Search

Molecular gas in the central region of the starburst galaxy NGC 253 is complicated because of the combination of different gas kinematics, including stellar feedback, noncircular motions, and magnetic activity. In paper I, we used the latest multitransition, multi-isotope CO-line data sets obtained with the Atacama Large Millimeter/submillimeter Array to classify the molecular gas within the central 850 pc in radius into 28 coherent structures, and we proposed a three-dimensional geometric structure for the gas in the central region of this galaxy. In this paper, we utilize intensity ratios between 12CO (J = 3–2), 12CO (J = 1–0), HCN (J = 4–3), and SiO (J = 2–1) to investigate the physical properties of each coherent structure, comparing the distributions with (proto-)star clusters, known as superbubble candidates, and stars. We found that the excitation state of the gas tends to increase toward the galactic center, and we identified high-excitation states in the gas with noncircular motions. From analyses of HCN (J = 4–3) and SiO (J = 2–1), we found that the dense, star-forming clumps that drive the starburst reside either at the points connecting any two streams or in the central region where two oppositely directed, accretion flows terminate. Based on analyses of the intensity ratios and the proposed geometric structure, we suggest that some superbubbles may instead be the overlap of the coherent structures rather than an expanding shell. From all these results, we propose a scenario in which the starburst was triggered by cloud–cloud collisions caused by interactions of streams and/or by magnetic flotation and/or efficient accretion along a stellar bar. [ABSTRACT FROM AUTHOR]

The super star cluster NGC1569-B has recently been observed to have an extremely high [Ba/Fe]. We consider that the observed high [Ba/Fe] (⁠|$\sim 1.3$|⁠) is due to the chemical enrichment of giant molecular clouds by either collapsars, neutron star mergers, or magnetorotational supernovae, and thereby investigate which of the three polluters can best reproduce the observed [Ba/Fe]. Since it is found that collapsars can best reproduce such an extremely high Ba abundance, we numerically investigate the star cluster formation in NGC1569 using chemodynamical simulations of merging dwarf galaxies with chemical enrichment by collapsars. The principal results are as follows: First, a cluster of the same scale as NGC1569-B was found to match both the observed [Ba/Fe] and [Fe/H] values, the best cluster having [Ba/Fe] |$=1.3\pm 0.2$| and [Fe/H] |$=-0.7\pm 0.2$|⁠. This simulation used a core-collapse supernova per collapsar rate of 70, a standard initial mass function and an initial metallicity of [Fe/H] = −1.5. Secondly, a prediction of the Eu abundance of NGC1569-B is made: [Eu/Fe] |$=1.9\pm 0.2$|⁠. These results are shown to be invariant under a change in the orbit parameters used for the merger. The need for a merger to promote the star formation that leads to the synthesis of the Ba and the star cluster formation is confirmed. Collapsars can not only better explain [Ba/Fe] but also be consistent with the observed star formation rate and stellar mass of the dwarf galaxy. [ABSTRACT FROM AUTHOR]

We report results on the internal dynamical evolution of old star clusters located in the outer regions of the Small Magellanic Cloud (SMC). Because the SMC has been imprinted with evidence of tidal interaction with the Large Magellanic Cloud (LMC), we investigated at what extend such an interaction has produced extra-tidal structures or excess of stars beyond the clusters' tidal radii. For that purpose, we used the Survey of the Magellanic Stellar History DR2 data sets to build number density radial profiles of suitable star clusters, and derived their structural and internal dynamics parameters. The analysed stellar density profiles do not show any evidence of tidal effects caused by the LMC. On the contrary, the Jacobi volume of the selected SMC star clusters would seem underfilled, with a clear trend toward a smaller percentage of underfilled volume as their deprojected distance to the SMC centre increases. Moreover, the internal dynamical evolution of SMC star clusters would seem to be influenced by the SMC gravitational field, being star clusters located closer to the SMC centre in a more advanced evolutionary stage. We compared the internal dynamical evolution of SMC old star clusters with those of LMC and Milky Way globular clusters, and found that Milky Way globular clusters have dynamical evolutionary paths similar to LMC/SMC old star clusters located closer to their respective galaxy's centres. Finally, we speculate with the possibility that globular clusters belonging to Magellanic Clouds like-mass galaxies have lived a couple of times their median relaxation times. [ABSTRACT FROM AUTHOR]

As gravitational wave detections increase the number of observed compact binaries (consisting of neutron stars or blacks), we begin to probe the different conditions producing these binaries. Most studies of compact remnant formation focus either on stellar collapse from the evolution of field binary stars in gas-free environments or on the formation of stars in clusters where dynamical interactions capture the compact objects, forming binaries. But a third scenario exists. In this paper, we study the fate of massive stars formed, accrete gas, and evolve in the dense discs surrounding supermassive black holes. We calculate the explosions produced and compact objects formed by the collapse of these massive stars. Nucleosynthetic yields may provide an ideal, directly observable, diagnostic of the formation and fate of these stars in active galactic nuclei. We present a first study of the explosive yields from these stars, comparing these yields with the observed nucleosynthetic signatures in the discs around supermassive stars with quasars. We show that, even though these stars tend to form black holes, their rapid rotation leads to discs that can eject a considerable amount of iron during the collapse of the star. The nucleosynthetic yields from these stars can produce constraints on the number of systems formed in this manner, but further work is needed to exploit variations from the initial models presented in this paper. [ABSTRACT FROM AUTHOR]

This study focuses on the open cluster NGC 6834. Despite being well-known, its primary parameters show considerable variation. As an intermediate-age cluster (50–80 million years), it is expected to host a prominent population of B-type stars, including numerous Be stars. Fifteen B-type variable stars have already been identified in the vicinity of the cluster, with four confirmed as Be stars through spectroscopic observations. Our spectrophotometric analysis aims to better understand the general and specific properties of NGC 6834, with particular emphasis on its B-type stars. Photometric data were gathered from gri images obtained with the Gemini North telescope, which was completed with several photometric catalogues. Additionally, 109 optical spectra for 89 stars in the cluster's region were obtained using the GMOS-MOS spectrograph. From the photometric data, we derive the foreground cluster colour excess, E (B − V) = 0.62. Through photometric diagrams, including traditional colour–colour and colour–magnitude diagrams and those incorporating the reddening-free Q parameter, we identify new stars with |$\mathrm{H}\alpha$| emission. Spectral classification of 89 stars is achieved, along with estimations of basic physical parameters, and projected rotational velocities for 64 B-type stars. We identify five new Be stars: NGC 6834 3, 9, and 118 as cluster members, and NGC 6834 23 and 121 as non-members. The research contributes to a deeper understanding of the structure of NGC 6834 and its B-type star population. [ABSTRACT FROM AUTHOR]

