First, we construct (in as much as is possible) a uniform comparison of binary fractions in the same separation ranges for five different regions (Taurus, Ophiuchus/L1688, Chamaeleon I, IC 348 and the ONC). In this paper, we approach the problem of examining differences between primordial binary populations with a two-fold method. A major aspect of this problem is that it is difficult to compare the observed binary populations of different regions due to differences in the separation range probed and the sensitivity to lower mass companions between different surveys. The problem is that it is difficult to ‘reverse engineer’ the current binary population of a cluster to determine the primordial population (reverse population synthesis, Kroupa 1995b). However, Parker, Goodwin & Allison (2011) suggest that even with dynamical processing the primordial binary populations of Taurus and the ONC were probably different (see also Kroupa & Petr-Gotzens 2011 Marks et al. Kroupa (1995a, b) showed that it is possible to process a Taurus-like primordial binary population into an Orion-like evolved population very quickly. In a dense environment, encounters are common and binaries will tend to be destroyed ( Heggie 1975 Hills 1975).

However, we know that dense regions will process their primordial binary populations and what we see at later times may not reflect the primordial population. 2007) and approximately twice that seen in the field ( Duquennoy & Mayor 1991 Fischer & Marcy 1992 Raghavan et al. 1993) is significantly higher than the binary fraction of the Orion Nebula Cluster (hereafter ONC, Prosser et al. Most famously, the binary fraction of Taurus ( Leinert et al. Indeed, differences between the binary populations of different clusters have been observed. A more promising route might be to search for differences between primordial binary populations – if two regions produce very different binary populations, then this suggests that star formation was different between these regions (see Duchêne et al. Such a result is rather surprising and interesting, but it means that determinations of the IMF are unable to probe the universality, or otherwise, of star formation. There is no evidence that the initial mass function (IMF) of stars varies systematically between different environments (see e.g. One of the major unsolved problems in star formation is the universality of the process: is the difference between small, local star-forming regions such as Taurus (∼10 2 M ⊙) and massive starburst clusters like 30 Doradus (∼10 5 M ⊙) merely one of the level of star formation, or is there something fundamentally different between these two extremes?

How stars form is extremely interesting in itself, but also has huge implications for understanding galaxy formation and evolution, and planet formation.

Star formation is one of the outstanding problems in astrophysics. Methods: numerical, binaries: general, stars: formation, stars: kinematics and dynamics, open clusters and associations: general 1 Introduction This could suggest that most stars, including M dwarfs, form in binaries. Therefore, if star formation is universal, then the initial conditions must be clumpy and with a high (but not 100 per cent) binary fraction. Only an initially clumpy (fractal) distribution with an initial total binary fraction of 73 per cent (17 per cent in the range 62–620 au) is able to reproduce all of the observations (albeit not very satisfactorily). With N-body simulations, we attempt to find a set of initial conditions that are able to reproduce the density, morphology and binary fractions of all five clusters. We find that the trend of decreasing binary fraction with cluster density is solely due to the high binary fraction of Taurus the other clusters show no obvious trend over a factor of nearly 20 in density. We have applied the same mass ratio (flux ratios of Δ K≤ 2.5) and primary mass cuts (∼0.1–3.0 M ⊙) to each cluster and therefore have directly comparable binary statistics for all five clusters in the separation range 62–620 au, and for Taurus, Chamaeleon I and Ophiuchus in the range 18–830 au. We have collated multiplicity data for five clusters (Taurus, Chamaeleon I, Ophiuchus, IC 348 and the Orion Nebula Cluster).

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