Impact of magnetic braking on high-mass close binary formation

Abstract

Combining numerical simulations and analytical modeling, we investigate whether close binary systems form by the effect of magnetic braking. Using magnetohydrodynamics simulations, we calculate the cloud evolution with a sink, for which we do not resolve the binary system or binary orbital motion to realize long-term time integration. Then, we analytically estimate the binary separation using the accreted mass and angular momentum obtained from the simulation. In unmagnetized clouds, wide binary systems with separations of >100 au form, in which the binary separation continues to increase during the main accretion phase. In contrast, close binary systems with separations of <100 au can form in magnetized clouds. Since the efficiency of magnetic braking strongly depends on both the strength and configuration of the magnetic field, they also affect the formation conditions of a close binary. In addition, the protostellar outflow has a negative impact on close binary formation, especially when the rotation axis of the prestellar cloud is aligned with the global magnetic field. The outflow interrupts the accretion of gas with small angular momentum, which is expelled from the cloud, while gas with large angular momentum preferentially falls from the side of the outflow on to the binary system and widens the binary separation. This study shows that a cloud with a magnetic field that is not parallel to the rotation axis is a favorable environment for the formation of close binary systems.

The role of magnetic fields in the formation of multiple massive stars

“…Ideal MHD is found to overestimate the magnetic field strength in the densest (ρ 10 −15 g cm −3 ) regions (Masson et al 2016), impacting both disk formation and outflows (Commerçon et al 2022). Nevertheless, the present study supports the interpretation of Harada et al (2021) that magnetic fields, in the form of magnetic braking, could play a prominent role in the formation of massive close binaries -at the condition of a coherent rotation structure such as a circumbinary disk -, at least initially. Further in time, however, gas accretion equally widens binaries in the hydrodynamical and in the magnetized cases, in the absence of a circumbinary accretion structure.…”

“…Magnetic fields may also play a role in the orbital separation of binaries. Recently, Harada et al (2021) reported a numericaland-analytical study of the impact of magnetic braking on massive close binaries, in the ideal MHD framework. They show that the orbital separation is larger in the hydrodynamical case than when magnetic fields are introduced (their Eq.…”

Discovery of a 500 au Protobinary in the Massive Prestellar Core G11.92–0.61 MM2

“…With ample fuel available within the gas-rich protocluster environment, the protostellar masses (and luminosities) are expected to increase with time. Accretion will also affect the binary separation, which can increase or decrease depending on turbulence, magnetic field strength, and the presence of outflows, with magnetic fields promoting the formation of close high-mass binary systems (e.g., Lund & Bonnell 2018;Harada et al 2021;Ramírez-Tannus et al 2021).…”

Discovery of a 500 au Protobinary in the Massive Prestellar Core G11.92-0.61 MM2

“…Accretion will also affect the binary separation, which can increase or decrease depending on turbulence, magnetic field strength, and the presence of outflows, with magnetic fields promoting the formation of close high-mass binary systems (e.g. Lund & Bonnell 2018;Harada et al 2021;Ramírez-Tannus et al 2021). Future high-resolution observations of MM2E/MM2W -at shorter wavelengths to better measure the protostellar luminosities, at longer wavelengths to search for line emission in a regime where the dust is optically thin, and in full polarization to measure the magnetic fieldwill provide a powerful test case for models of high-mass binary formation.…”