The binary content of multiple populations in NGC 3201

¹ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
e-mail: s.kamann@ljmu.ac.uk
² Institute for Astrophysics, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
³ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ Leibniz-Institute for Astrophysics, An der Sternwarte 16, 14482 Potsdam, Germany

Received: 4 October 2019
Accepted: 5 February 2020

Abstract

We investigate the binary content of the two stellar populations that coexist in the globular cluster NGC 3201. Previous studies of binary stars in globular clusters have reported higher binary fractions in their first populations (P1, having field-like abundances) compared to their second populations (P2, having anomalous abundances). This is interpreted as evidence for the latter forming more centrally concentrated. In contrast to previous studies, our analysis focusses on the cluster centre, where comparable binary fractions between the populations are predicted because of short relaxation times. However, we find that even in the centre of NGC 3201, the observed binary fraction of P1 is higher (23.1 ± 6.2)% compared to (8.2 ± 3.5)% in P2. Our results are difficult to reconcile with a scenario in which the populations only differ in their initial concentrations, but instead suggests that the populations also formed with different fractions of binary stars.