The role of cluster age on the onset of multiple populations in stellar clusters

AbstractThe origin of the chemical anomalies in star clusters is still an open question, although much effort has been employed both from a theoretical and observational point of view. The exploration of the dependence of such multiple stellar populations based on certain cluster properties (e.g. mass, age, metallicity) has represented a compelling line of investigation so far. Here I report an overview of the results obtained from our latest surveys aimed at characterising the phenomenon of chemical variations in star clusters that are much younger with respect to the ancient globular clusters. The fundamental question we are asking is whether these abundance patterns are only restricted to the old massive clusters; and if not, is there a difference between young and old objects?

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The accreted nuclear clusters of the Milky Way

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3407 ◽

2020 ◽

Vol 500 (2) ◽

pp. 2514-2524

Author(s):

Joel Pfeffer ◽

Carmela Lardo ◽

Nate Bastian ◽

Sara Saracino ◽

Sebastian Kamann

Keyword(s):

Globular Clusters ◽

Milky Way ◽

Dwarf Galaxy ◽

Star Clusters ◽

Host Galaxy ◽

The Galaxy ◽

Nuclear Clusters ◽

The Common ◽

Massive Clusters ◽

Peculiar Case

ABSTRACT A number of the massive clusters in the halo, bulge, and disc of the Galaxy are not genuine globular clusters (GCs) but instead are different beasts altogether. They are the remnant nuclear star clusters (NSCs) of ancient galaxies since accreted by the Milky Way. While some clusters are readily identifiable as NSCs and can be readily traced back to their host galaxy (e.g. M54 and the Sagittarius Dwarf galaxy), others have proven more elusive. Here, we combine a number of independent constraints, focusing on their internal abundances and overall kinematics, to find NSCs accreted by the Galaxy and trace them to their accretion event. We find that the true NSCs accreted by the Galaxy are: M54 from the Sagittarius Dwarf, ω Centari from Gaia-Enceladus/Sausage, NGC 6273 from Kraken, and (potentially) NGC 6934 from the Helmi Streams. These NSCs are prime candidates for searches of intermediate-mass black holes (BHs) within star clusters, given the common occurrence of galaxies hosting both NSCs and central massive BHs. No NSC appears to be associated with Sequoia or other minor accretion events. Other claimed NSCs are shown not to be such. We also discuss the peculiar case of Terzan 5, which may represent a unique case of a cluster–cluster merger.

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UV-Observations of Young Populous Clusters in the Large Magellanic Cloud

Symposium - International Astronomical Union ◽

10.1017/s0074180900039929 ◽

1984 ◽

Vol 108 ◽

pp. 55-56

Author(s):

Edward H. Geyer ◽

Angelo Cassatella

Keyword(s):

Globular Clusters ◽

Main Sequence ◽

Large Magellanic Cloud ◽

Time Interval ◽

Sequence Evolution ◽

Stellar Clusters ◽

T Tauri ◽

Red Supergiants ◽

Evolution Phase ◽

Massive Clusters

The young populous star clusters give evidence for the ‘explosive’ star formation in the Magellanic Clouds which took place in the time interval 5·106 yrs to < 108 yrs agoe. They are also key objects for the understanding of the formation of massive stellar clusters, because they are still situated close to their ‘birthplace’ in the parent galaxy and are dynamically not relaxed (Geyer et al. 1979). Their HRD-morphology shows most of the member stars in the upper Main Sequence range with only a few massive yellow and red supergiants. The lower massive stars are still in the pre-main-sequence evolution phase (‘T-Tauri state’), which cannot be observed at the MC's distances. Thus in the uv-spectral range the blue stars with (B-V) < 0.1 on the upper MS contribute to the uv-fluxes. In the optical spectral regions the bright ‘blue’ globular clusters seem not be embedded in remanent interstellar matter, though neighbouring loose stellar aggregates of similar age are in many cases surrounded by dense HII-regions. This rises the questions wether the starformation process in such massive clusters was so efficient that no remanent matter was left over, or was this material blown away by the uv-radiation of the numerous OB-member stars?

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Multiple populations in integrated light spectroscopy of intermediate-age clusters

Monthly Notices of the Royal Astronomical Society Letters ◽

10.1093/mnrasl/slz130 ◽

2019 ◽

Vol 489 (1) ◽

pp. L80-L85 ◽

Cited By ~ 4

Author(s):

Nate Bastian ◽

Christopher Usher ◽

Sebastian Kamann ◽

Carmela Lardo ◽

Søren S Larsen ◽

...

