The multiplicity of massive stars

TL;DR

This paper reviews the multiplicity of massive stars, focusing on binary properties, observational methods, and statistical distributions, highlighting the prevalence and characteristics of OB star binaries in different environments.

Contribution

It provides a comprehensive overview of the parameter space, binary fractions, and distribution functions of massive star binaries, with new insights into their period and mass-ratio distributions.

Findings

Binary fraction of OB stars is approximately 44%.

Period distribution is bi-modal with a break around 10 days.

Mass-ratio distribution is uniform between 0.2 and 1.0.

Abstract

Binaries are excellent astrophysical laboratories that provide us with direct measurements of fundamental stellar parameters. Compared to single isolated star, multiplicity induces new processes, offering the opportunity to confront our understanding of a broad range of physics under the extreme conditions found in, and close to, astrophysical objects. In this contribution, we will discuss the parameter space occupied by massive binaries, and the observational means to investigate it. We will review the multiplicity fraction of OB stars within each regime, and in different astrophysical environments. In particular we will compare the O star spectroscopic binary fraction in nearby open clusters and we will show that the current data are adequately described by an homogeneous fraction of f~0.44. We will also summarize our current understanding of the observed parameter distributions of O+OB spectroscopic binaries. We will show that the period distribution is overabundant in short period binaries and that it can be described by a bi-modal Oepik law with a break point around P~10d. The distribution of the mass-ratios shows no indication for a twin population of equal mass binaries and seems rather uniform in the range 0.2< q=M_2/M_1<1.0.