The mismatch between the mass function of the Milky Way's embedded clusters (ECs) and that of open clusters (OCs) raises the question of whether each OC originates from a single EC. In this work, we explore a scenario in which OCs form as a result of post-gas expulsion coalescence of ECs within the same parental molecular cloud. We model this process using N -body simulations of ECs undergoing expansion due to gas expulsion. Our initial conditions are based on the observed spatial, kinematic, and mass distributions of ECs in three representative massive star-forming regions. Initially, ECs are isolated. After further expansion, interactions between ECs begin, mutually influencing their evolution. We examine this process as a function of gas expulsion time-scales, spatial separations between ECs, and their relative velocities. Our results demonstrate that, within a reasonable range of these parameters, the coalescence of ECs is robust and largely insensitive to initial conditions. The mass of ECs plays a critical role in the coalescence process. More massive ECs form stable gravitational cores, which greatly facilitate coalescence and help the resulting cluster resist expansion and Galactic tidal forces. Additionally, the number of ECs also enhances coalescence. The current mass distribution of clumps in the Milky Way suggests that directly forming massive ECs is challenging. However, the coalescence of multiple low-mass ECs can account for the observed parameter space of OCs in the Milky Way. [ABSTRACT FROM AUTHOR]

In their early, formative stages star clusters can undergo rapid dynamical evolution leading to strong gravitational interactions and ejection of "runaway" stars at high velocities. While O/B runaway stars have been well studied, lower-mass runaways are so far very poorly characterized, even though they are expected to be much more common. We carried out spectroscopic observations with MAG2-MIKE to follow-up 27 high priority candidate runaways consistent with having been ejected from the Orion Nebula Cluster (ONC) |$\gt 2.5$| Myr ago, based on Gaia astrometry. We derive spectroscopic youth indicators (Li and H |$\alpha$|⁠) and radial velocities, enabling detection of bona fide runaway stars via signatures of youth and 3D traceback. We successfully confirmed 11 of the candidates as low-mass Young Stellar Objects (YSOs) on the basis of our spectroscopic criteria and derived radial velocities (RVs) with which we performed 3D traceback analysis. Three of these confirmed YSOs have kinematic ejection ages |$\gt 4\:$| Myr, with the oldest being 4.7 Myr. Assuming that these stars indeed formed in the ONC and were then ejected, this yields an estimate for the overall formation time of the ONC to be at least |$\sim 5\:$| Myr, i.e. about 10 free-fall times, and with a mean star formation efficiency per free-fall time of |$\bar{\epsilon }_{\rm ff}\lesssim 0.05$|⁠. These results favour a scenario of slow, quasi-equilibrium star cluster formation, regulated by magnetic fields and/or protostellar outflow feedback. [ABSTRACT FROM AUTHOR]

Observed low-mass galaxies with nuclear star clusters (NSCs) can host accreting massive black holes (MBH). We present simulations of dwarf galaxies (⁠|$M_{\mathrm{baryon}} \sim 0.6\!-\!2.4 \times 10^8 \rm \, M_\odot$|⁠) at solar mass resolution (⁠|$0.5\rm \, M_\odot \lt \mathit{ m}_{\mathrm{gas}} \lt 4 \rm \, M_\odot$|⁠) with a multiphase interstellar medium (ISM) and investigate the impact of NSCs on MBH growth and nuclear star formation (SF). The griffin simulation model includes non-equilibrium low temperature cooling, chemistry and the effect of H  ii regions and supernovae (SNe) from massive stars. Individual stars are sampled down to 0.08  |$\rm M_\odot$| and their non-softened gravitational interactions with MBHs are computed with the regularized ketju integrator. MBHs with masses in the range of |$10^2 \!-\! 10^5 \, \rm M_\odot$| are represented by accreting sink particles without feedback. We find that the presence of NSCs boost nuclear SF (i.e. NSC growth) and MBH accretion by funneling gas to the central few parsecs. Low-mass MBHs grow more rapidly on |$\sim 600$| Myr time-scales, exceeding their Eddington rates at peak accretion. MBH accretion and nuclear SF is episodic (i.e. leads to multiple stellar generations), coeval and regulated by SN explosions. On 40–60 Myr time-scales the first SN of each episode terminates MBH accretion and nuclear SF. Without NSCs, low-mass MBHs do not grow and MBH accretion and reduced nuclear SF become irregular and uncorrelated. This study gives the first insights into the possible co-evolution of MBHs and NSCs in low-mass galaxies and highlights the importance of considering dense NSCs in galactic studies of MBH growth. [ABSTRACT FROM AUTHOR]

Radiation pressure from Lyman- |$\alpha$| (Ly |$\alpha$|⁠) scattering is a potentially dominant form of early stellar feedback, capable of injecting up to |$\sim 100 \, \times$| more momentum into the interstellar medium (ISM) than ultraviolet continuum radiation pressure and stellar winds. Ly |$\alpha$| feedback is particularly strong in dust-poor environments and is thus especially important during the formation of the first stars and galaxies. As upcoming galaxy formation simulations incorporate Ly |$\alpha$| feedback, it is crucial to consider processes that can limit it to avoid placing Lambda-cold dark matter in apparent tension with recent JWST observations indicating efficient star formation at Cosmic Dawn. We study Ly |$\alpha$| feedback using a novel analytical Ly |$\alpha$| radiative transfer solution that includes the effects of continuum absorption, gas velocity gradients, Ly |$\alpha$| destruction (e.g. by |$2p \rightarrow 2s$| transitions), ISM turbulence, and atomic recoil. We verify our solution for uniform clouds using extensive Monte Carlo radiative transfer (MCRT) tests, and resolve a previous discrepancy between analytical and MCRT predictions. We then study the sensitivity of Ly |$\alpha$| feedback to the aforementioned effects. While these can dampen Ly |$\alpha$| feedback by a factor |$\lesssim \textrm {few} \times 10$|⁠ , we find it remains |$\gtrsim 5 - 100 \, \times$| stronger than direct radiation pressure and therefore cannot be neglected. We provide an accurate fit for the Ly |$\alpha$| force multiplier |$M_{\rm F}$|⁠ , suitable for implementation in subgrid models for galaxy formation simulations. Our findings highlight the critical role of Ly |$\alpha$| feedback in regulating star formation at Cosmic Dawn, and underscore the necessity of incorporating it into simulations to accurately model early galaxy evolution. [ABSTRACT FROM AUTHOR]