Keyword(s):

Globular Clusters ◽

Light Element ◽

Large Magellanic Cloud ◽

Element Abundance ◽

Multiple Populations ◽

Individual Stars ◽

Abundance Patterns ◽

Nearby Galaxies ◽

Massive Clusters ◽

Light Element Abundance

ABSTRACT The presence of star-to-star light-element abundance variations (also known as multiple populations, MPs) appears to be ubiquitous within old and massive clusters in the Milky Way and all studied nearby galaxies. Most previous studies have focused on resolved images or spectroscopy of individual stars, although there has been significant effort in the past few years to look for multiple population signatures in integrated light spectroscopy. If proven feasible, integrated light studies offer a potential way to vastly open parameter space, as clusters out to 10s of Mpc can be studied. We use the Na D lines in the integrated spectra of two clusters with similar ages (2–3 Gyr) but very different masses: NGC 1978 (∼3 × 105 M⊙) in the Large Magellanic Cloud and G114 (1.7 × 107 M⊙) in NGC 1316. For NGC 1978, our findings agree with resolved studies of individual stars that did not find evidence for Na spreads. However, for G114, we find clear evidence for the presence of multiple populations. The fact that the same anomalous abundance patterns are found in both the intermediate age and ancient globular clusters lends further support to the notion that young massive clusters are effectively the same as the ancient globular clusters, only separated in age.

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Population synthesis of black hole binary mergers from star clusters

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz3584 ◽

2020 ◽

Vol 492 (2) ◽

pp. 2936-2954 ◽

Cited By ~ 13

Author(s):

Fabio Antonini ◽

Mark Gieles

Keyword(s):

Black Hole ◽

Globular Clusters ◽

Initial Conditions ◽

Star Clusters ◽

Population Synthesis ◽

Wide Range ◽

Binary Mergers ◽

Cluster Properties ◽

Black Hole Binary ◽

Merger Rate

ABSTRACT Black hole (BH) binary mergers formed through dynamical interactions in dense star clusters are believed to be one of the main sources of gravitational waves (GWs) for Advanced LIGO and Virgo. Here, we present a fast numerical method for simulating the evolution of star clusters with BHs, including a model for the dynamical formation and merger of BH binaries. Our method is based on Hénon’s principle of balanced evolution, according to which the flow of energy within a cluster must be balanced by the energy production inside its core. Because the heat production in the core is powered by the BHs, one can then link the evolution of the cluster to the evolution of its BH population. This allows us to construct evolutionary tracks of the cluster properties including its BH population and its effect on the cluster and, at the same time, determine the merger rate of BH binaries as well as their eccentricity distributions. The model is publicly available and includes the effects of a BH mass spectrum, mass-loss due to stellar evolution, the ejection of BHs due to natal and dynamical kicks, and relativistic corrections during binary–single encounters. We validate our method using direct N-body simulations, and find it to be in excellent agreement with results from recent Monte Carlo models of globular clusters. This establishes our new method as a robust tool for the study of BH dynamics in star clusters and the modelling of GW sources produced in these systems. Finally, we compute the rate and eccentricity distributions of merging BH binaries for a wide range of cluster initial conditions, spanning more than two orders of magnitude in mass and radius.

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On the initial mass-radius relation of stellar clusters

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab2514 ◽

2021 ◽

Author(s):

Nick Choksi ◽

J M Diederik Kruijssen

Keyword(s):

High Pressure ◽

Globular Clusters ◽

High Redshift ◽

Cluster Formation ◽

Stellar Clusters ◽

Population Synthesis ◽

Star Forming ◽

Massive Clusters ◽

Good Agreement ◽

Environmental Dependence

Abstract Young stellar clusters across nearly five orders of magnitude in mass appear to follow a power-law mass-radius relationship (MRR), $R_{\star }\propto M_{\star }^{\alpha }$, with α ≈ 0.2 − 0.33. We develop a simple analytic model for the cluster mass-radius relation. We consider a galaxy disc in hydrostatic equilibrium, which hosts a population of molecular clouds that fragment into clumps undergoing cluster formation and feedback-driven expansion. The model predicts a mass-radius relation of $R_{\star }\propto M_{\star }^{1/2}$ and a dependence on the kpc-scale gas surface density $R_{\star }\propto \Sigma _{\rm g}^{-1/2}$, which results from the formation of more compact clouds (and cluster-forming clumps within) at higher gas surface densities. This environmental dependence implies that the high-pressure environments in which the most massive clusters can form also induce the formation of clusters with the smallest radii, thereby shallowing the observed MRR at high-masses towards the observed $R_{\star }\propto M_{\star }^{1/3}$. At low cluster masses, relaxation-driven expansion induces a similar shallowing of the MRR. We combine our predicted MRR with a simple population synthesis model and apply it to a variety of star-forming environments, finding good agreement. Our model predicts that the high-pressure formation environments of globular clusters at high redshift naturally led to the formation of clusters that are considerably more compact than those in the local Universe, thereby increasing their resilience to tidal shock-driven disruption and contributing to their survival until the present day.

