Context. Most stars form as part of a star cluster. The most massive clusters in the Milky Way exist in two groups -loose and compact clusters -with significantly different sizes at the end of the star formation process. After their formation, both types of clusters expand up to a factor 10-20 within the first 20 Myr of their development. Gas expulsion at the end of the star formation process is usually regarded as the only possible process that can lead to such an expansion during this early period of development. Aims. We investigate the effect of gas expulsion by a direct comparison between numerical models and observed clusters and concentrate on clusters with masses >10 3 M . For these clusters the initial conditions before gas expulsion, the characteristic cluster development, its dependence on cluster mass, and the star formation efficiency (SFE) are investigated. Methods. We performed N-body simulations of the cluster expansion process after gas expulsion and compared the results with observations. Results. We find that the expansion processes of the observed loose and compact massive clusters are driven by completely different physical processes. As expected, the expansion of loose massive clusters is largely driven by the gas loss due to the low SFE of ∼30%. One new revelation is that all the observed massive clusters of this group seem to have a very similar size of 1-3 pc at the onset of expansion. It is demonstrated that compact clusters have a much higher effective SFE of 60-70% and are as a result much less affected by gas expulsion. Their expansion is mainly driven by stellar ejections caused by interactions between the cluster members. The reason ejections are so efficient in driving cluster expansion is that they occur dominantly from the cluster centre and over an extended period of time. During the first 10 Myr the internal dynamics of loose and compact clusters thus differ fundamentally.

Context. Field stars are not always single stars, but can often be found in bound double systems. Since binary frequencies in the birth places of stars, young embedded clusters, are sometimes even higher than on average the question arises of how binary stars form in young dense star clusters and how their properties evolve to those observed in the field population. Aims. We assess, the influence of stellar dynamical interactions on the primordial binary population in young dense cluster environments. Methods. We perform numerical N-body simulations of the Orion nebula cluster like star cluster models including primordial binary populations using the simulation code nbody6 ++ . Results. We find two remarkable results that have yet not been reported: The first is that the evolution of the binary frequency in young dense star clusters is independent predictably of its initial value. The time evolution of the normalized number of binary systems has a fundamental shape. The second main result is that the mass of the primary star is of vital importance to the evolution of the binary. The more massive a primary star, the lower the probability that the binary is destroyed by gravitational interactions. This results in a higher binary frequency for stars more massive than 2 M compared to the binary frequency of lower mass stars. The observed increase in the binary frequency with primary mass is therefore most likely not due to differences in the formation process but can be entirely explained as a dynamical effect. Conclusions. Our results allow us to draw conclusions about the past and the future number of binary systems in young dense star clusters and demonstrate that the present field stellar population has been influenced significantly by its natal environments.

Most stars form in a cluster environment. These stars are initially surrounded by discs from which potentially planetary systems form. Of all cluster environments starburst clusters are probably the most hostile for planetary systems in our Galaxy. The intense stellar radiation and extreme density favour rapid destruction of circumstellar discs via photoevaporation and stellar encounters. Evolving a virialized model of the Arches cluster in the Galactic tidal field we investigate the effect of stellar encounters on circumstellar discs in a prototypical starburst cluster. Despite its proximity to the deep gravitational potential of the Galactic centre only a moderate fraction of members escapes to form an extended pair of tidal tails. Our simulations show that encounters destroy one third of the circumstellar discs in the cluster core within the first 2.5 Myr of evolution, preferentially affecting the least and most massive stars. A small fraction of these events causes rapid ejection and the formation of a weaker second pair of tidal tails that is overpopulated by disc-poor stars. Two predictions arise from our study: (i) If not destroyed by photoevaporation protoplanetary discs of massive late B-and early O-type stars represent the most likely hosts of planet formation in starburst clusters. (ii) Multiepoch Kand L-band photometry of the Arches cluster would provide the kinematically selected membership sample required to detect the additional pair of disc-poor tidal tails.

