Testing the universality of star formation - I. Multiplicity in nearby star-forming regions

TL;DR

This study compares binary star statistics across five nearby star-forming regions using uniform criteria, finding that initial conditions are likely clumpy with a high binary fraction, supporting the universality of star formation.

Contribution

It demonstrates that only a clumpy initial distribution with high binary fractions can reproduce observed cluster properties, suggesting universal star formation conditions.

Findings

Binary fraction decreases with cluster density mainly due to Taurus.

Clumpy initial conditions with high binary fraction reproduce observations.

Most stars, including M-dwarfs, likely form in binaries.

Abstract

We have collated multiplicity data for five clusters (Taurus, Chamaeleon I, Ophiuchus, IC348, and the Orion Nebula Cluster). We have applied the same mass ratio (flux ratios of delta K <= 2.5) and primary mass cuts (~0.1-3.0 Msun) 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 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. 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. 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). Therefore, if star formation is universal the initial conditions must be clumpy and with a high (but not 100 per cent) binary fraction. This could suggest that most stars, including M-dwarfs, form in binaries.