On the properties of young multiple stars

We present numerical results on the properties of young binary and multiple stellar systems. Our analysis is based on a series of smoothed particle hydrodynamics (SPH) + N ‐body simulations of the fragmentation of small molecular clouds, which fully resolve the opacity limit for fragmentation. These simulations demonstrate that multiple star formation is a major channel for star formation in turbulent flows. We have produced a statistically significant number of stable multiple systems, with component separations in the range ∼1–10 3 au . At the end of the hydrodynamic stage (0.5 Myr), we find that ≈60 per cent of stars and brown dwarfs are members of multiples systems, with about a third of these being low‐mass, weakly bound outliers in wide eccentric orbits. Our results imply that in the stellar regime most stars are in multiples (≈80 per cent) and that this fraction is an increasing function of primary mass. After N ‐body integration to 10.5 Myr, the percentage of bound objects has dropped to about 40 per cent, this decrease arising mostly from very low‐mass stars and brown dwarfs that have been released into the field. Brown dwarfs are never found to be very close companions to stars (the brown dwarf desert at very small separations), but one case exists of a brown dwarf companion at intermediate separations (10 au). Our simulations can accommodate the existence of brown dwarf companions at large separations, but only if the primaries of these systems are themselves multiples. We have compared the outcome of our simulations with the properties of real stellar systems as deduced from the infrared colour–magnitude diagram of the Praesepe cluster and from spectroscopic and high‐resolution imaging surveys of young clusters and the field. We find that the spread of the observed main sequence of Praesepe in the 0.4– 1 M ⊙ range appears to require that stars are indeed commonly assembled into high‐order multiple systems. Similarly, observational results from Taurus and ρ Ophiuchus, or moving groups such as TW Hydrae and MBM 12, suggest that companion frequencies in young systems can indeed be as high as we predict. The comparison with observational data also illustrates two problems with the simulation results. First, low mass ratio ( q < 0.2) binaries are not produced by our models, in conflict with both the Praesepe colour–magnitude diagram and independent evidence from field binary surveys. Secondly, very low‐mass stars and brown dwarf binaries appear to be considerably underproduced by our simulations.