduces the observed RSG luminosity ranges in rich clusters well. While the luminosity increase as consequence of mass transfer is more limited, it may help to increase the number of overluminous RSGs. However, our results also demonstrate that binary effects alone are insufficient to account for the number of RSGs found with luminosities of up to 3 times those predicted by current single-star models. We discuss observational accuracy, rotational mixing, age spread, and intrinsic RSG variability as possible explanations. Further observations of RSGs in young open clusters, in particular studies of their intrinsic brightness variability, appear crucial for disentangling these effects.

Formation of massive star clusters with and without iron abundance spreads in a dwarf galaxy merger

Abstract To study the formation of star clusters and their properties in a dwarf–dwarf merging galaxy, we have performed a numerical simulation of a dwarf–dwarf galaxy merger by using the Tree+GRAPE $N$-body/SPH code ASURA. In our simulation, 13 young star clusters are formed during the merger process. We show that our simulated star clusters can be divided into two types: with and without [Fe$/$H] abundance variations. The former is created by a seed star cluster (the first-generation stars) formed in compressed gas. These stars contaminate the surrounding gas by Type II supernovae. At that time, the energy injection is insufficient to induce an outflow of the surrounding gas. After that, the contaminated gas falls into the seed, thereby forming a new generation of stars from the contaminated gas. We also show that most star clusters are formed in the galactic central region after the second encounter and fall into the galactic center due to dynamical friction within several hundred Myr. As a result, close encounters and mergers between the clusters take place. Although the clusters with shallower gravitational potential are tidally disrupted by these close encounters, others survive and finally merge at the center of the merged dwarf galaxies to create a nuclear star cluster. Therefore, the nuclear star cluster comprises various stellar components in ${[\rm Fe/H]}$ abundance and age. We discuss our work in the context of observations and demonstrate the diagnostic power of high-resolution simulations in the context of star cluster formation.

Contribution of young massive stellar clusters to the Galactic diffuse γ-ray emission

Contribution of young massive stellar clusters to the Galactic diffuse γ-ray emission

Unraveling the Kinematic and Morphological Evolution of the Small Magellanic Cloud

Abstract We model the kinematics of the Small Magellanic Cloud (SMC) by analyzing the proper motions (PMs) from Gaia DR3 of nine different stellar populations, including young main-sequence (MS) stars (<2 Gyr), red giant branch stars, red clump stars, red giants with line-of-sight velocities, and three groups of star clusters. This analysis is carried out using a robust Markov Chain Monte Carlo method, to derive up to seven kinematic parameters. We trace the evolution from a nonrotating flattened elliptical system, as mapped by the old population, to a rotating highly stretched disk structure, as denoted by the young MS stars and clusters (<400 Myr). We estimate that the inclination i (∼58°–82°) decreases and the position angle Θ (∼180°–240°) increases with age. We estimate an asymptotic velocity of ∼49–89 km s−1 with a scale radius of ∼6–9 kpc for the young MS populations, with velocity dispersion of ∼11 km s−1, suggesting a rotation-supported disk structure. Our models estimate a line-of-sight extension of ∼30 kpc, in agreement with observations. We identify four regions of the SMC showing anomalies in the residual PM: the East Anomaly, the Southeast Anomaly (SEA), the South Anomaly, and the West Anomaly. The SEA appears like an infalling feature and is identified for the first time. The tidal imprints observed in the residual PM of the SMC suggest that its evolution is considerably shaped by the recent interaction with the Large Magellanic Cloud.

Into the Mystic: the MUSE view of the ionized gas in the Mystic Mountains in Carina

ABSTRACT We present optical integral field unit observations of the Mystic Mountains, a dust pillar complex in the centre of the Carina Nebula that is heavily irradiated by the nearby young massive cluster Trumpler 14. With the continuous spatial and spectral coverage of data from the Multi-Unit Spectroscopic Explorer (MUSE), we measure the physical properties in the ionized gas including the electron density and temperature, excitation, and ionization. MUSE also provides an excellent view of the famous jets HH 901, 902, and 1066, revealing them to be high-density, low-ionization outflows despite the harsh environment. HH 901 shows spatially extended [C i] emission tracing the rapid dissociation of the photoevaporating molecular outflow in this highly irradiated source. We compute the photoevaporation rate of the Mystic Mountains and combine it with recent Atacama Large Millimeter Array observations of the cold molecular gas to estimate the remaining lifetime of the Mystic Mountains and the corresponding shielding time for the embedded protostars. The longest remaining lifetimes are for the smallest structures, suggesting that they have been compressed by ionizing feedback. Our data do not suggest that star formation in the Mystic Mountains has been triggered but it does point to the role that ionization-driven compression may play in enhancing the shielding of embedded stars and discs. Planet formation models suggest that the shielding time is a strong determinant of the mass and orbital architecture of planets, making it important to quantify in high-mass regions like Carina that represent the type of environment where most stars form.

