A stellar census in globular clusters with MUSE: Binaries in NGC 3201

TL;DR

This study uses multi-epoch MUSE spectroscopy and advanced simulations to measure binary star frequencies, analyze their properties, and identify black hole candidates in the globular cluster NGC 3201, revealing insights into binary evolution and cluster dynamics.

Contribution

It introduces a new statistical method for detecting RV variations, infers the true binary fraction, and reports the first orbital fits for a significant binary sample in a globular cluster.

Findings

Binary fraction in NGC 3201 is approximately 6.75%.

Over half of blue straggler stars are in binaries.

Two new black hole candidates were discovered.

Abstract

We utilize multi-epoch MUSE spectroscopy to study binaries in the core of NGC 3201. Our sample consists of 3553 stars with 54883 spectra in total comprising 3200 main-sequence stars up to 4 magnitudes below the turn-off. Each star in our sample has between 3 and 63 (with a median of 14) reliable radial velocity (RV) measurements within five years of observations. We introduce a statistical method to determine the probability of a star showing RV variations based on the whole inhomogeneous RV sample. Using HST photometry and an advanced dynamical MOCCA simulation of this specific GC we overcome observational biases that previous spectroscopic studies had to deal with. This allows us to infer a binary frequency in the MUSE FoV and enables us to deduce the underlying true binary frequency of (6.75+-0.72) % in NGC 3201. The comparison of the MUSE observations with the MOCCA simulation suggests a significant fraction of primordial binaries. We can also confirm a radial increase of the binary fraction towards the GC centre due to mass segregation. We discovered that in our sample at least (57.5+-7.9) % of blue straggler stars (BSS) are in a binary system. For the first time in a study of GCs, we were able to fit Keplerian orbits to a significant sample of 95 binaries. We present the binary system properties of eleven BSS and show evidence that two BSS formation scenarios, the mass transfer in binary (or triple) star systems and the coalescence due to binary-binary interactions, are present in our data. We also describe the binary and spectroscopic properties of four sub-subgiant (or red straggler) stars. Furthermore, we discovered two new black hole (BH) candidates with minimum masses (Msini) of (7.68+-0.50) M_sun, (4.4+-2.8) M_sun, and refine the minimum mass estimate on the already published BH to (4.53+-0.21) M_sun. These BHs are consistent with an extensive BH subsystem hosted by NGC 3201.