IAUS 316: Formation, evolution, and survival of massive star clusters

Massive star clusters deserve special attention because they are one of the few bridges that connect the interstellar medium, star formation and evolution, mass and energy feedback to evolution of galaxies, and cosmology. These systems are ideal laboratories to probe very localized processes within interstellar gas clouds as well as large-scale dynamics of interstellar matter in interplay with the formation, evolution, and death of stars. Populous clusters are the key sites of high-mass star formation (i.e., the high-mass end of the IMF). They host large numbers of compact and exotic objects that form preferentially in dense environments, and they are fundamental benchmarks to study stellar physics and evolution and to explore dynamics in dense systems. They hold clues to how star formation and evolution as well as chemical and dynamical evolution of stellar systems influence each other and vary from one environment to another, from the present to the early Universe. They are the witnesses of the formation, assembly, and evolution of galaxies and of their substructures across time. They play a role in hierarchical cosmology as well as in the reionization of the intergalactic medium. These very complex systems are a prime example of astronomical objects currently being studied in every wavelength region, from radio to x-rays (and even gamma rays), and are also relevant for gravitational wave detectors.

IAU Conferences on related topics were held in 2009 at the GA in Rio (IAU Symposium 266 “Star Clusters – Basic Galactic Building Blocks Throughout Time and Space”) and in 2012 at the GA in Beijing (IAU SpS 1 “Origin and Complexity of Massive Star Clusters). Other non-IAU supported meetings on star clusters were also organized over the past decade and they were all well attended, indicating that this is a field in rapid evolution and the professional community is requesting on-going discussion on this topic.

In fact, these conferences were largely motivated by the recent advances in numerical modelling, by the new stellar population synthesis models, by new indications of presence of intermediate-mass black holes, and by the breakthrough results of the high resolution observations of stellar clusters both with the HST and from the ground. For example, the discovery of multiple stellar populations in Galactic globular clusters, which is now embraced by the community at large, has totally revolutionized the classical picture of these objects and has shed a new light on massive star clusters long thought to be simple systems of coeval stars born out of homogeneous material.

This drastic paradigm shift has raised a number of fundamental questions on the physical processes that drive the formation and internal evolution of massive star clusters, on the star cluster initial mass function, and on the contribution of star clusters of various sizes and ages to the general galactic field stellar population and to the reionization of the intergalactic medium at high redshift. This has consequently led to significant investments both on the observational and theoretical sides over the past couple of years in order to understand these very complex objects in a broad astrophysical context. Most of the important issues are still at an embryonic stage and require a very wide range of expertise. Among the crucial points one can quote for example the role for the early dynamical and chemical evolution of star clusters of stellar feedback on the intracluster medium, the connection between the detected numbers of exotic objects (millisecond pulsars, cataclysmic variables) and both chemical and dynamical evolution, the combination between multiple population studies and kinematics of single stars as well as various structural parameters for star clusters, or the connection between nuclear star clusters and the globular cluster system of individual galaxies.

Therefore, we strongly feel that interaction among traditionally segregated fields in physics, astrophysics, and cosmology, both on the theoretical and the observational sides, is urgently needed to make significant progress on the understanding of massive star clusters. We believe that a new Symposium is perfectly timely to examine the fundamental physical processes sustaining the formation, evolution, disruption or survival of massive star clusters, at the very same moment when we enter a golden age for observations and numerical multi dimensional simulations. This is required to ensure cross-fertilisation and networking between specialists of the relevant fields and to define coordinated, innovative, and cutting-age theoretical, numerical, and observational developments in order to tackle inter-related open issues.
In this context, the goals of the proposed Symposium are:

• To review state-of-the-art data on massive star clusters across space (from local to high redshift) and time (from currently forming to cosmological relics);
• To present the current theoretical knowledge and discuss the open questions on the many and diverse physical processes that drive the formation, evolution, disruption or survival of massive star clusters in different environment and at various cosmic times;
• To address the question of the formation of multiple populations and of their impact on the intracluster medium and on the dynamics of their host clusters;
• To define innovative and cutting-age high-resolution, multi-scale numerical simulations in order to tackle inter-related open issues, and to confront e.g. new observations of star forming places with hydro-simulations of star forming turbulent clouds;
• To create new synergy among the large surveys with both ground-based telescopes and the ESA and NASA space missions Spitzer, Herschel, HST, and GAIA;
• To prepare the ground for future observations with ALMA and later SKA, JWST, and the next multi-object spectroscopic facilities on very large and extremely large telescopes, which will open new windows on super star clusters as well as on Galactic and extragalactic stellar systems and globular clusters.These objectives require the collective input from specialists (observers and theorists) working on very different domains: stellar evolution, interstellar matter magnetohydrodynamics, stellar dynamics, formation and evolution of galaxies, cosmology, multidimensional numerical simulations, N-body simulations, and multi-wavelength high-precision photometry, spectroscopy, and astrometry.

This is the reason why we think that the prime location for this Symposium is the GA, which will be attended by a large audience of observers and theoreticians, with expertise in different but relevant fields, whom we are aiming at attracting to our Symposium.We propose to leverage this wide presence of specialists to proactively address the goals above, and reach some specific conclusions on issues that the community still feels are controversial, such as:

• Which physical properties determine the mode of star cluster formation? What are the initial and boundary conditions that will lead to massive and not-so-massive cluster formation? What is the relationship with the pre-cluster molecular cloud cores? Can we find pre-cluster molecular cloud cores of massive clusters?
• How important are early disruption processes (including "infant mortality") and does their importance depend on environment?
• How well do current models explain the presence of multiple populations in massive clusters, and how multiple generation stars are connected to the properties and evolution of the natal cloud cores?
• How important is the role of binary stars and what is the impact on cluster models? How does the binary mass fraction evolve with time during the life of the clusters? Do binary mergers play a role?
• What are the correlations between the presence of various compact objects in globular clusters and other structural parameters? Is there a correlation between the initial mass of a cluster and the mass of its most massive star?
• Which fraction of stars form in bound clusters and how does this vary with environment, metallicity, and time?