Emergence of high-mass stars in complex fiber networks (EMERGE)

Context. Identified as parsec-size, gas clumps at the junction of multiple filaments, hub-filament systems (HFS) play a crucial role during the formation of young clusters and high-mass stars. These HFS still appear to be detached from most galactic filaments when compared in the mass–length (M–L) phase space. Aims. We aim to characterize the early evolution of HFS as part of the filamentary description of the interstellar medium (ISM). Methods. Combining previous scaling relations with new analytic calculations, we created a toy model to explore the different physical regimes described by the M–L diagram. Despite its simplicity, our model accurately reproduces several observational properties reported for filaments and HFS, such as their expected typical aspect ratio (A), mean surface density (Σ), and gas accretion rate (ṁ). Moreover, this model naturally explains the different mass and length regimes populated by filaments and HFS, respectively. Results. Our model predicts a dichotomy between filamentary (A ≥ 3) and spheroidal (A < 3) structures connected to the relative importance of their fragmentation, accretion, and collapse timescales. Individual filaments with low accretion rates are dominated by an efficient internal fragmentation. In contrast, the formation of compact HFS at the intersection of filaments triggers a geometric phase-transition, leading to the gravitational collapse of these structures at parsec-scales in ~1–2 Myr. In addition, this process also induces higher accretion rates.

Fast-rotating A- and F-type Stars with Hα Emissions in NGC 3532: Candidate UV-dim Stars?

Abstract Extended main-sequence stars that are dim in the ultraviolet passbands of the Hubble Space Telescope (UV-dim stars) are found in several young and intermediate-age star clusters in the Magellanic Clouds (MCs). The obscuring of the dust in the disks of stars expelled as a result of fast rotation has been suggested to be responsible for the appearance of UV-dim stars and to play an important role in the formation of extended main sequences. In this paper, we report a population of A- and F-type stars that show Hα emission features in their spectra in the young (~340 Myr old) neighboring Galactic star cluster NGC 3532. By fitting the observed absorption profiles, we found that most Hα emitters are fast-rotating stars, indicating that they form decretion disks by fast rotation like Be stars. As A- and F-type stars dominate the extended main-sequence turnoff regions of intermediate-age clusters, their appearance provides observational evidence to support the dust extinction scenario for these clusters, and they might be the counterparts of UV-dim stars that are detected in remote MC star clusters such as NGC 1783.

Black Hole Accretion and Spin-up through Stellar Collisions in Dense Star Clusters

Abstract Dynamical interactions in dense star clusters could significantly influence the properties of black holes, leaving imprints on their gravitational-wave signatures. While previous studies have mostly focused on repeated black hole mergers for spin and mass growth, this work examines the impact of physical collisions and close encounters between black holes and (noncompact) stars. Using Monte Carlo N-body models of dense star clusters, we find that a large fraction of black holes retained upon formation undergo collisions with stars. Within our explored cluster models, the proportion of binary black hole mergers affected by stellar collisions ranges from 10%–60%. If all stellar-mass black holes are initially nonspinning, we find that up to 40% of merging binary black holes may have components with dimensionless spin parameter χ ≳ 0.2 because of prior stellar collisions, while typically about 10% have spins near χ = 0.7 from prior black hole mergers. We demonstrate that young star clusters are especially important environments, as they can produce collisions of black holes with very massive stars, allowing for significant spin-up of the black holes through accretion. Our predictions for black hole spin distributions from these stellar collisions highlight their sensitivity to accretion efficiency, underscoring the need for detailed hydrodynamic calculations to better understand the accretion physics following these interactions.

An In-depth Investigation of the Primordial Cluster Pair ASCC 19 and ASCC 21

Abstract Utilizing Gaia data from the literature, we report a new young (∼8.9 Myr) cluster pair, ASCC 19 and ASCC 21, located near the Orion star-forming complex. The clusters are separated by a 3D distance of 27.00 ± 7.51 pc. Both clusters share a common age (Log(age) = 6.95 ± 0.05), similar radial velocities (RV = 21.34 ± 4.47 km s−1 for ASCC 19 and RV = 20.05 ± 3.86 km s−1 for ASCC 21), and comparable metallicities ([Fe/H] = −0.14 ± 0.25 dex for ASCC 19 and [Fe/H] = −0.12 ± 0.04 dex for ASCC 21, from LAMOST-DR11). These similarities suggest that the clusters likely originated from the fragmentation of the same molecular cloud, forming a primordial cluster pair. Furthermore, the formation of the two clusters is attributed to the coalescence of multiple subclusters, as inferred from the distribution analysis between metal abundances and distances to clusters’ centers. Neither cluster shows significant mass segregation. Their members with RVs exceeding 100 km s−1 are young variables. Additionally, a tidal interaction between the clusters is observed. Comparisons of the Roche radius with tidal radii, as well as velocity difference with orbital velocity, suggest that the pair is an unbound system, that is, a double cluster. Finally, orbital motion simulations show that the clusters will not merge into a single system.

