Binary properties of the globular cluster 47 Tuc (NGC 104). A dearth of short-period binaries

TL;DR

This study uses spectroscopic data to analyze binary star properties in 47 Tuc, revealing a very low fraction of short-period binaries and no massive dark companions, challenging existing models of stellar evolution.

Contribution

It provides the first large spectroscopic survey of binaries in 47 Tuc, quantifying binary fractions and period distributions, and highlights discrepancies with theoretical predictions.

Findings

Binary fraction of (2.4±1.0)% in 47 Tuc

Higher binary fraction among blue stragglers

Absence of short-period and massive dark companion binaries

Abstract

Spectroscopic observations of binary stars in globular clusters are essential to shed light on the poorly constrained period, eccentricity, and mass ratio distributions and to develop an understanding of the formation of peculiar stellar objects. 47 Tuc (NGC 104) is one of the most massive Galactic globular clusters, with a large population of blue stragglers and with many predicted but as-yet elusive stellar-mass black holes. This makes it an exciting candidate for binary searches. We present a multi-epoch spectroscopic survey of 47 Tuc with the VLT/MUSE integral field spectrograph to determine radial velocity variations for 21,699 stars. We find a total binary fraction in the cluster of $(2.4\pm1.0)\%$, consistent with previous photometric estimates, and an increased binary fraction among blue straggler stars, approximately three times higher than the cluster average. We find very few binaries with periods below three days, and none with massive dark companions. A comparison with predictions from state-of-the-art models shows that the absence of such short-period binaries and of binaries with massive companions is surprising, highlighting the need to improve our understanding of stellar and dynamical evolution in binary systems.