As self-gravitating systems, dense star clusters exhibit a natural diffusion of energy from their innermost to outermost regions, leading to a slow and steady contraction of the core until it ultimately collapses under gravity. However, in spite of the natural tendency toward “core collapse,” the globular clusters (GCs) in the Milky Way exhibit a well-observed bimodal distribution in core radii separating the core-collapsed and non-core-collapsed clusters. This suggests an internal energy source is at work, delaying the onset of core collapse in many clusters. Over the past decade, a large amount of work has suggested that stellar black holes (BHs) play a dynamically-significant role in clusters throughout their entire lifetimes. Here we review our latest understanding of BH populations in GCs and demonstrate that, through their dynamical interaction with their host cluster, BHs can naturally explain the distinction between core-collapsed and non-core-collapsed clusters through a process we call “black hole burning.”

A growing number of studies are revealing that many Milky Way globular clusters possess extended stellar structures beyond their traditional boundaries. Just how ubiquitous these structures are, and how they originate, are key questions to explore. In this contribution, we present a Bayesian technique that we have developed to separate probable members of globular clusters from the dominant Milky Way fore/background at large clustercentric radii and hence facilitate quantitative analyses of these intriguing structures. We demonstrate the promise of our method by showing how it recovers the known extended features around Palomar 5 and NGC 7089.

Integrated light (IL) spectroscopy enables studies of stellar populations beyond the Milky Way and its nearest satellites. In this paper, I will review how IL spectroscopy reveals essential information about globular clusters and the assembly histories of their host galaxies, concentrating particularly on the metallicities and detailed chemical abundances of the GCs in M31. I will also briefly mention the effects of multiple populations on IL spectra, and how observations of distant globular clusters help constrain the source(s) of light-element abundance variations. I will end with future perspectives, emphasizing how IL spectroscopy can bridge the gap between Galactic and extragalactic astronomy.

In this work we present our new estimates of the fundamental parameters of the open cluster Collinder 463, based on Gaia astrometric and PanSTARRS photometric data. In addition to updating previously available parameter values we highlight the existence of an extended stellar “halo” which is closely related to the presently known star cluster.

Dense stellar systems in general and star clusters in particular have recently regained the interest of the extragalactic and even cosmology communities, due to the role they could play as actors and probes of re-ionization, galactic archeology and the dark matter content of galaxies, among many others. In the era of the exploitation and the preparation of large stellar surveys (Gaia, APOGEE, 4MOST, WEAVE), of the detection of gravitational waves mostly originating from dense regions like the cores of clusters (Ligo, LISA), and in an always more holistic view of galaxy formation (HARMONI, Euclid, LSST†), a complete theory on the formation and evolution of clusters is needed to interpret the on-going and forthcoming data avalanche. In this context, the community carries an effort to model the aspects of star cluster formation and evolution in galactic and even cosmological context. However, it is not always easy to understand the caveats and the shortcuts taken in theories and simulations, and their implications on the conclusions drawn. I take the opportunity of this document to highlight three of these topics and discuss why some shortcuts taken by the community are or could be misleading.

We have done N-body simulations with N up to 106, with the aim to determine whether fluctuations in the force field of a globular cluster are caused by nearby or distant encounters. We find that distant encounters are insignificant, in agreement with Chandrasekhar’s expectations, contrary to general opinion.

Using data from the core of 47 Tuc we have identified stars in different evolutionary stages in the colour-magnitude diagram, and used the effects of mass segregation on their radial distribution to study the evolution and origin of blue stragglers (BSS). We separate the BSS into 2 samples by their magnitude and find considerable differences in their distribution. Bright BSS are more centrally concentrated with mass estimates over twice the turn-off mass suggesting an origin involving a triple or multiple star system. The distribution of the faint BSS is close to that of the main-sequence (MS) binaries pointing to these stars as their likely progenitors. Using MESA models, we calculate the expected number of stars in each evolutionary stage and compare it with the observed number of stars. Results indicate that BSS have a post-MS evolution comparable to that of a normal star of the same mass and a MS-BSS lifetime of about 200 – 300 Myr.

