The Formation and Evolution of Wide-orbit Stellar Multiples In Magnetized Clouds

Abstract

Stars rarely form in isolation. Nearly half of the stars in the Milky Way have a companion, and this fraction increases in starforming regions. However, why some dense cores and filaments form bound pairs while others form single stars remains unclear. We present a set of three-dimensional, gravo-magnetohydrodynamic simulations of turbulent star-forming clouds, aimed at understanding the formation and evolution of multiple-star systems formed through large scale ( 10 3 AU) turbulent fragmentation. We investigate three global magnetic field strengths, with global mass-to-flux ratios of µ φ = 2, 8, and 32. The initial separations of protostars in multiples depends on the global magnetic field strength, with stronger magnetic fields (e.g., µ φ = 2) suppressing fragmentation on smaller scales. The overall multiplicity fraction (MF) is between 0.4 − 0.6 for our strong and intermediate magnetic field strengths, which is in agreement with observations. The weak field case has a lower fraction. The MF is relatively constant throughout the simulations, even though stellar densities increase as collapse continues. While the MF rarely exceeds 60% in all three simulations, over 80% of all protostars are part of a binary system at some point. We additionally find that the distribution of binary spin mis-alignment angles is consistent with a randomized distribution. In all three simulations, several binaries originate with wide separations and dynamically evolve to 10 2 AU separations. We show that a simple model of mass accretion and dynamical friction with the gas can explain this orbital evolution.

Architecture of Hierarchical Stellar Systems and Their Formation

“…Other mechanisms (disk instability and dynamical interactions) also play a substantial role, and several mechanisms may combine to create a given system. This emerging picture is further confirmed in the simulations by Lee et al [6] and Kuffmeier et al [7] who studied smaller clouds with a higher resolution. In these simulations, companions form sequentially, approach the main star, and migrate to closer orbits owing to the continuing gas accretion and dynamical friction.…”

“…The final stellar masses result from the accretion of gas on time scales much longer than the free-fall time of their natal envelopes and may be unrelated to the initial masses of the small-scale clumps from which the protostars form. Continued accretion on to newly formed binaries shrinks their orbits [6,38]. The origins of the stellar mass distribution (the initial mass function, IMF, and the system mass function, SMF) and its relation to multiplicity are reviewed by Lee et al [39].…”

“…The last stage of the collapse is set by the opacity limit to fragmentation (∼10 au) which also defines the minimum separation of binaries that can be formed by CF. Numerical simulations of an isolated cloud collapse driven by internal turbulence show how several protostars usually are born from the individual over-densities [6,43]. Given that the gravitational collapse creates two-and onedimensional structures, the protostars can form in linear configurations along filaments.…”

Orbital and Mass Constraints of the Young Binary System IRAS 16293-2422 A

“…Misaligned configurations for the rotation axis of individual circumstellar disks, the circumbinary material, and the orbital motion naturally arise in simulations where turbulence is included in the starforming cloud (Offner et al 2010;Bate 2018;Lee et al 2019). For instance, members of a multiple system might form a few thousands of astronomical unit apart, from gas with different angular momentum, and later move closer to form a bound tight binary (or higher multiplicity) system (Offner et al 2016;Bate 2018;Kuffmeier et al 2019;Lee et al 2019). Misalignment can also be the product of binary formation in an elongated structure whose minor axis is misaligned with the initial rotation axis (Bonnell et al 1992).…”

The close binary fraction as a function of stellar parameters in APOGEE: a strong anticorrelation with α abundances

“…One possible explanation is that at least 10 per cent of protostellar disks become massive or cool enough to fragment early in their accretion evolution, regardless of their chemical composition or the final primary mass. Another possibility is that metal-rich and/or low-mass disks are entirely unsusceptible to fragmentation, and the floor of a 10 per cent close binary fraction is actually due to the small fraction of cores that fragment on large scales and subsequently decay to a < 10 au via dynamical friction or exchange interactions (Lee et al 2019;Bate 2019). In the future, measurements of how the close binary fraction of M-dwarfs changes with Fe and α will help differentiate between these two scenarios.…”