The formation details of globular clusters (GCs) are still poorly understood due to their old ages and the lack of detailed observations of their formation. A large variety of models for the formation and evolution of GCs have been created to improve our understanding of their origins, based on GC properties observed at |$z=0$|⁠. We present the first side-by-side comparison of six current GC formation models with respect to their predictions for the GC ages and formation redshifts in Milky Way (MW)-like galaxies. We find that all the models are capable of forming most of the surviving GCs at more than |$10 \,\mathrm{G}{\rm {yr}}$| ago, in general agreement with the observation that most GCs are old. However, the measured MW GC ages are still systematically older than those predicted in the galaxies of four of the models. Investigating the variation of modelled GC age distributions for general MW-mass galaxies, we find that some of the models predict that a significant fraction of MW-mass galaxies would entirely lack a GC population older than |$10 \,\mathrm{G}{\rm {yr}}$|⁠ , whereas others predict that all MW-mass galaxies have a significant fraction of old GCs. This will have to be further tested in upcoming surveys, as systems without old GCs in that mass range are currently not known. Finally, we show that the models predict different formation redshifts for the oldest surviving GCs, highlighting that models currently disagree about whether the recently observed young star clusters at high redshifts could be the progenitors of today's GCs. [ABSTRACT FROM AUTHOR]

• False star filtering based on unsupervised learning. • Unsupervised learning by density-based clustering with morphological features. • Camera motion estimation using maximum likelihood estimator. • High accuracy and robustness to noise. • Preprocessing to increase accuracy in star identification by false star filtering. • Preprocessing to reduce complexity in star identification by motion estimation. Star sensors serve as sophisticated instruments for determining spacecraft attitude, offering high accuracy to meet complicated scientific demands. However, their accuracy can be compromised by various sources of noise in the captured image, such as false stars arising from reflective objects or solar flares. Filtering out these false stars is essential for enhancing accuracy and reducing computational complexity. This study introduces an algorithm designed to identify and filter false stars while also estimating camera motion parameters, thus improving attitude determination performance. The algorithm operates by detecting isomorphic feature vectors via density-based clustering that is employed to discern false stars. Moreover, the variance in slope angles of true star pairs facilitates the derivation of an affine transformation matrix through a maximum likelihood estimator. It is a standalone algorithm that can be integrated into any star identification method to increase robustness to false stars while providing motion parameters to be used in recursive star identification algorithms to reduce complexity. The algorithm's effectiveness is evaluated through experiments on 1000 pairs of time-sequential simulated star images, in which the sensor parameters are taken from the SharjahSat-1 project, while also taking position and brightness noise effects into account. [ABSTRACT FROM AUTHOR]

Ehlers and Kundt [J. Ehlers and W. Kundt, Exact solutions of the gravitational field equations, in Gravitation: An Introduction to Current Research, ed. L. Witten (Wiley, New York, 1962)] argued in favor of the velocity effect: particles initially at rest hit by a burst of gravitational waves should fly apart with constant velocity after the wave has passed. Zel’dovich and Polnarev [Y. B. Zel’dovich and A. G. Polnarev, Radiation of gravitational waves by a cluster of superdense stars, Astron. Zh. 51 (1974) 30, [Sov. Astron. 18 (1974) 17]] suggested instead that waves generated by flyby would merely displace the particles, as illustrated here by further examples. [ABSTRACT FROM AUTHOR]

Cosmic rays (CRs) are accelerated in diverse astrophysical objects like supernova remnants, massive star clusters, or pulsars. Fermi acceleration mechanisms built a power-law distribution controlled by the ratio of the acceleration to escape timescales in the acceleration site. Hence, escape is an essential mechanism to establish the particle distribution at cosmic-ray sources and to control the flux of cosmic rays injected into the galaxy. Different models have tried to account for the escape process. However, all show some limitations due to the complexity of the particle release mechanism, usually involving 3D geometry, with specific magnetic turbulence properties linked to the process itself. The escape process is also time dependent and results from the interplay of particle acceleration and injection efficiency in the astrophysical source. Once injected into the interstellar medium, freshly released particles are channelled by the ambient magnetic field, which is itself turbulent. In a simplified view, we mainly focus on the propagation of CRs along 1D magnetic flux tubes before turbulent motions start to mix them over a turbulent coherence length, and then we further question this assumption. Close to their sources, one can also expect cosmic rays to harbour higher pressure with respect to their mean value in the interstellar medium. This intermittency in the CR distribution is prone to trigger several types of kinetic and macro instabilities, among which the resonant streaming instability has been the most investigated. In this article, we review recent observational and theoretical studies treating cosmic-ray escape and propagation in the vicinity of their source. We will consider three main astrophysical contexts: association with massive star clusters, gamma-ray halos around pulsars, and, more specifically, supernova remnants. In particular, we discuss in some detail the cosmic-ray cloud (CRC) model, which has been widely used to investigate CR propagation in the environment of supernova remnants. The review also discusses recent studies on CR-induced feedback over the interstellar medium surrounding the sources associated with the release process, as well as alternative types of driven instabilities. [ABSTRACT FROM AUTHOR]

Asteroseismic masses of globular cluster (GC) stars are invaluable to investigate stellar evolution. Previously, only two GCs have been seismically studied. We present new detections of solar-like oscillations in the clusters M9 and M19, focusing on two key areas: stellar mass-loss and GC multiple populations. Using K2 photometry, we detect solar-like oscillations in stars on the red giant branch and early asymptotic giant branch. We measure an integrated mass-loss for M9 of |$0.16\pm 0.02$| (rand) |$^{+0.03}_{-0.03}$| (sys) |$~\mathrm{M}_{\odot }$| and M19 of |$0.33\pm 0.03$| (rand) |$^{+0.09}_{-0.07}$| (sys) |$~\mathrm{M}_{\odot }$|⁠. Comparing these to the mass-loss estimates from previous seismically studied clusters, we derive a preliminary relationship between stellar mass-loss and metallicity for Type I GCs. We find that the mass-loss for M19 – a Type II GC – is significantly larger, suggesting Type II clusters follow a different mass-loss-metallicity trend. We also examine the mass distributions in each evolutionary phase for evidence of a bimodality that could indicate mass differences between sub-populations. While no clear bimodality is observed, there is tentative evidence suggesting the presence of two mass populations. Classification through spectroscopic abundances into the sub-populations is needed to verify these findings. This study reinforces that asteroseismology of GC stars provides an excellent testbed for studying stellar evolution. However, to advance the field we need high-quality photometry of more GCs, a goal that could be realised with the upcoming Roman Telescope. [ABSTRACT FROM AUTHOR]

