| Time | Speaker | Title |
|---|---|---|
| 09:00 – 09:30 |  | Welcome and Registration |
| 09:30 – 10:00 |  | Opening |
| 10:00 – 10:30 | Myung Gyoon Lee | Globular Clusters in the Universe  [PDF] |
| 10:30 – 11:00 |  | Break |
| Session 1: Black Holes and Pulsars in Dense Clusters 1 | ||
| 11:00 – 11:30 | Michela Mapelli | Growing black holes via collisions |
| 11:30 – 12:00 | Holger Baumgardt | Massive black holes in star clusters [pdf] |
| 12:00 – 12:15 | Sara Rastello | Close encounters between stars and compact objects in young star clusters |
| 12:15 – 12:30 | Abbas Askar | Formation and growth of intermediate-mass black holes in dense star clusters |
| 12:30 – 14:00 |  | Lunch |
| Session 2: Galactic and Extragalactic Star Clusters, Tidal Tails I | ||
| 14:00 – 14:30 | Anna Lisa Varri | The phase space of star clusters |
| 14:30 – 14:45 | Cristiano Ugolini | The assembly of intermediate black holes with complementary approaches: Dragon II and BPop |
| 14:45 – 15:00 | Abylay Bissekenov | Evolution of star clusters with initial bulk rotation via N-body |
| 15:00 – 15:15 | Jayanand Mauriya | The role of the stellar rotations and binaries in the origin of extended Main Sequence Turn-Off in open cluster |
| 15:15 – 15:30 | Steffani Grondin | Unraveling Common Envelope Evolution: A New Window from Star Clusters |
| 15:30 – 16:00 |  | Break |
| 16:00 – 18:00 | Poster Sparkler | |
| Â | End of the Day | |
Globular Clusters in the Universe
Globular clusters are a proto-type of dense stellar systems that host a number of different compact binaries, including binary black holes and neutron stars. These compact binaries are important sources of gravitational wave radiation. There are about 200 globular clusters in the Milky Way Galaxy, but many more beyond the Milky Way. They are therefore an excellent laboratory to study the nature of compact binaries in relation with gravitational wave observations. I present an overview of the properties of globular clusters in the Universe: from the old globular clusters in the Milky Way Galaxy to the young massive clusters in high-z galaxies, and from galaxy globular clusters to intracluster globular clusters.
Massive black holes in star clusters
In my talk I will give an overview about the dynamical evidence for the presence of black holes in star clusters, from stellar mass black holes to intermediate-mass and supermassive black holes. I will also talk about possible processes by which IMBHs and SMBHs in clusters have formed and grown in mass.
Close encounters between stars and compact objects in young star clusters
In dense star clusters, close encounters between stars and compact objects such as stellar-mass black holes or neutron stars can lead to a wide range of outcomes, from the disruption of stars to the formation of quiescent binary systems, like those recently discovered thanks to Gaia. The former are transient events known as “micro-Tidal Disruption Events” (micro-TDEs). To date, micro-TDEs have not yet been observed, but they are promising multi-messenger sources predicted to be detected by future gravitational wave (GW) observatories. In this contribution, I will discuss micro-TDEs originating in young star clusters from a dynamical perspective. I have performed a suite of high-precision direct N-body simulations of massive collisional young star clusters (YSCs) using the state-of-the-art PeTar code. PeTar is an N-body code coupled with up-to-date stellar population synthesis codes, which are essential for modeling stars and black hole progenitors. I will present results on the population of micro-TDEs formed in YSCs through hyperbolic-parabolic encounters between single stars and black holes, as well as black hole binaries (BBHs).
Formation and growth of intermediate-mass black holes in dense star clusters
Black holes with masses ranging from a few hundred to hundreds of thousands of solar masses, known as intermediate-mass black holes (IMBHs), remain elusive, and their formation pathways are uncertain. Dense star clusters have long been proposed as promising sites for both the formation and growth of IMBHs. In this talk, I will present recent results from comprehensive MOCCA Monte Carlo simulations of dense star clusters, focusing specifically on the conditions conducive to IMBH formation. Our simulations indicate that IMBHs predominantly form during the initial few million years of cluster evolution via runaway stellar mergers, resulting in the creation of very massive stars that subsequently collapse into IMBHs. Following their formation, these IMBHs initially grow significantly, primarily through mergers with stellar-mass black holes and other stars, and subsequently continue to grow gradually through mergers with low-mass stars and white dwarfs. I will discuss the physical processes and assumptions underlying IMBH formation and growth in these simulations, highlighting key caveats, uncertainties, and dependencies on initial cluster properties. Additionally, I will briefly explore the implications of mergers leading to IMBH growth for observations of gravitational waves and electromagnetic transient events.
The phase space of star clusters
Several astrometric, spectroscopic, and morphological studies have recently exposed the true phase space richness of globular star clusters in the local universe. Now, the challenge for the community consists of understanding the origin of such complexity by exploring the emergence of this class of stellar systems in the early universe. I will discuss various collisional dynamical processes of interest and highlight relevant phenomenological and observational imprints in phase space.
