Magnetized Binary Star Formation

Zhao, Bo, Astronomy - Graduate School of Arts and Sciences, University of Virginia
Li, Zhi-Yun, Department of Astronomy, University of Virginia

The majority of stars reside in multiple systems, especially binaries. The formation and early evolution of binaries is a longstanding problem in star formation that is not yet fully understood. In particular, how the magnetic field observed in the star-forming cores of molecular clouds shapes the binary characteristics remains relatively unexplored. In this thesis, we present a systematic study of magnetized binary formation using the Enzo magnetohydrodynamics code. We investigate separately the initial core fragmentation leading to the formation of two binary seeds, and the binary orbital evolution during the subsequent protostellar mass accretion phase. The famous “magnetic flux problem” in star formation is also studied numerically in detail.

Star-forming dense cores are observed to be significantly magnetized. If the magnetic flux threading the cores is dragged into the stars, the stellar field would be orders of magnitude stronger than observed; most of the core magnetic flux must be decoupled from the matter that enters the star. In our simulations, we find that the accumulation of the decoupled flux near the accreting protostar leads to a magnetic pressure buildup. The high pressure is released anisotropically along the path of least resistance. It drives a low density expanding region in which the decoupled magnetic flux is expelled. In the presence of an initial core rotation, the structure presents an obstacle to the formation of a rotationally supported disk, in addition to magnetic braking, by acting as a rigid magnetic wall that prevents the rotating gas from completing a full orbit around the central object.

In the study of the magnetized binary formation during the protostellar mass accretion phase, we demonstrate numerically that a magnetic field of the observed strength can drastically change two of the basic quantities that characterize a binary system: the orbital separation and mass ratio of the two components. We find that in dense cores magnetized to a realistic level, the angular momentum of the material accreted by the protobinary is greatly reduced by magnetic braking. Accretion of strongly braked material shrinks the protobinary separation by a large factor compared to the non-magnetic case. The magnetic braking also changes the evolution of the mass ratio of unequal mass protobinaries by producing material of low specific angular momentum that accretes preferentially onto the more massive primary star rather than the secondary. In addition, the magnetic field greatly modifies the morphology and dynamics of the protobinary accretion flow.

We also investigate the effects of the misalignment between the magnetic field and rotation axis on the properties of the protobinaries; such a misalignment was recently revealed by millimeter interferometric observations of protostellar systems. Somewhat surprisingly, we find that the misaligned magnetic field is more efficient at tightening the binary orbit compared to the aligned field. The main reason is that the misalignment weakens the magnetically-driven outflow, which allows more material to accrete onto the binary. The additional mass being accreted onto the binary carries insufficient specific angular momentum, which leads to an even tighter binary. A large field rotation misalignment also helps produce rotationally-supported circumbinary disks even for relatively strong magnetic fields, by weakening the magnetically-dominated structure close to the binary that is responsible for strong magnetic braking in the aligned case.

Finally, we present preliminary results on simulations that follow both the (magnetized) core fragmentation into binary seeds and the subsequent protobinary evolution during the mass accretion phase. We find that fragmentation can occur in a strongly magnetized core as long as the initial density perturbation is large. The resulting binary seeds generally have a large separation and eccentricity. After the first close approach, the binary can either merge or dynamically relax to a stable orbit depending on the magnetic field strength and orientation. Additional simulations and more detailed analysis are needed to obtain a complete picture of how binary systems form and evolve in magnetized clouds.

PHD (Doctor of Philosophy)
star formation, binary system, magnetic field
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