Some ultra diffuse galaxies (UDGs) reveal many more globular clusters (GCs) than classical dwarf galaxies of the same stellar mass. These UDGs, with a mass in their GC system (⁠|$M_{\rm GC}$|⁠) approaching 10 per cent of their host galaxy stellar mass (⁠|$M_{\ast }$|⁠), are also inferred to have high halo mass to stellar mass ratios (⁠|$M_{\rm halo}/M_{\ast }$|⁠). They have been dubbed Failed Galaxies. It is unknown what role high GC formation efficiencies and/or low destruction rates play in determining the high |$M_{\rm GC}/M_{\ast }$| ratios of some UDGs. Here we present a simple model, which is informed by recent JWST observations of lensed galaxies and by a simulation in the literature of GC mass loss and tidal disruption in dwarf galaxies. With this simple model, we aim to constrain the effects of GC efficiency/destruction on the observed GC richness of UDGs and their variation with the integrated stellar populations of UDGs. We assume no ongoing star formation (i.e. quenching at early times) and that the disrupted GCs contribute their stars to those of the host galaxy. We find that UDGs, with high |$M_{\rm GC}/M_{\ast }$| ratios today, are most likely the result of very high GC formation efficiencies combined with modest rates of GC destruction. The current data loosely follow the model that ranges from the mean stellar population of classical dwarfs to that of metal-poor GCs as |$M_{\rm GC}/M_{\ast }$| increases. As more data becomes available for UDGs, our simple model can be refined and tested further. [ABSTRACT FROM AUTHOR]

High-resolution imaging and strong gravitational lensing of high-redshift galaxies have enabled the detection of compact sources with properties similar to nearby massive star clusters. Often found to be very young, these sources may be globular clusters detected in their earliest stages. In this work, we compare predictions of high-redshift (⁠|$z \sim 1$| –10) star cluster properties from the E-MOSAICS simulation of galaxy and star cluster formation with those of the star cluster candidates in strongly lensed galaxies from JWST and Hubble Space Telescope (HST) imaging. We select galaxies in the simulation that match the luminosities of the majority of lensed galaxies with star cluster candidates observed with JWST. We find that the luminosities, ages, and masses of the brightest star cluster candidates in the high-redshift galaxies are consistent with the E-MOSAICS model. In particular, the brightest cluster ages are in excellent agreement. The results suggest that star clusters in both low- and high-redshift galaxies may form via common mechanisms. However, the brightest clusters in the lensed galaxies tend to be |$\approx 1$| – |$1.5 \, \rm {mag}$| brighter and |$\approx 0.5$|  dex more massive than the median E-MOSAICS predictions. We discuss the large number of effects that could explain the discrepancy, including simulation and observational limitations, stellar population models, cluster detection biases, and nuclear star clusters. Understanding these limitations would enable stronger tests of globular cluster formation models. [ABSTRACT FROM AUTHOR]

Supermagnified stars are gravitationally lensed individual stars that are located close to a caustic of a lensing galaxy cluster, and have their flux magnified by a large enough factor (typically |${\sim} 1000$|⁠) to make them detectable with present telescopes. The maximum magnification is limited by microlensing caused by intracluster stars or other compact objects, which create a network of corrugated critical lines with an angular width proportional to the surface density of microlenses. We consider a set of nine cases of supermagnified stars reported in the literature, and derive an upper limit on the surface density of compact objects, such as primordial black holes, that might be present as a fraction of the dark matter in addition to known intracluster stars. Any such additional compact objects would widen the corrugated critical line network and therefore the width of the distribution of supermagnified stars around the modelled critical lines of the lens. We find that any compact objects, including primordial black holes, with masses above |${\sim} 10^{-6} \, {\rm M}_\odot$| (below which the microcaustics are closer together than the typical angular size of supermagnified stars) cannot account for more than |${\sim} 2$|  per cent of the dark matter. [ABSTRACT FROM AUTHOR]

Comparing the dynamical and stellar masses of Milky Way (MW) globular clusters (GCs) reveals a discrepancy exceeding a factor of 2. Since this substantial invisible mass is concentrated in the cluster centre, it is attributed to stellar remnants. The majority of mass in remnants consists of white dwarfs (WDs). Allocating over half of a GC's current mass to WDs could significantly restrict the dynamical evolution scenarios governing stellar clusters. As the most massive stars in GCs, black holes (BHs) exert a substantial effect on the escape rate of lower mass stars, such as WDs. This paper aims to identify which scenarios of BH natal kicks can accurately reproduce the notable dark remnant fraction observed in MW GCs. We compare the observed remnant fraction of MW GCs with a comprehensive grid of direct N -body simulations while adjusting the natal kick received by BHs. Our results reveal that simulations employing low natal kicks to BHs are the only ones capable of mirroring the remnant fraction of MW GCs. According to the Spitzer instability, the presence of a BH population prompts the formation of a BH sub-system (BHSub) at the centre of a star cluster. The BHSub serves as an energetic power plant, continually releasing kinetic energy through few-body encounters between single and binary BHs, and transferring the generated energy to the entire stellar population. This energy induces a significant difference in the ejection rate of stellar remnants and luminous stars, ultimately increasing the fraction of dark remnants within the star cluster. [ABSTRACT FROM AUTHOR]

Understanding the formation of multiple populations in globular clusters (GCs) represents a challenge for stellar population studies. Nevertheless, the outermost cluster regions, likely to hold clues about the initial configuration of GC stars, remain underexplored. We use synthetic spectra reflecting the chemical compositions of first- and second-population (1P, 2P) stars in 47 Tucanae to identify spectral regions sensitive to these populations. This led us to define new photometric bands that effectively distinguish 1P and 2P giant stars using Gaia XP spectra. Testing these filters, we constructed the pseudo two-colour diagrams dubbed chromosome maps (ChMs) and, for the first time, identified 1P and 2P stars in the cluster's outermost regions and beyond its tidal radius. We constructed similar diagrams for NGC 3201, NGC 6121, NGC 6752, and NGC 6397, thus exploring GCs with different metallicities. The ChMs effectively distinguished multiple populations in the outer regions of all clusters, except for the metal-poor NGC 6397. Our findings, together with literature results from more-internal regions, show that the 2P stars of 47 Tucanae are more-centrally concentrated than the 1P. A similar pattern is seen for 2P stars with extreme chemical composition of NGC 3201. The multiple populations of NGC 6121, and NGC 6752 share the same radial distributions. These radial behaviours are consistent with the GC formation scenarios where 2P stars originate in the central regions. Noticeably, results on NGC 3201 are in tension with the conclusion from recent work that its 1P is more centrally concentrated than the 2P and might form with more central concentration. [ABSTRACT FROM AUTHOR]