The assembly of intermediate black holes with complementary approaches: Dragon II and BPop
We investigate the formation and growth of intermediate-mass black holes (IMBHs) using two complementary approaches. First, we analyze the DRAGON II direct N-body simulations to identify dominant IMBH assembly pathways and quantify their feedback on host-cluster structural evolution. Second, we leverage our semi-analytical code BPop, into which we’ve implemented proxies calibrated on DRAGON II results, to carry out large-scale, statistical studies of IMBH formation across a broad range of cluster initial conditions. This hybrid strategy combines the precision of direct N-body simulations with the extended parameter- space covered with of semi-analytics approaches. In this way, we can explore cluster whose properties remain inaccessible to purely N-body methods, therefore shedding new light on IMBH demographics in dense stellar systems.
Evolution of star clusters with initial bulk rotation via N-body
Young star clusters can inherit bulk rotation from the molecular clouds from which they have formed. This rotation can affect the long-term evolution of a star cluster and its constituent stellar populations. In this study, we aim to characterize the effects of different degrees of initial rotation on star clusters with primordial binaries. The simulations are performed using NBODY6++GPU. We find that initial rotation strongly affects the early evolution of star clusters. Rapidly-rotating clusters show angular momentum transport from the inner parts to the outskirts, resulting in a core-collapse. Angular momentum transport is accompanied by a highly elongated bar-like structure morphology. The effects of bulk rotation are reduced on the timescale of two-body relaxation. Rotating and non-rotating clusters experience changes in the direction of angular momentum near the dissolution and early evolution due to the tidal field, respectively. We present synthetic observation of simulated clusters for comparison with future observations in filters of Gaia, CSST, and HST. This work shows the effects of bulk rotation on systems with primordial binaries and could be used for identification of rotation signature in observed open clusters.
The role of the stellar rotations and binaries in the origin of extended Main Sequence Turn-Off in open cluster
The physical mechanism(s) leading to the origin of the extended Main Sequence Turn Off (eMSTO) in the Galactic open star clusters is currently one of the fascinating research topics. The presence of eMSTO in the open clusters has been attributed to various factors, such as spread in rotational rates, binary stars, and dust-like extinction from stellar excretion disc. The theories explaining the presence of the eMSTO in the open clusters are still emerging, and keen attention is needed to develop an in-depth understanding of its origin. We comprehensively analyze the eMSTO present in the open cluster NGC 2355. We utilize spectra from the Gaia-ESO surveys’ archives to estimate the stars’ atmospheric parameters and projected rotational velocity, v sin i. We find that the red part of the eMSTO is preferentially occupied by fast-rotating stars with a mean v sin i value of 135.3±4.6 km/s whereas blue eMSTO hosts slow-rotating stars with an average v sin i equal to 81.3±5.6 km/s. This suggests the presence of the eMSTO in NGC 2355 is possibly related to the spread in rotation rates of stars. Leveraging existing ultraviolet data from the Swift survey and optical data, we find no evidence of the dust-like extinction from the fast-rotating eMSTO stars. The tidal interaction theory explaining the spread in rotation rates proposes that all the eMSTO stars were initially fast-rotating until tidal locking in the binaries caused some stars to slow down. Another hypothesis suggests that the star-disc interaction during the pre-main-sequence (PMS) phase may cause the spread in the rotation rates of the eMSTO stars. We use synchronization time and spatial distribution of the eMSTO stars to investigate the most likely mechanism responsible for the presence of the eMSTO in the cluster. Based on our analysis of the results, we conclude that the star-disc interaction in the PMS phase of the stars is the most likely reason for the origin of the eMSTO in NGC 2355. The lower main-sequence stars beyond the eMSTO region of NGC 2355 are slow-rotating (mean v sin i = 26.5±1.3 km/s) possibly due to the magnetic braking of their rotations. In the talk, I will discuss these results and their physical interpretation in detail to address the critical questions currently at the forefront of eMSTO research.
Unraveling Common Envelope Evolution: A New Window from Star Clusters
Close compact object binaries are the precursors to Type Ia supernovae and gravitational wave events. While most short-period binaries are believed to have evolved through at least one common envelope (CE) phase, our understanding of CE evolution remains one of the largest unresolved issues in stellar astrophysics, mainly due to the lack of observational benchmarks that connect post-CE parameters with their pre-CE initial conditions. Identifying post-CE systems in star clusters can circumvent this issue by providing an independent constraint on the system’s age, but until recently, there were only two white dwarf-main sequence (WD+MS) post-CE systems associated with a star cluster. In this talk, I will describe our discovery of the first population of candidate WD+MS binaries in Milky Way star clusters. First, I will discuss our new catalog of 52 WD+MS candidate binaries in 38 open star clusters identified through multi-wavelength observations and supervised machine learning. Next, I will detail the ongoing follow-up characterization of a subset of systems that has led to the confirmation of new WD+MS post-CE systems in clusters. Finally, I will outline how we can expand this sample by using novel star cluster dynamics simulations (Corespray) to link field binaries with their birth clusters. These efforts will ultimately provide fundamental observational constraints on one of the most uncertain yet crucial phases of binary evolution.