The Observed Distribution of Spectroscopic Binaries from the Anglo-Australian Planet Search

TL;DR

This paper reports the discovery and analysis of sixteen spectroscopic binary systems from the Anglo-Australian Planet Search, revealing their orbital properties, companion mass distribution, and implications for binary formation mechanisms.

Contribution

It provides the first detailed characterization of binary companions in this survey, including potential brown dwarfs and eccentricity distributions, and compares binary and planetary mass distributions.

Findings

Binary companion mass distribution is flat up to 12 years periods.

Approximately 43% binary fraction across a wide metallicity range.

Distribution contrasts with the rising planetary mass distribution below 10MJ.

Abstract

We report the detection of sixteen binary systems from the Anglo-Australian Planet Search. Solutions to the radial velocity data indicate that the stars have companions orbiting with a wide range of masses, eccentricities and periods. Three of the systems potentially contain brown-dwarf companions while another two have eccentricities that place them in the extreme upper tail of the eccentricity distribution for binaries with periods less than 1000 d. For periods up to 12 years, the distribution of our stellar companion masses is fairly flat, mirroring that seen in other radial velocity surveys, and contrasts sharply with the current distribution of candidate planetary masses, which rises strongly below 10MJ. When looking at a larger sample of binaries that have FGK star primaries as a function of the primary star metallicity, we find that the distribution maintains a binary fraction of ~43$\pm$4% between -1.0 to +0.6 dex in metallicity. This is in stark contrast to the giant exoplanet distribution. This result is in good agreement with binary formation models that invoke fragmentation of a collapsing giant molecular cloud, suggesting this is the dominant formation mechanism for close binaries and not fragmentation of the primary star's remnant proto-planetary disk.