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Star clusters: age, metallicity and extinction from integrated spectra

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309991517 ◽

2009 ◽

Vol 5 (S266) ◽

pp. 403-406

Author(s):

Rosa M. González Delgado ◽

Roberto Cid Fernandes

Keyword(s):

Globular Clusters ◽

Star Clusters ◽

Magellanic Clouds ◽

High Spectral Resolution ◽

Evolutionary Models ◽

Spectral Models ◽

Integrated Optical ◽

Pros And Cons ◽

Cluster Properties ◽

Comparison Of The Results

AbstractIntegrated optical spectra of star clusters in the Magellanic Clouds and a few Galactic globular clusters are fitted using high-resolution spectral models for single stellar populations. The goal is to estimate the age, metallicity and extinction of the clusters, and evaluate the degeneracies among these parameters. Several sets of evolutionary models that were computed with recent high-spectral-resolution stellar libraries (MILES, GRANADA, STELIB), are used as inputs to the starlight code to perform the fits. The comparison of the results derived from this method and previous estimates available in the literature allow us to evaluate the pros and cons of each set of models to determine star cluster properties. In addition, we quantify the uncertainties associated with the age, metallicity and extinction determinations resulting from variance in the ingredients for the analysis.

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The e-MERLIN Cyg OB2 radio survey (COBRaS)

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311010696 ◽

2010 ◽

Vol 6 (S272) ◽

pp. 306-307

Author(s):

Raman K. Prinja ◽

Danielle Fenech

Keyword(s):

Thermal Radiation ◽

Mass Loss ◽

Northern Hemisphere ◽

Globular Clusters ◽

Massive Stars ◽

Star Clusters ◽

Formation Dynamics ◽

Super Star Clusters ◽

Massive Clusters ◽

Radio Survey

AbstractThe e-MERLIN Cyg OB2 Radio Survey (COBRaS) is designed to exploit e-MERLIN's enhanced capabilities to conduct uniquely probing, targeted deep-field mapping of the massive Cyg OB2 association in our Galaxy. The project aims to deliver (between 2010 to 2013) the most detailed radio census for the most massive OB association in the northern hemisphere, offering direct comparison to not only massive clusters in general, but also young globular clusters and super star clusters. With the COBRaS Legacy project we will assemble a uniform dataset of lasting value that is critical for advancing our understanding of current astrophysical problems in the inter-related core themes of (i) mass loss and evolution of massive stars, (ii) the formation, dynamics and content of massive OB associations, and (iii) the frequency of massive binaries and the incidence of non-thermal radiation.

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The dynamical effect of white dwarf kicks in star clusters

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309991190 ◽

2009 ◽

Vol 5 (S266) ◽

pp. 318-319

Author(s):

Harvey B. Richer

Keyword(s):

Globular Clusters ◽

Star Clusters ◽

Core Collapse ◽

Dynamical Models ◽

Intermediate Mass ◽

Heating Source ◽

The Core ◽

Central Heating ◽

Massive Clusters ◽

Hubble Time

AbstractGlobular star clusters generally have large cores, i.e., rc/rh (the ratio of core to half-light radii) exceeds 0.3 for more than 50% of the Galactic globular clusters. In the absence of a central heating source, dynamical models suggest that massive clusters will contract, typically on a timescale shorter than a Hubble time, and exhibit a compact core. To explain the disagreement between observations and theory, intermediate-mass mass black holes have been invoked to explain the core structure. Recent observations, however, have failed to definitively prove their existence in clusters. A new scenario, involving a natal kick given to white dwarfs may provide the required heating and help clusters avoid or delay core collapse.

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Spatially Resolving the Star Formation Histories of Three Nearby Nuclear Star Clusters

The Astronomical Journal ◽

10.3847/1538-3881/ac282e ◽

2021 ◽

Vol 162 (6) ◽

pp. 281

Author(s):

Christian H. Hannah ◽

Anil C. Seth ◽

Dieu D. Nguyen ◽

Antoine Dumont ◽

Nikolay Kacharov ◽

...

Keyword(s):

Globular Clusters ◽

Stellar Population ◽

Star Clusters ◽

Formation Mechanisms ◽

Full Spectrum ◽

Spatially Resolved ◽

Open Question ◽

Star Formation Histories ◽

Spectrum Fitting

Abstract The formation of nuclear star clusters (NSCs) remains an open question. In this work, we use spatially-resolved HST/STIS spectroscopic observations of three nearby NSCs (hosted by NGC 5102, NGC 5206, and NGC 205) to constrain their formation histories by exploring radial variations of the stellar populations within each cluster. Utilizing full-spectrum fitting, we find substantial age and metallicity gradients within the central 0.″9 (16 pc) of the NSC in NGC 5102 where populations near the center are young/metal-rich (age ∼400 Myr and [M/H] ∼ −0.4) and become older/metal-poor at larger radii (mean age ∼1 Gyr and mean [M/H] ∼ −1.6 in the radial range [0.″3, 0.″9]). This behavior suggests that the young/metal-rich population at the center was formed from a period of in situ formation, while the older/metal-poor populations were likely formed by inspiraled globular clusters. The two broad populations observed in the NGC 5102 NSC (young/metal-rich and old/metal-poor) appear to be linked to the transition between the two morphological components of the NSC derived from the surface-brightness profile in Nguyen et al. (2018). The radial ranges explored in NGC 5206 and NGC 205 were much smaller due to poor data quality; in NGC 5206 we find a similar metallicity gradient to NGC 5102 (but with much lower significance), while the data for NGC 205 is too poor to reach any conclusions. Overall, this data highlights the links between the morphological and stellar population complexity of NSCs and their formation mechanisms.

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