Unexpected Formation Modes of the First Hard Binary in Core Collapse

TL;DR

This paper uses direct N-body simulations to challenge traditional assumptions about how the first hard binary forms during star cluster core collapse, revealing more complex and varied formation modes than previously thought.

Contribution

It demonstrates that the formation of the first hard binary often involves fewer stars and more complex interactions than the standard three-body scenario, due to the small core size at collapse.

Findings

Many three-body interactions produce a hard binary from scratch

More than three stars can be involved simultaneously in binary formation

Core contains only about five stars at deep collapse, invalidating homogeneous background assumptions

Abstract

The conventional wisdom for the formation of the first hard binary in core collapse is that three-body interactions of single stars form many soft binaries, most of which are quickly destroyed, but eventually one of them survives. We report on direct N-body simulations to test these ideas, for the first time. We find that both assumptions are often incorrect: 1) quite a few three-body interactions produce a hard binary from scratch; 2) and in many cases there are more than three bodies directly and simultaneously involved in the production of the first binary. The main reason for the discrepancies is that the core of a star cluster, at the first deep collapse, contains typically only five or so stars. Therefore, the homogeneous background assumption, which still would be reasonable for, say, 25 stars, utterly breaks down. There have been some speculations in this direction, but we demonstrate this result here explicitly, for the first time.