title: Investigating the Spectroscopic Binary Population of Omega Centauri with MUSE
Creators: name: Wragg, F
Publisher: name: Liverpool John Moores University
Year: 2024

description: Globular clusters are a crucial component of their host galaxies. They are ubiquitous in both distant and local galaxies, including our own Milky Way. Despite their importance, however, some fundamental questions about globular cluster formation and evolution are still unanswered: how many black holes are retained in clusters, and could more massive clusters host an illusive intermediate mass black hole (IMBH)? How do multiple populations form, creating populations of stars each with distinct chemistry? Can any links be made between young and ancient star clusters? And can galactic archaeology identify the population of accreted globular clusters, including bright, massive nuclear star clusters (NSCs) that form in the centres of their host galaxies, and track the merger history of the galaxy? A key topic to understanding globular clusters is binary stars. Binary systems are present at the birth of the cluster and act to delay the later stages of cluster evolution. They can also provide insights on the properties of the cluster, namely the number and masses of black holes within the cluster centre, and the formation conditions and evolution of multiple populations. And yet, for many of the most complex and unusual clusters in the Milky Way, the binary fraction, and the demographics of binary stars, are still unknown. NGC 5139 (ω Cen), is the closest candidate of a NSC that has been stripped of its host galaxy in the Milky Way. Despite extensive studies through the last decades, many open questions about the cluster remain, including the properties of the binary population. In this study we use multi-epoch spectroscopy from the Multi Unit Spectroscopic Explorer (MUSE) to identify binary systems in ω Cen. The observations span 8 years, with a total of 312 248 radial velocity measurements for 37 225 stars. Following the removal of known photometric variables, and using dedicated simulations, we identify 275 stars that show radial velocity variations, corresponding to a discovery fraction of 1.4±0.1%, and show that our data is sensitive to 70±10% of the binaries expected in the sample. By combining these results, we calculate a completeness-corrected binary fraction of 2.1±0.4% in the central region of ω Cen. We find similar binary fractions for all stellar evolutionary stages covered by our data, the only notable exception being the blue straggler stars, which show an enhanced binary fraction. We also find no distinct correlation with distance from the cluster centre, indicating a limited amount of mass segregation within the half-light radius of ω Cen. By using a combination of spectroscopy and photometry, we then investigate the binary fraction across the various stellar populations of ω Cen. By separating our sample into populations grouped by metallicity and light element abundance respectively, we separate the red giant branch of ω Cen into 9 chemically distinct populations. Overall, we find no statistically significant trend in binary fraction across the different populations. This result may be due to the effect of cluster relaxation, which will erase the distributions of populations over time.