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

Context. Nuclear star cluster (NSC) mergers, involving the fusion of dense stellar clusters near the centres of galaxies, play a pivotal role in shaping galactic structures. The distribution of stellar orbits has significant effects on the formation and characteristics of extreme mass ratio inspirals (EMRIs). Aims. In this study, we address the orbital distribution of stars in merging NSCs and the subsequent effects on supermassive black hole binary (SMBHB) evolution. Methods. We ran dedicated direct-summation N-body simulations with different initial conditions to do a detailed study of the resulting NSC after their progenitors had merged. Results. Our findings reveal that prograde stars form a flattened structure, while retrograde stars have a more spherical distribution. The axial ratios of the prograde component vary based on the presence and mass ratio of the SMBHs. The fraction of prograde and retrograde stars depends on the merger orbital properties and the SMBH mass ratio. The interactions of retrograde stars with the SMBHB affect the eccentricity and separation evolution of the binary. Our analysis reveals a strong correlation between the angular momentum and eccentricity of the SMBH binary. This relationship could serve as a means to infer information about the stellar dynamics surrounding the binary. We find that prograde orbits are particularly close to the binary of SMBHs, a promising fact regarding EMRI production. Moreover, prograde and retrograde stars have different kinematic structures, with the prograde stars typically rotating faster than the retrograde ones. The line-of-sight velocity and velocity dispersion, as well as the velocity anisotropy of each NSC, depend on the initial merger orbital properties and SMBH mass ratios. The prograde and retrograde stars always show different behaviours. Conclusions. The distribution of stellar orbits and the dynamical properties of each kinematic population can potentially be used as a way to tell the properties of the parent nuclei apart, and has an important impact on expected rates of EMRIs, which will be detected by future gravitational wave observatories such as the Laser Interferometer Space Antenna (LISA). [ABSTRACT FROM AUTHOR]

Most stars are born in the crowded environments of gradually forming star clusters. Dynamical interactions between close-passing stars and the evolving ultraviolet radiation fields from proximate massive stars are expected to sculpt the protoplanetary discs (PPDs) in these clusters, potentially contributing to the diversity of planetary systems that we observe. Here, we investigate the impact of cluster environment on disc demographics by implementing simple PPD evolution models within N -body simulations of gradual star cluster formation, containing 50 per cent primordial binaries. We consider a range of star formation efficiency per free-fall time, |$\epsilon _{\rm ff}$|⁠ , and mass surface density of the natal cloud environment, |$\Sigma _{\rm cloud}$|⁠ , both of which affect the overall duration of cluster formation. We track the interaction history of all stars to estimate the dynamical truncation of the discs around stars involved in close encounters. We also track external photoevaporation of the discs due to the ionizing radiation field of the nearby high- and intermediate-mass (⁠|$\gt 5\,{\rm M}_\odot$|⁠) stars. We find that |$\epsilon _{\rm ff}$|⁠ , |$\Sigma _{\rm cloud}$|⁠ , and the presence of primordial binaries have major influences on the masses and radii of the disc population. In particular, external photoevaporation has a greater impact than dynamical interactions in determining the fate of discs in our clusters. [ABSTRACT FROM AUTHOR]

The stellar mass distribution in star-forming regions, stellar clusters and associations, the initial mass function (IMF), appears to be invariant across different star-forming environments, and is consistent with the IMF observed in the Galactic field. Deviations from the field, or standard, IMF, if genuine, would be considered strong evidence for a different set of physics at play during the formation of stars in the birth region in question. We analyse N -body simulations of the evolution of spatially and kinematically substructured star-forming regions to identify the formation of binary star clusters, where two (sub)clusters which form from the same Giant Molecular Cloud orbit a common centre of mass. We then compare the mass distributions of stars in each of the subclusters and compare them to the standard IMF, which we use to draw the stellar masses in the star-forming region from which the binary cluster(s) form. In each binary cluster that forms, the mass distributions of stars in one subcluster deviates from the standard IMF, and drastically so when we apply similar mass resolution limits as for the observed binary clusters. Therefore, if a binary subcluster is observed to have an unusual IMF, this may simply be the result of dynamical evolution, rather than different physical conditions for star formation in these systems. [ABSTRACT FROM AUTHOR]

The chemically peculiar (CP) stars of the upper main sequence are excellent astrophysical laboratories to test the diffusion, mass-loss, rotational mixing, and pulsation in the (non-)presence of a stable local magnetic field. These processes are time-dependent. The age estimation of Galactic field stars suffers from several limitations. Therefore, studying members of star clusters overcomes these difficulties. We matched the most recently published catalogues of star clusters and CP stars. For the matching, we used the newest Gaia Data Release. We also used the |$\Delta$| a photometry tool to further distinguish between the CP subgroups. We found 595 CP stars in 408 star clusters of all ages. Furthermore, we report on misclassified metallic line stars (Am or CP1) and objects with no CP classification. The distribution of magnetic and non-magnetic CP stars on the main sequence seems different. We do not detect very young and very old CP stars showing rotationally induced variability. CP members of star clusters help to study all relevant processes responsible for this phenomenon in more detail. Still, a larger sample is desired to put tighter constraints on models. [ABSTRACT FROM AUTHOR]

In order to explore the differences in testicular development in ducks with different semen volumes, in this study, adult Gaoyou ducks were taken as the research object. These ducks were divided into two groups：high semen volume and low semen volume groups. The weight and testicular weight of the two groups of ducks were measured, respectively. ELISA method was used to detect testosterone levels in the blood of the two groups. Microscopic observation and measurement of the area of the seminiferous tubules and the number of interstitial cells per unit area were performed after paraffin sectioning on the two groups of duck testicular tissue. The differences between the two groups were compared and correlation analysis was conducted. Finally, fluorescence quantitative PCR was used to detect the expression levels of FSHR, LHR, AMH, and StAR in the two groups of duck testicular tissues. The results showed that the testicular weight, testicular volume, and testosterone levels in the high semen volume group were significantly higher than those in the low semen volume group. There was no significant difference in body weight, testicular index, seminiferous tubule area, and number of interstitial cells per unit field between the two groups. Correlation analysis showed that semen volume was significantly positively correlated with testicular volume. While testosterone content was significantly positively correlated with testicular weight and volume. The expression levels of testicular development related genes FSHR, LHR, AMH, and StAR. were significantly higher in the high semen volume group than those in the low semen volume group. These results indicated that the semen volume in male ducks was related to testicular weight and testosterone content. And testicular development related genes FSHR, LHR, AMH and StAR may play important roles in duck testicular development and spermatogenesis. [ABSTRACT FROM AUTHOR]

We present 22 GHz imaging of regions of massive star formation within the Local Wolf–Rayet Galaxy Sample, a NSF Karl G. Jansky Very Large Array survey of 30 local galaxies showing spectral features of Wolf–Rayet (WR) stars. These spectral features are present in galaxies with young super star clusters (SSCs), and are an indicator of large concentrations of massive stars. We present a catalog of 92 individually identified regions of likely free–free emission associated with potential young SSCs located in these WR galaxies. The free–free fluxes from these maps allow extinction-free estimates of the Lyman continuum rates, masses, and luminosities of the emission regions. Thirty-nine of these regions meet the minimum Lyman continuum rate to contain at least once SSC, and 29 of these regions could contain individual SSCs massive enough to test specific theories on star formation and feedback inhibition in SSCs, requiring follow-up observations at higher spatial resolution. The resulting catalog provides sources for future molecular line and IR studies into the properties of SSC formation. [ABSTRACT FROM AUTHOR]