RR Lyrae variables are powerful tools to study their host stellar populations. Globular clusters and dwarf galaxies are old and usually host this type of variables. With a growing number of low luminosity objects discovered in the halo of the Milky Way, classifying stars clusters and galaxies has become more challenging. In this study, we examine the properties of RR Lyrae stars in globular clusters and dwarf galaxies in the Local Group. We construct a catalog of RR Lyrae variables in the Local Group globular clusters and dwarf galaxies from previously published data and compare the properties of RR Lyrae variables between those two types of stellar systems. Our goal is to search for a physical difference in the properties of RR Lyrae variables in those two classes of stellar systems. We also analyze the global trend of RRLs in these systems to understand more about their formation and evolution history.

As a result of the slow action of two-body encounters, globular clusters develop mass segregation and attain a condition of only partial energy equipartition even in their central, most relaxed regions. Realistic numerical simulations show that, during the process, a radially-biased anisotropy profile slowly builds up, mimicking that resulting from incomplete violent relaxation. Commonly used dynamical models, such as the one-component King models, cannot describe these properties. Here we show that simple two-component models based on a distribution function originally conceived to describe elliptical galaxies, recently truncated and adapted to the context of globular clusters, can describe in detail what is observed in complex and realistic numerical simulations.

The Fornax Deep Survey (FDS) is a multi-band imaging survey of the Fornax cluster of galaxies, executed with the ESO VLT Survey Telescope (VST). The survey is designed to reach unprecedented surface brightness and point-source magnitude depth over one virial radius of the cluster. The scientific objectives of the survey are numerous: the study of the galaxy luminosity function, derivation of galaxy scaling relations, determination of the properties of compact stellar systems, an accurate determination of distances and 3-D geometry of the Fornax cluster, analysis of diffuse stellar light and galaxy interactions, etc.

In this contribute we give an overview on the interest of the survey on globular clusters (GC) populations, and present a report the status of the study of GCs also providing some preliminary results of our analysis, with particular regard to the two-dimensional distribution of GC candidates over ∼20 sq. degree area of Fornax centered on NGC 1399.

Globular clusters (GCs) display anomalous light-elements abundances (HeCNONaMgAl), resembling the yields of hot-hydrogen burning, but there is no consensus yet on the origin of these ubiquitous multiple populations. We present a model in which a super-massive star (SMS, ≳103 M⊙) forms via stellar collisions during GC formation and pollutes the intra-cluster medium. The growth of the SMS finds a balance with the wind mass loss rate, such that the SMS can produce a significant fraction of the total GC mass in processed material, thereby overcoming the so-called mass-budget problem that plagues other models. Because of continuous rejuvenation, the SMS acts as a ‘conveyer-belt’ of hot-hydrogen burning yields with (relatively) low He abundances, in agreement with empirical constraints. Additionally, the amount of processed material per unit of GC mass correlates with GC mass, addressing the specific mass budget problem. We discuss uncertainties and tests of this new self-enrichment scenario.

We study runaway stellar collisions in primordial star clusters and formation of intermediate mass black holes (IMBHs). Using cosmological simulations, we identify eight atomic-cooling halos in which the star clusters form. We follow stellar and dark matter (DM) dynamics for 3Myr using hybrid N-body simulations. We find that the runaway stellar collisions occur in all star clusters and IMBHs with masses ∼400–1900M⊙ form. Performing additional N-body simulations, we explore evolutions of the IMBHs in the star clusters for 15 Myr. The IMBH masses grow via stellar tidal disruption events (TDEs) to ∼700–2500 M⊙. The TDE rates are ∼0.3–1.3 Myr−1. DM motions affect the star cluster evolutions and reduce the TDE rates. The IMBHs may subsequently grow to SMBHs by gas supply through galaxy mergers or large-scale gas inflows, or they may remain within or around the clusters.

