The binary content of multiple populations in NGC 3201

TL;DR

This study examines the binary star fractions in the two stellar populations of NGC 3201, revealing unexpected differences in the cluster's center that challenge previous assumptions about their formation.

Contribution

It provides new evidence that the two populations in NGC 3201 formed with different binary fractions, even in the cluster's core, contrary to prior expectations.

Findings

P1 binary fraction: 23.1%

P2 binary fraction: 8.2%

Results challenge models assuming similar binary fractions in the core.

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 focuses on the cluster centre, where comparable binary fractions between the populations are predicted because of the 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 where the populations only differ in their initial concentrations, but instead suggests that the populations also formed with different fractions of binary stars.