# Forming Pop III binaries in self-gravitating disks: how to keep the orbital angular momentum

## TL;DR

This study uses 3D hydrodynamics simulations to explore how Population III binary stars form in self-gravitating disks, highlighting the roles of gravitational torque and tidal disruption in angular momentum transfer.

## Contribution

It introduces a controlled simulation approach to understand the diversity of fragment outcomes in Pop III disk fragmentation, emphasizing angular momentum extraction mechanisms.

## Key findings

- Survivor binaries expand while accreting disk gas.
- Disk spiral torque and tidal disruption are key to angular momentum removal.
- A gap-opening window exists depending on envelope mass.

## Abstract

The disk fragmentation is a possible process leading to the formation of Population III stellar binary systems. However, numerical simulations show diverse fates of the fragments; some evolve into stable binaries and others merge away with a central star. To clarify the physics behind such diversity, we perform a series of three dimensional hydrodynamics simulations in a controlled manner. We insert a point particle mimicking a fragment in a self-gravitating disk, where the initial mass and position are free parameters, and follow the orbital evolution for several tens of orbits. The results show great diversity even with such simple experiments. Some particles shortly merge away after migrating inward, but others survive as the migration stalls with the gap-opening in the disk. We find that our results are well interpreted postulating that the orbital angular momentum is extracted by (i) the gravitational torque from the disk spiral structure, and (ii) tidal disruption of a gravitationally-bound envelope around the particle. Our analytic evaluations show the processes (i) and (ii) are effective in an outer and inner part of the disk respectively. There is a window of the gap-opening in the middle, if the envelope mass is sufficiently large. These all agree with our numerical results. We further show that the binaries, which appear for the "survival" cases, gradually expand while accreting the disk gas. Our theoretical framework is freely scalable to be applied for the present-day star and planet formation.