The primordial binary population II: Recovering the binary population for intermediate mass stars in Sco OB2

TL;DR

This study analyzes the binary star population in Sco OB2, revealing a high binary fraction among intermediate-mass stars and providing detailed distributions of their properties, emphasizing the importance of multiplicity in star formation.

Contribution

It offers the first comprehensive characterization of the binary population in Sco OB2, accounting for observational biases and deriving distributions of binary parameters for intermediate-mass stars.

Findings

Binary fraction > 70% with high confidence

Mass ratio distribution f(q) ~ q^-0.4

Semi-major axis distribution consistent with Opik's law

Abstract

We characterize the binary population in the young and nearby OB association Scorpius OB2 using available observations of visual, spectroscopic, and astrometric binaries with intermediate-mass primaries. We take into account observational biases by comparing the observations with simulated observations of model associations. The available data indicate a large binary fraction (> 70% with 3sigma confidence), with a large probability that all intermediate mass stars in Sco OB2 are part of a binary system. The binary systems have a mass ratio distribution of the form f(q) ~ q^-0.4. Sco OB2 has a semi-major axis distribution of the form f(log a) ~ constant (Opik's law), in the range 5-5e6 Rsun. The log-normal period distribution of Duquennoy & Mayor results in too few spectroscopic binaries, even if the model binary fraction is 100%. Sco OB2 is a young association with a low stellar density; its current population is expected to be very similar to the primordial population. The fact that practically all stars in Sco OB2 are part of a binary (or multiple) system demonstrates that multiplicity is a fundamental factor in the star formation process, at least for intermediate mass stars.