High-density cusps of compact remnants are expected to form around supermassive black holes (SMBHs) in galactic nuclei via dynamical friction and two-body relaxation. Due to the high density, binaries in orbit around the SMBH can frequently undergo close encounters with compact remnants from the cusp. This can affect the gravitational wave merger rate of compact binaries in galactic nuclei. We investigated this process by means of high accuracy few-body simulations, performed with a novel Monte Carlo approach. We find that, around a SgrA*-like SMBH, three-body encounters increase the number of mergers by a factor of 3. This occurs because close encounters can reorient binaries with respect to their orbital plane around the SMBH, increasing the number of Kozai-Lidov induced mergers. We obtain a binary black hole merger rate of ГMW = 1.6 × 10−6 yr−1 per Milky Way-like nucleus.

The Large Magellanic Cloud (LMC) is the closest massive satellite of the Milky Way (MW), and its proximity allows us to study its stellar populations with great detail, both with resolved photometry and spectroscopy. In turn, this is crucial to unveil its star formation and chemical enrichment histories, and also to investigate the effects that gravitational interactions with other systems (as the Small Magellanic Cloud (SMC) and the MW) may induce on an irregular galaxy. The LMC is characterized by a still on-going star formation activity, as traced by the wide range of ages and metallicities of its stellar populations. However, most of the information about the chemistry and the kinematics of this galaxy has been obtained from low-resolution spectra, which do not allow to draw firm conclusions on many crucial open questions. In particular, (1) we still miss a homogeneous determination of the LMC metallicity distribution; (2) the metal-poor component is still poorly known and described; and (3) we have no conclusive information on the existence of metallicity gradients, which would suggest to spatially inhomegeneous star formation events. To properly address these issues, we analysed nearly 500 high-resolution FLAMES spectra of red giant stars belonging to the LMC field, the largest set of high- resolution spectra of LMC stars analysed so far in a homogeneous way.

According to earlier investigations by Turner and co-authors, P Cygni could be a member of a hypothetical, sparsely populated open cluster. The star lies near the east boundary of this hypothetical cluster. There is another known open cluster, IC 4996, in the vicinity of P Cygni. The same authors believe that the above mentioned two clusters are connected to each other and they could represent a double cluster. As P Cygni is a hypergiant and consequently has very strong and variable stellar wind, so a cluster membership can enable us to determine the age, distance, and reddening of the star relatively precisely. We used new data of different catalogues, for example, PPMXL and Gaia and tried to resolve the problem.

Three-body interactions of stellar-mass binaries with intermediate mass black holes (IMBHs) in nuclei of globular clusters may produce specific features that may serve as an independent indicator of existence of the IMBHs. By means of direct N-body integrations we follow the dynamical evolution of globular clusters of moderate extension and mass with 50% binary population over a time span of ≍ 0.8 Gyr and compare the cases with and without the primordial binaries as well as with and without the IMBH. We show that (i) presence of the IMBH leads to rapid formation of a density cusp regardless of the initial binary fraction, (ii) binary rich clusters with the IMBH produce high velocity escapers at a rate of ≍ 0.1 Myr−1 and (iii) clusters hosting an IMBH together with high number of binaries form a denser halo of marginally unbound stars than clusters that lack either the IMBH or the binary population.

We discuss a meta-analysis of the association of abundance variations in globular cluster stars with the present-day stellar mass and metallicity of globular clusters. Using data for 42 globular clusters that are well-sampled from either or both of prior literature studies and the APOGEE survey, we confirm prior findings that increasing aluminum abundance variations in globular clusters are positively correlated with increasing present-day stellar mass or decreasing metallicity. We also demonstrate that the ratio of aluminum abundance variations to either nitrogen abundance variations or sodium abundance variations is itself positively correlated with decreasing metallicity and increasing stellar mass of globular clusters. This suggests that there were at least two non-supernovae chemical polluters that were active in the early universe.