Stellar clusters are critical constituents within galaxies: They are the result of highest-density star formation, and through their spatially and temporally correlated feedback, they regulate their host galaxy evolution. We present a novel numerical method to model star clusters as individual units of star formation using sink particles. In our method, star clusters grow via gas accretion and via merging with less massive clusters. We describe the implementation in the radiation hydrodynamics code GIZMO and run a large grid of marginally bound, turbulent clouds of 107 M ⊙ to explore the effect of modeling ingredients on the evolution of the clouds and the star clusters. We find both gas accretion and mergers to be critical processes to form star clusters of masses up to ∼105–106 M ⊙, while ionizing radiation is the main feedback mechanism regulating the growth of star clusters. The majority of our star clusters assemble their mass in 0.3–2.6 Myr, and the most massive ones take ∼10 Myr. By removing high-density gas by accretion, our sink-based cluster formation prescription allows the newly formed star clusters to inject their stellar feedback in less dense environments. This makes feedback more efficient at ionizing and disrupting the cloud than if we were to use a standard star formation approach, indicating that our numerical method is the missing critical step to model the interplay between star clusters and their host galaxies. [ABSTRACT FROM AUTHOR]

The bimodal metallicity distribution of globular clusters (GCs) in massive galaxies implies two distinct subpopulations: metal-poor and metal-rich. Using the recent data of Gaia we highlighted three distinct dissimilarities between metal-poor and metal-rich GCs in the Milky Way (MW). Half-mass (light) radii of metal-poor GCs exhibit, on average, |$\simeq 52 \pm$| 5 (⁠|$60 \pm$| 3) per cent more expansion than metal-rich ones. Furthermore, the lack of metal-poor GCs at low Galactocentric distances (⁠|$R_\mathrm{G}$|⁠) follows a triangular pattern in |$R_\mathrm{G}$| –[Fe/H] space, indicating that GCs with lower metallicities appear further away from the Galactic centre. Metal-poor GCs are more susceptible to destruction by the tidal field in the inner part of the MW. We perform a series of N -body simulations of star clusters, to study the impact of the BHs' natal kicks on the long-term evolution of low- and high-metallicity GCs to explain these observational aspects. We found that the retention of BHs inside the cluster is crucial to reproducing the observed dissimilarities. The heavier and less expanded BH subsystem (BHSub) in metal-poor clusters leads to more intense few-body encounters, injecting more kinetic energy into the stellar population. Consequently, they experience larger expansion and higher evaporation rates rather than metal-rich clusters. The higher energy production within the BHSub of metal-poor GCs causes them to dissolve before a Hubble time near the Galactic centre, leading to a triangular pattern in |$R_\mathrm{G}$| –[Fe/H] space. [ABSTRACT FROM AUTHOR]

The γ Cas type stars are a group of Be stars with unusually hard X-ray emission and an X-ray luminosity of LX ~ 1031–1033 erg/s, which is higher than for typical Be stars but less than for massive X-ray binaries with Be components. The temperature of the X-ray emitting plasma reaches values of 10–20 keV or even more, assuming that they emit thermal X-rays. To test the hypotheses on the X-rays formation from this group of stars, the variability of the X-ray and optical emission of γ Cas type stars is analyzed. Regular components of X-ray brightness variations and H, He and FeII line profile variations in spectra of such stars are revealed. The periods of optical and X-ray variability are close and correspond to typical periods of non-radial pulsations (NRPs) of Be stars. That suggests modulation of the wind structure of a Be star as a result of NRPs. [ABSTRACT FROM AUTHOR]

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

The article explores the lesser-known short story "In the Avu Observatory" by H. G. Wells, which contains meaningful allusions to Shakespeare's Hamlet. The protagonist, Woodhouse, encounters a mysterious and violent creature in a remote observatory in Borneo, leading to a brutal and transformative struggle reminiscent of Hamlet's own trials. Through these allusions, Wells adds depth and resonance to the story, highlighting themes of existential change and enlightenment in the face of the unknown. The narrative culminates in a climactic encounter that leaves Woodhouse forever changed, echoing Hamlet's own journey of self-discovery and acceptance of life's mysteries. [Extracted from the article]

High-velocity stars and peculiar G objects orbit the central supermassive black hole (SMBH) Sagittarius A* (Sgr A*). Together, the G objects and high-velocity stars constitute the S cluster. In contrast with theoretical predictions, no binary system near Sgr A* has been identified. Here, we report the detection of a spectroscopic binary system in the S cluster with the masses of the components of 2.80 ± 0.50 M⊙ and 0.73 ± 0.14 M⊙, assuming an edge-on configuration. Based on periodic changes in the radial velocity, we find an orbital period of 372±3 days for the two components. The binary system is stable against the disruption by Sgr A* due to the semi-major axis of the secondary being 1.59±0.01 AU, which is well below its tidal disruption radius of approximately 42.4 AU. The system, known as D9, shows similarities to the G objects. We estimate an age for D9 of 2. 7 − 0.3 + 1.9 × 1 0 6 yr that is comparable to the timescale of the SMBH-induced von Zeipel-Lidov-Kozai cycle period of about 106 yr, causing the system to merge in the near future. Consequently, the population of G objects may consist of pre-merger binaries and post-merger products. The detection of D9 implies that binary systems in the S cluster have the potential to reside in the vicinity of the supermassive black hole Sgr A* for approximately 106 years. S-cluster is a group of high-velocity stars near the supermassive black hole in the center of our Galaxy, and binary stars have been predicted in the S-cluster, but not detected. Here, the authors show detection of a spectroscopic binary in the S-cluster. [ABSTRACT FROM AUTHOR]

We present age estimates for over 8100 subgiant branch (SGB) stars in Omega Centauri (ω Cen) to study its star formation history. Our large data set, which combines multi-wavelength Hubble Space Telescope photometry with MUSE metallicities, provides an unprecedented opportunity to measure individual stellar ages. We do this by fitting each star's photometry and metallicity with theoretical isochrones that are embedded with an empirical [C + N + O]–[Fe/H] relation specific to ω Cen. The bulk of the stars have ages between 13 and 10 Gyr, with the mean stellar age being 12.08 ± 0.01 Gyr and the median age uncertainty being 0.68 Gyr. From these ages we construct the most complete age–metallicity relation for ω Cen to date. We find that the mean age of stars decreases with increasing metallicity and find two distinct streams in the age–metallicity plane, hinting at different star formation pathways. We derive an intrinsic spread in the ages of 0.75 ± 0.01 Gyr for the whole cluster, with the age spread showing a clear increase with metallicity. We verify the robustness of our age estimations by varying isochrone parameters and constraining our systematics. We find the C + N + O relation to be the most critical consideration for constraining the age–metallicity relation. We also present an SGB chromosome map with age information. In the future these stellar ages could be combined with chemical abundances to study age differences in subpopulations and uncover the chemical evolution history of this massive nuclear star cluster. [ABSTRACT FROM AUTHOR]

