|dc.description.abstracteng||The natural constants in the universe are adjusted so that dense and massive star forming regions can end up in gravitational bound star clusters, called globular clusters. These objects are found in all galaxies and are building blocks of galaxies and the universe. The kinematic properties of globular clusters cannot be explained by that of a population consisting only of single stars. In fact, an important aspect of these clusters are binary star systems, as they alter the dynamical evolution. To date, there are some estimates of the binary fraction of Galactic globular clusters, but very little is known about the properties of binary systems themselves. With their high mass, black holes can also have an important impact on the cluster evolution. However, black holes are extremely difficult to detect unless they are actively accreting.
In order to gain insights into the binary properties, such as orbital period, multi-epoch spectroscopic observations of individual stars in a given cluster are generally needed. However, classical spectrographs only offer limited capabilities when it comes to probing the crowded central regions of globular clusters where most of the binary systems are expected to be found. This is where the Multi Unit Spectroscopic Explorer (MUSE), a second generation instrument mounted on one of the Very Large Telescopes (VLTs), comes into play. With MUSE, the spectroscopy of individual stars in cluster cores became not only feasible, but also observationally efficient. This work makes use of multi-epoch MUSE observations of 27 Galactic globular clusters, including more than 380000 stars and 1400000 individual spectra, to derive radial velocities that allow the identification and characterisation of binary systems.
For the first time in astronomy, three binary systems, each composed of a black hole and a visible stellar companion, were discovered by this blind spectroscopic survey. A fit of a Keplerian orbit to these systems allowed us to infer the properties which are accessible by the radial velocity method. These findings helped to understand the retention fraction of black holes and how they affect the dynamical properties of globular clusters. The identification of these three black holes in NGC 3201 provided evidence that the cluster hosts an extensive subsystem of black holes including at least 40 objects. Black holes, as well as the binary systems, provide the cluster with an extra source of energy that explains why the core of NGC 3201 is not as dense as expected for a 12 Gyr old simple stellar population.
The pilot study on the globular cluster NGC 3201 shows what can be learned about the binary content of a globular cluster when using state-of-the-art observations and simulations. A new statistical method was developed to infer the binary probability of individual stars based on noisy and sparse radial velocities taking data from all stars of a given cluster into account. With the help of a sophisticated MOCCA globular cluster model of NGC 3201, the total binary fraction of 6.8 %, freed from many observational biases, was derived for this cluster. This is the most accurate estimate obtained to date. The best fitting MOCCA model also suggests that NGC 3201 was born with a large binary fraction (≥ 50 %) and that the present day binary population consists mainly of primordial binaries. For the first time in a study of globular clusters, Keplerian orbits to a significant sample of 95 binaries were obtained. The periods, eccentricities, and derived minimum companion masses of these systems give insights into the binary population of NGC 3201. The combination of Hubble Space Telescope (HST) photometry and literature data with the MUSE star sample of NGC 3201 revealed the binary nature and spectral properties of peculiar objects such as blue straggler stars, sub-subgiants, and eclipsing binaries. This showed a high blue straggler binary fraction of at least 58 % and evidence for two blue straggler formation scenarios in NGC 3201. On the one hand, blue stragglers are formed involving mass transfer in a binary or triple star system. On the other hand, blue stragglers are formed by the coalescence of two stars following the encounter of two binary systems.
It is now known that most Galactic globular clusters do not consist of only one primordial stellar population, but have instead multiple populations that differ, for example, in elemental abundances. NGC 3201 shows two distinct populations and it was a logical step to compare the binary contents of these populations with each other. We found, that the binary fraction of the first population is significantly higher than the fraction of the second population. Previous studies have shown similar results for the outer regions of other clusters, but this work was the first to find such results in a cluster centre. This challenges some theories that expect the binary fractions of different populations in cluster centres to be the same. In the core of NGC 3201 the most plausible explanation is that the populations were formed with different primordial binary fractions, which is conceivable if the second population has been formed within and after the already formed first population.||de