The Chandra X-ray Observatory has detected relatively hard X-ray emission from the central stars of several planetary nebulae (PNe). A subset has no known late-type companions, making it very difficult to isolate which of several competing mechanisms may be producing the X-ray emission. The central star of NGC 2392 is one of the most vexing members, with substantial indirect evidence for a hot white dwarf (WD) companion. Here we report on the results of a radial velocity (RV) monitoring campaign of its central star with the HERMES échelle spectrograph of the Flemish 1.2 m Mercator telescope. We discover a single-lined spectroscopic binary with an orbital period of 1.902208 ± 0.000013 d and an RV semi-amplitude of 9.96 ± 0.13 km s−1. The high degree of nebula ionisation requires a WD companion (M ≳ 0.6M⊙), which the mass-function supports at orbital inclinations ≲ 7°, in agreement with the nebula orientation of 9°. The hard component of the X-ray spectrum may be explained by the companion accreting mass from the wind of the Roche lobe filling primary, while the softer component may be due to colliding winds. A companion with a stronger wind than the primary could produce the latter and would be consistent with models of the observed diffuse X-ray emission detected in the nebula. The diffuse X-rays may also be powered by the jets of up to 180 km s−1, and active accretion would imply that they may be the first active jets of a post-common-envelope PN, potentially making NGC 2392 an invaluable laboratory to study jet formation physics. The 1.9 d orbital period rules out a double-degenerate merger leading to a Type Ia supernova, and the weak wind of the primary likely also precludes a single-degenerate scenario. We suggest that a hard X-ray spectrum, in the absence of a late-type companion, could be a powerful tool to identify accreting WD companions.

The detection of a neutron star merger by the Advanced Laser Interferometer Gravitational-Wave Observatory and Advanced Virgo gravitational wave detectors, and the subsequent detection of an electromagnetic counterpart have opened a new era of transient astronomy. With upgrades to the Advanced Laser Interferometer Gravitational-Wave Observatory and Advanced Virgo detectors and new detectors coming online in Japan and India, neutron star mergers will be detected at a higher rate in the future, starting with the O3 observing run which will begin in early 2019. The detection of electromagnetic emission from these mergers provides vital information about merger parameters and allows independent measurement of the Hubble constant. The Australian Square Kilometre Array Pathfinder is expected to become fully operational in early 2019, and its 30 deg2 field of view will enable us to rapidly survey large areas of sky. In this work we explore prospects for detecting both prompt and long-term radio emission from neutron star mergers with Australian Square Kilometre Array Pathfinder and determine an observing strategy that optimises the use of telescope time. We investigate different strategies to tile the sky with telescope pointings in order to detect radio counterparts with limited observing time, using 475 simulated gravitational wave events. Our results show a significant improvement in observing efficiency when compared with a naïve strategy of covering the entire localisation above some confidence threshold, even when achieving the same total probability covered.

We illustrate the extraordinary potential of the (far-IR) Origins Survey Spectrometer (OSS) on board the Origins Space Telescope (OST) to address a variety of open issues on the co-evolution of galaxies and AGNs. We present predictions for blind surveys, each of 1000 h, with different mapped areas (a shallow survey covering an area of 10 deg2 and a deep survey of 1 deg2) and two different concepts of the OST/OSS: with a 5.9 m telescope (Concept 2, our reference configuration) and with a 9.1 m telescope (Concept 1, previous configuration). In 1 000 h, surveys with the reference concept will detect from ∼1.9×106 to ∼8.7×106 lines from ∼4.8×105 to 2.7×106 star-forming galaxies and from ∼1.4×104 to ∼3.8×104 lines from ∼1.3×104 to 3.5×104 AGNs. The shallow survey will detect substantially more sources than the deep one; the advantage of the latter in pushing detections to lower luminosities/higher redshifts turns out to be quite limited. The OST/OSS will reach, in the same observing time, line fluxes more than one order of magnitude fainter than the SPICA/SMI and will cover a much broader redshift range. In particular it will detect tens of thousands of galaxies at z ≥ 5, beyond the reach of that instrument. The polycyclic aromatic hydrocarbons lines are potentially bright enough to allow the detection of hundreds of thousands of star-forming galaxies up to z ∼ 8.5, i.e. all the way through the reionisation epoch. The proposed surveys will allow us to explore the galaxy–AGN co-evolution up to z ∼ 5.5−6 with very good statistics. OST Concept 1 does not offer significant advantages for the scientific goals presented here.