This paper investigates the evolutionary phases of a collapsing dissipative star cluster in the presence of dark matter. For this purpose, the minimal coupling model of f (R , T) gravity is used as a candidate for exotic matter in star cluster. The collapse equation is derived from generalized dynamical equations which show the total conservation of energy within the cluster. In this context, the approach of Misner and Sharp is applied to study the phenomenon and differentiate between homologous as well as non-homologous gravitational collapse. The generalized dynamical equations are also connected to modified heat transport equations related to heat flux, viscosity and shear, which deals with Israel–Stewart theory (without removing the thermodynamics heat/viscous coupling coefficients). It is found that shear-free and non-dissipative fluid give homologous collapse. The dissipation of collapsing star cluster provides a relation between tidal forces and density inhomogeneity. It is theoretically predicted that the presence of exotic matter controlled the collapsing mechanism. [ABSTRACT FROM AUTHOR]

Stellar systems—globular and nuclear star clusters, elliptical and spiral galaxies and their surrounding dark matter haloes, and so on—are ubiquitous characters in the evolutionary tale of our Universe. This tutorial article is an introduction to the collective dynamical evolution of the very large numbers of stars and/or other self-gravitating objects that comprise such systems, i.e., their kinetic theory. We begin by introducing the basic phenomenology of stellar systems, and explaining why and when we must develop a kinetic theory that transcends the traditional two-body relaxation picture of Chandrasekhar. We then study the individual orbits that comprise stellar systems, how those orbits are modified by linear and nonlinear perturbations, how a system responds self-consistently to fluctuations in its own gravitational potential, and how one can predict the long-term evolutionary fate of a stellar system in both quasilinear and nonlinear regimes. Though our treatment is necessarily mathematical, we develop the formalism only to the extent that it facilitates real calculations. Each section is bolstered with intuitive illustrations, and we give many examples throughout the text of the equations being applied to topics of major astrophysical importance, such as radial migration, spiral instabilities, and dynamical friction on galactic bars. Furthermore, in the 1960s and 1970s, the kinetic theory of stellar systems was a fledgling subject which developed in tandem with the kinetic theory of plasmas. However, the two fields have long since diverged as their practitioners have focused on ever more specialized and technical issues. This tendency, coupled with the famous obscurity of astronomical jargon, means that today relatively few plasma physicists are aware that their knowledge is directly applicable in the beautiful arena of galaxy evolution, and relatively few galactic astronomers know of the plasma-theoretic foundations upon which a portion of their subject is built. Yet, once one has become fluent in both Plasmaish and Galacticese, and has a dictionary relating the two, one can pull ideas directly from one field to solve a problem in the other. Therefore, another aim of this tutorial article is to provide our plasma colleagues with a jargon-light understanding of the key properties of stellar systems, to offer them the theoretical minimum necessary to engage with the modern stellar dynamics literature, to point out the many direct analogies between stellar- and plasma-kinetic calculations, and ultimately to convince them that stellar dynamics and plasma kinetics are, in a deep, beautiful and useful sense, the same thing. [ABSTRACT FROM AUTHOR]

Context. With the increasing number of discoveries of globular clusters in the inner Milky Way, the need for spectroscopic confirmation and further investigation of their stellar populations and chemodynamical properties has become crucial. Aims. Gran 5 is a newly reported low-mass globular cluster located close to the Galactic center, and it is thought to be an accreted object associated with the Gaia-Enceladus structure. This study aims to investigate the stellar populations of Gran 5 and their detailed chemical properties. Methods. We performed high-resolution near-infrared spectroscopy on seven stars in the field of Gran 5 using IGRINS on the Gemini-South telescope. Results. We identified six stars as cluster members and reveal that they are divided into two stellar populations with different metallicities, with mean [Fe/H] values of −0.76 dex and −0.55 dex, respectively. In addition, the chemodynamical properties of Gran 5 agree with those of in situ globular clusters. Conclusions. Our findings represent the first detection of two stellar populations with different metallicities in a low-mass globular cluster. This suggests that the metallicity variation in Gran 5 may have arisen from processes different from those in other globular clusters with metallicity variation, or that it may have lost a substantial amount of its initial mass during its evolution. [ABSTRACT FROM AUTHOR]

This paper discusses the phenomenon of evolving spherically symmetric cluster of stars in the presence of an exotic matter. To discuss evolutionary mechanism, we use the Starobinsky model of f(R) gravity as exotic matter and the structure scalars as evolutional parameters. We study various evolution modes such as isotropic pressure, quasi-homologous evolution, density homogeneity, and geodesic nature. The stability of homogeneous density of baryonic and non-baryonic matter is discussed using dissipation, tidal forces, anisotropic pressures, expansion and shear-effects. It is observed that the dark matter has remarkable impact on the evolutionary changes. Also, it is shown that the dissipation factor produces density inhomogeneity in expanding clusters with shear effects. We observe that high curvature geometry enhances quasi-homologous evolution in the presence of shear. We use star Her X-1 as test star to discuss physical behavior of Starobinsky model. It is found that the density of dark matter overcomes the density of matter for large values of dark matter parameter n. [ABSTRACT FROM AUTHOR]

We report the detection of high-energy gamma-ray emission towards the G305 star-forming region. Using almost 15 yr of observation data from Fermi Large Area Telescope, we detected an extended gamma-ray source in this region with a significance of |$\sim 13 \sigma$|⁠. 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 of 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We suggest that "Godzilla" of the lensed Sunburst galaxy (z = 2.37) is a young super star cluster powering a nebula of gravitationally trapped stellar ejecta. Employing Hubble Space Telescope photometry and spectroscopy from the Very Large Telescope (VLT) MUSE and VLT/X-Shooter, we infer the physical and chemical properties of the cluster and nebula, finding that Godzilla is young, 4–6 Myr; massive, 2 × 106 M ⊙ (1000/ μ); of stellar metallicity, Z ≃ 0.25 Z ⊙; and has a compact far-UV component of ≲1 pc (1000/ μ), where μ is the flux magnification factor. The gas is significantly enriched with N and He, indicating stellar wind material, and has highly elevated O relative to the stellar metallicity, indicating entrainment of core-collapse supernova (CCSN) ejecta. The high density, n e ≃ 107−8 cm−3, implies a highly pressurized intracluster environment. We propose that the pressure results from CCSN-driven supersonic turbulence in warm, self-shielding gas, which has accumulated in the cluster center after runaway radiative cooling and is dense enough to resist removal by CCSNe. The nebula gas shows subsolar C/O, Ne/O, and Si/O, which may reflect the CCSN element yields for initial stellar masses >40 M ⊙. A comparison to element yield synthesis models for young star clusters shows that the gas abundances are consistent with complete retention and mixture of stellar winds and CCSN ejecta until the inferred cluster age. The inferred O and He enhancement may have implications for the formation of multiple stellar populations in globular clusters, as stars formed from this gas would contradict the observed abundances of second-population stars. [ABSTRACT FROM AUTHOR]

