The Early Evolution of Star Clusters and their Stellar Populations

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Star clusters are groups of tens to millions of stars. They are found in all galaxies and are used to trace galaxy formation and structure. However, our understanding of the early stages of a cluster's life is quite incomplete. Young star clusters are hard to observe, as they are often far away and always enshrouded in gas and dust. Most models assume that star clusters were formed very quickly, with all stars born at exactly the same time, in a spherical system, and that all the gas from which these stars formed is no longer present. Observationally, we know none of these assumptions are correct, but they were used in order to make previous simulations more tractable. However, because of our expertise in different areas of stellar astrophysics, my group is now poised to make significant advances in our understanding of the early evolution of clusters and their stars. Models of massive star clusters have assumed that their stars all formed at the same time, in the same place, and out of the same material. Recently, however, observations are showing us that these clusters have multiple stellar populations. The expectation is that some of the shorter-lived stars of the first generation produced and polluted the gas which then formed the second generation, but not in the same way that generations of stars are formed in the rest of the galaxy. Open questions still remain about the properties and behaviour of the second generation gas in young clusters, how and where the stars formed, and how the two generations interacted. In addition, star clusters host many different kinds of "stellar exotica", stars which do not evolve in the same way as normal single stars like our Sun. Recent advances in modelling these populations have shown that both dynamical interactions and binary evolution are required to understand the properties of such systems in clusters.Studying very young clusters, cracking the "multiple populations" problem and modelling stellar exotica share some of the same challenges. They are multifaceted problems: in order correctly interpret the observations, we must understand stars and how they evolve. At the same time, we must use our understanding of hydrodynamics to follow the gas in the system, for example as mass is transferred from one star to another, or as gas is emitted by the polluters to create a second generation in clusters. We are never dealing with just one isolated star, but must consider the gravitational forces of many stars. Therefore, stellar dynamics must also be included. Each of these areas of astrophysics has its own natural timescale, distance scale, and energy scale. Therefore, to first approximation, each area can be applied to star clusters independently. Because of this, our understanding of stellar clusters and their stellar populations is quite good, but it is not complete. Thanks to the recent wealth of observational data, we now understand that we must move to more complicated multi-timescale, multi-physics modelling.In this proposal, I describe how this novel approach to stellar astrophysics will be applied to the study of very young stellar clusters. We weill combine stellar dynamics with hydrodynamical models of the gas lost by stars in clusters to understand how it can form a second generation in the presence of the first generation stars. We will investigate the structural properties of very young clusters, prompted by X-ray and infrared observations of such regions in the nearby galaxy. We are one of the few groups in the world who can provide this comprehensive approach to modelling the early evolution of star clusters, and we expect the next 5 years to be particularly fruitful.