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Spectroscopy of Binaries in Globular Clusters Dissertation for the award of the degree “Doctor rerum naturalium” (Dr.rer.nat.) of the Georg-August-Universität Göttingen within the doctoral program PROPHYS of the Georg-August University School of Science (GAUSS) submitted by Benjamin David Giesers from Oldenburg Göttingen, 2019 Thesis Commitee Prof. Dr. Stefan Dreizler Sonnenphysik und Stellare Astrophysik, Institut für Astrophysik, Georg-August-Universität Göttingen, Germany Dr. Sebastian Kamann Star formation and stellar populations, Astrophysics Research Institute, Liverpool John Moores University, United Kingdom Dr. Tim-Oliver Husser Stellare Astrophysik, Institut für Astrophysik, Georg-August-Universität Göttingen, Germany Members of the Examination Board Reviewer: Prof. Dr. Stefan Dreizler Sonnenphysik und Stellare Astrophysik, Institut für Astrophysik, Georg-August-Universität Göttingen, Germany Second Reviewer: Prof. Dr. Ulrich Heber Stellare Astrophysik, Dr. Karl Remeis Sternwarte, Friedrich-Alexander-Universität Erlangen- Nürnberg, Germany Further members of the Examination Board: Dr. Saskia Hekker Stellar Ages and Galactic Evolution, Max-Planck-Institut für Sonnensystemforschung, Göttin- gen, Germany Prof. Dr. Wolfram Kollatschny Extragalaktische Astrophysik, Institut für Astrophysik, Georg-August-Universität Göttingen, Germany Prof. Dr. Wolfgang Glatzel Sonnenphysik und Stellare Astrophysik, Institut für Astrophysik, Georg-August-Universität Göttingen, Germany PD Dr. Jörn Große-Knetter Kern- und Atomphysik, II. Physikalisches Institut, Georg-August-Universität Göttingen, Ger- many Date of the oral examination: 13 December 2019 Abstract The natural constants in the universe are adjusted so that dense and massive star forming re- gions 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 spec- troscopic 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 spectro- scopy of individual stars in cluster cores became not only feasible, but also observationally effi- cient. This work makes use of multi-epoch MUSE observations of 27 Galactic globular clusters, including more than 380 000 stars and 1 400 000 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 Kep- lerian 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 iii 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. iv Acknowledgments I would like to thank Stefan Dreizler very much for enabling me to explore the different aspects of astrophysics. He is responsible for the multitude of opportunities I had during my Bachelor, Master, and PhD projects. I also give special thanks to my additional supervisors Sebastian Kamann and Tim-Oliver Husser, not only for their helpful and patient support, but for all the work done beforehand, without which this work would not have been possible. The opportunities allowed me to collaborate internationally with Guillem Anglada Escudé and Abbas Askar. Both have greatly contributed to the significance of my scientific work. I would like to thank the MUSE consortium for building such a wonderful instrument and organising several inspiring meetings all over Europe with many helpful scientific discussions. I also like to thank ESO for the preparation of the press release (especially Richard Hook) and the unique experience at the Paranal Observatory, which I will never forget. A special thanks goes to the co-authors of my papers, Abbas Askar, Guillem Anglada Es- cudé, Jarle Brinchmann, Peter Weilbacher, Marcella Carollo, Martin Roth, Martin Wendt, Lutz Wisotzki, Nate Bastian, Fabian Göttgens, Marilyn Latour, Tim-Oliver Husser, Sebastian Kamann, and Stefan Dreizler. Finally, I would like to thank a selection of people who have accompanied me on my scientific journey: Jantje Freudenthal, Marvin Böhm, Wolfram Kollatschny, Roland Bacon, Rick Hessman, Arash Bahramian, Robert Mathieu, and Stan Lai. Göttingen, October 2019 Benjamin Giesers v Contents 1. Introduction 1 1.1. Globular clusters . .2 1.1.1. Cluster formation . .4 1.1.2. Cluster evolution . .4 1.1.3. Binaries . .6 1.1.4. Blue stragglers . 10 1.1.5. Black holes . 12 1.1.6. Multiple populations . 14 1.1.7. Simulations . 16 1.2. Multiple star systems and binaries . 17 1.2.1. Observational techniques . 18 1.2.2. The radial velocity technique of binary systems . 19 1.3. Integral-field spectroscopy and MUSE . 21 1.4. MUSE survey of globular clusters . 22 1.4.1. Pilot study . 22 1.4.2. A stellar census of globular clusters with MUSE . 23 1.5. Aims of this work . 26 2. Publications 29 B. Giesers, S. Dreizler, T.-O. Husser, S. Kamann, G. Anglada Escudé, J. Brinchmann, C. M. Carollo, M. M. Roth, P. M. Weilbacher, and L. Wisotzki. A detached stellar- mass black hole candidate in the globular cluster NGC 3201. In Monthly Notices of the Royal Astronomical Society, volume 475, pages L15-L19, March 2018. 31 B. Giesers, S. Kamann, S. Dreizler, T.-O. Husser, A. Askar, F. Göttgens, J. Brinchmann, M. Latour, P. M. Weilbacher, M. Wendt, and M. M. Roth. A stellar census in globular clusters with MUSE: Binaries in NGC 3201. In Astronomy & Astrophysics, volume 632, pages A3, December 2019. 37 S. Kamann, B. Giesers., N. Bastian, J. Brinchmann, S. Dreizler, F. Göttgens, T.-O. Husser, M. Latour, P. M. Weilbacher, and L. Wisotzki. The binary content of multiple popula- tions in NGC 3201. In Astronomy & Astrophysics, volume 635, pages A65, March 2020. 57 3. Conclusions & Outlook 63 3.1. A new statistical method . 63 vii Contents 3.2. The binary fraction of NGC 3201 . 64 3.3. Orbital parameters of 95 binaries . 64 3.4.
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