Copyright of Basrah Journal of Science / Magallat Al-Barat Li-L-ulum is the property of Republic of Iraq Ministry of Higher Education & Scientific Research (MOHESR) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

II Zw 40 is a starburst dwarf and merger product, and holds a radio-infrared supernebula excited by thousands of embedded OB stars. We present here observations of three aspects of the supernebula: maps of the K and KU radio continuum that trace dense ionized gas with spatial resolution |${\sim} 0.1^{\prime \prime }$|⁠ , a spectral data cube of the [S  iv ] |$10.5\,\mu$| m emission line that measures the kinematics of the ionized gas with velocity resolution 4.5 km s |$^{-1}$|⁠ , and an ALMA (Atacama Large Millimeter Array) spectral cube of the CO(3-2) line that probes the dense warm molecular gas with spatial and velocity resolution comparable to the ionized gas. The observations suggest that the supernebula is the overlap, collision or merger of two star clusters, each associated with an elongated molecular cloud. We accordingly modelled the supernebula with simulations of colliding clusters. The model that best agrees with the data is a grazing collision that has distorted the gas and stars to create the distinctive structures observed. These models may have wide applicability in the cluster-rich regions of young starbursts. [ABSTRACT FROM AUTHOR]

Intermediate-mass black hole (IMBH) mergers with masses 104–106 M ⊙ are expected to produce gravitational waves detectable by the Laser Interferometer Space Antenna (LISA) with high signal-to-noise ratios from the present day to cosmic dawn. IMBH mergers are expected to take place within dwarf galaxies; however, the dynamics, timescales, and effect on their hosts are largely unexplored. In a previous study, we examined how IMBHs would pair and merge within nucleated dwarf galaxies. IMBHs in nucleated hosts evolve very efficiently, forming a binary system and coalescing within a few hundred million years. Although the fraction of dwarf galaxies (107 M ⊙ ≤ M ⋆ ≤ 1010 M ⊙) hosting nuclear star clusters is between 60% and 100%, this fraction drops to 20%–70% for lower-mass dwarfs (M ⋆ ≈ 107 M ⊙), with the largest drop in low-density environments. Here, we extend our previous study by performing direct N -body simulations to explore the dynamics and evolution of IMBHs within nonnucleated dwarf galaxies, under the assumption that IMBHs exist within these dwarfs. To our surprise, none of the IMBHs in our simulation suite merge within a Hubble time, despite many attaining high eccentricities e ∼ 0.7–0.95. We conclude that extremely low stellar density environments in the centers of nonnucleated dwarfs do not provide an ample supply of stars to interact with an IMBH binary, resulting in its stalling, in spite of triaxiality and high eccentricity, common means to drive a binary to coalescence. Our findings underline the importance of considering all detailed host properties to predict IMBH merger rates for LISA. [ABSTRACT FROM AUTHOR]

We adopt the two-point correlation function (2PCF) as a statistical tool to quantify the spatial clustering of halo stars for galaxy systems spanning a wide range in host halo virial mass ( 11.25 < log 10 M 200 c / M ⊙ < 15 ) and redshifts (0 < z < 1.5) from the IllustrisTNG simulations. Consistent with a previous study, we identify clear correlations between the strength of the 2PCF signals and galaxy formation redshifts, but over a much wider mass range. We find that such correlations are slightly stronger at higher redshifts and get weakened with the increase of host halo mass. We demonstrate that the spatial clustering of halo stars is affected by two factors: (1) the clustering gets gradually weakened as time passes (phase mixing); (2) newly accreted stars at more recent times would increase the clustering. For more massive galaxy systems, they assemble late, and the newly accreted stars would increase the clustering. The late assembly of massive systems may also help to explain the weaker correlations between the 2PCF signals and the galaxy formation redshifts in massive halos, as their 2PCFs are affected more by recently accreted stars, while formation redshift characterizes mass accretion on a much longer timescale. We find that the orbits of satellite galaxies in more massive halos maintain larger radial anisotropy, reflecting the more active accretion state of their hosts while also contributing to their stronger mass loss rates. [ABSTRACT FROM AUTHOR]

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 & Kroupa (2012, A&A, 543, A8) is significantly different from the relation suggested by Pfalzner et al. (2016, A&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 & 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. [ABSTRACT FROM AUTHOR]

Context. Blue large-amplitude pulsators (BLAPs) are a recently discovered group of hot stars pulsating in radial modes. Their origin needs to be explained, and several scenarios for their formation have already been proposed. Aims. We investigate whether BLAPs can originate as the product of a merger of two low-mass white dwarfs (WDs) and estimate how many BLAPs can be formed in this evolutionary channel. Methods. We used the Modules for Experiments in Stellar Astrophysics (MESA) code to model the merger of three different double extremely low-mass (DELM) WDs and the subsequent evolution of the merger product. We also performed a population synthesis of Galactic DELM WDs using the COSMIC code. Results. We find that BLAPs can be formed from DELM WDs provided that the total mass of the system ranges between 0.32 and 0.7 M⊙. BLAPs born in this scenario either do not have any thermonuclear fusion at all or show off-centre He burning. The final product evolves to hot subdwarfs and eventually finishes its evolution either as a cooling He WD or a hybrid He/CO WD. The merger products become BLAPs only a few thousand years after coalescence, and it takes them 20–70 thousand years to pass the BLAP region. We found the instability of the fundamental radial mode to be in fair agreement with observations, but we also observed instability of the radial first overtone. The calculated evolutionary rates of period change can be both positive and negative. From the population synthesis, we found that up to a few hundred BLAPs born in this scenario can exist at present in the Galaxy. Conclusions. Given the estimated number of BLAPs formed in the studied DELM WD merger scenario, there is a good chance to observe BLAPs that originated through this scenario. Since strong magnetic fields can be generated during mergers, this scenario could lead to the formation of magnetic BLAPs. This fits well with the discovery of two likely magnetic BLAPs whose pulsations can be explained in terms of the oblique rotator model. [ABSTRACT FROM AUTHOR]