Massive, wide binaries as tracers of massive star formation

TL;DR

The paper demonstrates through simulations that massive wide binaries are indicators of multiple star formation sites, and observations of Cyg OB2 support this, implying it formed from many subregions with a normal high-mass IMF.

Contribution

It introduces a method to infer star formation history from the presence of massive wide binaries using N-body simulations and observational data.

Findings

Any bound cluster tends to have about one massive wide binary.

Cyg OB2's high binary fraction indicates multiple star formation sites.

Cyg OB2 has a normal high-mass initial mass function.

Abstract

Massive stars can be found in wide (hundreds to thousands AU) binaries with other massive stars. We use $N$-body simulations to show that any bound cluster should always have approximately one massive wide binary: one will probably form if none are present initially; and probably only one will survive if more than one are present initially. Therefore any region that contains many massive wide binaries must have been composed of many individual subregions. Observations of Cyg OB2 show that the massive wide binary fraction is at least a half (38/74) which suggests that Cyg OB2 had at least 30 distinct massive star formation sites. This is further evidence that Cyg OB2 has always been a large, low-density association. That Cyg OB2 has a normal high-mass IMF for its total mass suggests that however massive stars form they 'randomly sample' the IMF (as the massive stars did not 'know' about each other).