Context. Surveys of binary populations in the solar neighbourhood have discovered that the periods of G-and M-type stars are log-normally distributed in the range 0.1−10 11 days. However, observations of young binary populations in various star-forming regions have instead inferred a log-uniform distribution. Some process(es) must clearly be responsible for this change in the period distribution over time. Most stars form in star clusters, so it is here that the(se) process(es) take place. Aims. In dense young clusters, two important dynamical processes occur: i) the gas-induced orbital decay of embedded binary systems and ii) the destruction of soft binaries in three-body interactions. The emphasis in this work is on orbital decay as its influence on the binary distribution in clustered environments has largely been neglected so far. Methods. We performed Monte Carlo simulations of binary populations to model the process of orbital decay due to friction between the gas and binary stars. In addition, the destruction of soft binaries in young dense star clusters was simulated using N-body modelling of binary populations. Results. It is known that the cluster dynamics destroy the number of wide binaries, but leave short-period binaries basically undisturbed. Here we demonstrate that this result is also valid for an initially log-uniform period binary distribution. In contrast, the process of orbital decay significantly reduces the number and changes the properties of short-period binaries, leaving wide binaries largely uneffected. Until now, it has been unclear whether the short period distribution of the field has remained unaltered since its formation. We show here, that if any alteration took place, then orbital decay is a prime candidate for this task. In combination, the dynamics of these two processes, convert even an initial log-uniform distribution into a log-normal period distribution. The probability is 94% that the evolved period distribution and the observed period distribution have been sampled from the same parent distribution. Conclusions. Our results provide a new picture for the development of the field binary population: binaries can be formed as a result of the star-formation process in star clusters with periods that are sampled from the log-uniform distribution. As the cluster evolves, short-period binaries merge to form single stars by means of gas-induced orbital decay, while the dynamical evolution in the cluster destroys wide binaries. The combination of these two equally important processes reshapes an initial log-uniform period distribution to the log-normal period distribution that is observed in the field.

Context. The expulsion of the unconverted gas at the end of the star formation process potentially leads to the expansion of the just formed stellar cluster and membership loss. The degree of expansion and mass loss depends largely on the star formation efficiency and scales with the mass and size of the stellar group when stellar interactions can be neglected. Aims. We investigate the circumstances in which stellar interactions between cluster members become so important that the fraction of bound stars after gas expulsion is significantly altered. Methods. The Nbody6 code is used to simulate the cluster dynamics after gas expulsion for different star formation efficiences. Concentrating on the most massive clusters observed in the Milky Way, we test to what extent the results depend on the model, i.e. stellar mass distribution, stellar density profile etc., and the cluster parameters, such as cluster density and size. Results. We find that stellar interactions leading to ejections are responsible for up to 20% mass loss in the most compact massive clusters in the Milky Way. Therefore, ejections are the prime mass loss process in these systems. Even in the loosely bound OB associations, stellar interactions are responsible for at least ∼5% mass loss. The main reason why the importance of encounters for massive clusters has been largely overlooked is because of the often used approach of a single-mass representation instead of a realistic distribution for the stellar masses. The density dependence on the encounter-induced mass loss is shallower than expected because of the increasing importance of few-body interactions in dense clusters compared to sparse clusters where 2-body encounters dominate.

Context. The recent realization that most stars form in clusters, immediately raises the question of whether star and planet formation are influenced by the cluster environment. The stellar density in the most prevalent clusters is the key factor here. Whether dominant modes of clustered star formation exist is a fundamental question. Using near-neighbour searches in young clusters, Bressert and collaborators claim this not to be the case. They conclude that -at least in the solar neighbourhood -star formation is continuous from isolated to densely clustered environments and that the environment plays a minor role in star and planet formation. Aims. We investigate under which conditions near-neighbour searches in young clusters can distinguish between different modes of clustered star formation. Methods. Model star clusters with different memberships and density distributions are set up and near-neighbour searches are performed. We investigate the influence of the combination of different cluster modes, observational biases, and types of diagnostic on the results. Results. We find that the specific cluster density profile, the relative sample sizes, the limitations of the observation, and the choice of diagnostic method decide, whether modelled modes of clustered star formation are detected by near-neighbour searches. For density distributions that are centrally concentrated but span a wide density range (for example, King profiles), separate cluster modes are only detectable under ideal conditions (sample selection, completeness) if the mean density of the individual clusters differs by at least a factor of ∼65. Introducing a central cut-off can lead to an underestimate of the mean density by more than a factor of ten especially in high density regions. The environmental effect on star and planet formation is similarly underestimated for half of the population in dense systems. Conclusions. Local surface-density distributions are a very useful tool for single cluster analyses, but only for high-resolution data. However, in a simultaneous analysis of a sample of cluster environments, it is found that effects of superposition suppress characteristic features very efficiently and thus promote erroneous conclusions. While multiple peaks in the distribution of the local surface density in star forming regions imply the existence of different modes of star formation, the converse conclusion is impossible. Equally, a smooth distribution is no proof of continuous star formation, because such a shape can easily hide modes of clustered star formation.

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