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

• Published in: The Astrophysical journal 2019-12, Vol.887 (2), p.232
• Main Authors: Lee, Aaron T., Offner, Stella S. R., Kratter, Kaitlin M., Smullen, Rachel A., Li, Pak Shing
• Format: Article
• Language: English
• Subjects: Astrophysical fluid dynamics; Astrophysics; Binary stars; Cloud formation; Companion stars; Computational fluid dynamics; Computer simulation; Deposition; Field strength; Filaments; Fluid flow; Fragmentation; Interstellar clouds; Interstellar medium; Magnetic fields; Magnetism; Magnetohydrodynamic simulation; Magnetohydrodynamic turbulence; Magnetohydrodynamics; Milky Way; Misalignment; Multiple stars; Orbital mechanics; Protostars; Simulation; Star formation; Stars; Stellar dynamics; Stellar evolution; Stellar system evolution; Stellar systems
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ab584b

Stars rarely form in isolation. Nearly half of the stars in the Milky Way have a companion, and this fraction increases in star-forming 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 ( 103 au) turbulent fragmentation. We investigate three global magnetic field strengths, with global mass-to-flux ratios of φ = 2, 8, and 32. The initial separations of protostars in multiples depend on the global magnetic field strength, with stronger magnetic fields (e.g., φ = 2) suppressing fragmentation on smaller scales. The overall multiplicity fraction (MF) is between 0.4 and 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 misalignment angles is consistent with a randomized distribution. In all three simulations, several binaries originate with wide separations and dynamically evolve to 102 au separations. We show that a simple model of mass accretion and dynamical friction with the gas can explain this orbital evolution.