The evolutionary tracks of young massive star clusters

Stars mostly form in groups consisting of a few dozen to several ten thousand members. For 30 years, theoretical models provide a basic concept of how such star clusters form and develop: they originate from the gas and dust of collapsing molecular clouds. The conversion from gas to stars being incomplete, the left over gas is expelled, leading to cluster expansion and stars becoming unbound. Observationally, a direct confirmation of this process has proved elusive, which is attributed to the diversity of the properties of forming clusters. Here we take into account that the true cluster masses and sizes are masked, initially by the surface density of the background and later by the still present unbound stars. Based on the recent observational finding that in a given star-forming region the star formation efficiency depends on the local density of the gas, we use an analytical approach combined with mbox{N-body simulations, to reveal} evolutionary tracks for young massive clusters covering the first 10 Myr. Just like the Hertzsprung-Russell diagram is a measure for the evolution of stars, these tracks provide equivalent information for clusters. Like stars, massive clusters form and develop faster than their lower-mass counterparts, explaining why so few massive cluster progenitors are found.

Modes of clustered star formation

The realization that most stars form in clusters, raises the question of whether star/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 et al. (2010) claim this not to be the case and conclude that star formation is continuous from isolated to densely clustered. We investigate under which conditions near-neighbour searches can distinguish between different modes of clustered star formation. Near-neighbour searches are performed for model star clusters investigating the influence of the combination of different cluster modes, observational biases, and types of diagnostic and find that the cluster density profile, the relative sample sizes, limitations in observations and the choice of diagnostic method decides whether modelled modes of clustered star formation are detected. For centrally concentrated density distributions spanning a wide density range (King profiles) separate cluster modes are only detectable if the mean density of the individual clusters differs by at least a factor of ~65. Introducing a central cut-off can lead to underestimating the mean density by more than a factor of ten. The environmental effect on star and planet formation is underestimated for half of the population in dense systems. A analysis of a sample of cluster environments involves effects of superposition that suppress characteristic features and promotes erroneous conclusions. While multiple peaks in the distribution of the local surface density imply the existence of different modes, the reverse conclusion is not possible. Equally, a smooth distribution is not a proof of continuous star formation, because such a shape can easily hide modes of clustered star formation (abridged)

Quasi-binarity of massive stars in young dense clusters - the case of the ONC

Observations indicate that in young stellar clusters the binary fraction for massive stars is higher than for solar mass stars. For the Orion Nebula Cluster (ONC) there is a binary frequency of ~ 50% for solar-mass stars compared to 70-100% for the massive O- and B-stars. We explore the reasons for this discrepancy and come up with two possible answers: a) a primordially higher binarity of massive stars could be inherent to the star formation process or b) the primordial binary rate might be the same for solar-mass and massive stars, but the higher capture cross section of the massive stars possibly leads to the formation of additional massive binaries in the early cluster development. Here we investigate the likelihood of the latter using the ONC as an example. N-body simulations are performed to track the capture events in an ONC-like cluster. We find that whereas low-mass stars rarely form bound systems through capture, the dynamics of the massive stars - especially in the first 0.5 Myrs - is dominated by a rapid succession of ``transient binary or multiple systems. In observations the transient nature of these systems would not be apparent, so that they would be rated as binaries. At 1-2 Myrs, the supposed age of the ONC, the ``transient massive systems become increasingly stable, lasting on average several 10^6 yrs. Despite the ONC being so young, the observed binary frequency for massive stars -- unlike that of solar-mass stars -- is not identical to the primordial binary frequency but is increased by at least 10-15% through dynamical interaction processes. This value might be increased to at least 20-25% by taking disc effects into account. The primordial binary frequency could well be the same for massive and solar mass stars because the observed difference can be explained by capture processes alone.