Vertical kinematics of the young Galactic clusters

ABSTRACT The young disc vertical phase is paramount in our understanding of Galaxy evolution. Analysing the vertical kinematics at different Galactic regions provides important information about the space–time variations of the Galactic potential. The vertical phase snail shell structure identified in Gaia Data Release 2 includes a wide age range. However, the structure of the $V_Z \ \mathrm{ versus} \ Z$ diagram appears linear when the analysis is limited to studying objects younger than 30 Ma. Based on the vertical velocity and height-over-disc maps obtained for a sample of young open clusters, this method also allows the matter density in the solar neighbourhood to be estimated using a completely different approach than previously found in the literature. We use two different catalogues of star clusters to confirm the previous result and study new age ranges. The linear pattern between $V_Z$ and Z shows different slopes, $\partial V_Z/\partial Z$, for various age groups. The results fit a simple model (harmonic oscillator) of in-plane decoupled vertical dynamics up to a certain age limit, corresponding to $\sim$30 Ma. This work also analyses the relationship between the local volumetric density of matter ($\rho _0$) and the disc vertical kinematics for different age ranges, all below 50 Ma. The best estimates of the effective volumetric mass density in the solar neighbourhood, 0.09–0.15 M$_\odot$ pc${}^{-3}$, agree with those given by other authors, assessing the reliability of the proposed dynamical model. These values are a minorant of the actual matter density in the region.

Evidence of the Amino Acids Tyrosine and Phenylalanine in the Interstellar Material of IC348 in Perseus.

We employed data from the Spitzer Space Telescope to investigate the presence of the aromatic amino acids tyrosine and phenylalanine in the interstellar gas of the young star cluster IC 348. Our analysis revealed emission lines in the observed spectrum that closely matched the strongest mid-infrared laboratory bands associated with tyrosine and phenylalanine in terms of wavelength and intensity. Through flux measurements, we estimated column densities along the line of sight toward the core of IC 348, ranging from 0.8-1.0 × 1011 cm-2. Additionally, these emission lines were evident in the combined spectra of more than 30 interstellar locations spanning various unrelated star-forming regions observed by Spitzer, indicating a widespread distribution of the molecules responsible for the emission throughout interstellar space. Prospective endeavors employing high spectral resolution mid-infrared searches for proteinogenic amino acids in protostars, protoplanetary disks, and the interstellar medium will play a pivotal role in elucidating the external origins of meteoritic amino acids and understanding the prebiotic conditions that laid the groundwork for life on early Earth.

Detecting the Black Hole Candidate Population in M51’s Young Massive Star Clusters: Constraints on Accreting Intermediate-mass Black Holes

Abstract Intermediate-mass black holes (102 < M BH < 105 M ⊙) are an open question in our understanding of black hole evolution and growth. They have long been linked to dense star cluster environments, thanks to cluster dynamics, but there are a limited number of secure detections. We leverage existing X-ray observations from the Chandra X-ray Observatory and optical catalogs from the Hubble Space Telescope (HST) as well as new radio observations from the Karl G. Jansky Very Large Array to search for any evidence of accreting black holes in young massive star clusters in the nearby galaxy M51. We find that of 44 bright (L X > 1038 erg s−1) X-ray point sources in M51, 24 had probable matches to objects including possible associated star clusters in the HST Legacy Extragalactic UV Survey catalog, seven of which were classified as contaminants (background galaxies or foreground stars). We explore the optical properties of the remaining 17 sources, including cluster age and mass estimates, and search for radio counterparts in the 8–12 GHz band. The lack of radio counterparts to X-ray sources we know to be associated with young massive star clusters in M51 suggests that we do not significantly detect hard-state intermediate-mass black holes (IMBHs) ~104 M ⊙ or above. However, more sensitive radio facilities, like the Square Kilometre Array and next-generation Very Large Array, may be able to provide evidence for IMBHs with masses down to ~103 M ⊙.

Disk-locking Regulates Stellar Rotation in Young Clusters: Insights from NGC 2264

Abstract Many young clusters possess extended main sequences, a phenomenon commonly ascribed to stellar rotation. However, the mechanism behind their very wide stellar rotation distributions remains unclear. A proposed explanation is that magnetic star–disk interaction can regulate stellar rotation, i.e., protostars with longer disk lifetimes will eventually evolve into slow rotators, and vice versa. To examine this hypothesis, we took the star-forming region NGC 2264, as a test bed. We have studied its high-mass pre-main-sequence and zero-age main-sequence (ZAMS) stars. We find that, on average, diskless pre-main-sequence stars rotate faster than their disk-bearing counterparts. The stellar rotation distribution of the ZAMS stars is similar to evolved young clusters. We conclude that disk-locking may play a crucial role in the rotational velocity distribution of intermediate-mass early-type stars. We suggest that the observed wide stellar rotation distributions in many young clusters can occur in their early stages.

Massive Clusters and OB Associations as Output of Massive Star Formation in Gaia Era

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.