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Massive Binary Stars and the Kinematics of Young Massive Clusters Vincent H´enault-Brunet Doctor of Philosophy The University of Edinburgh July 2013 Abstract Located in the Large Magellanic Cloud, R136 is a rare example of a nearby young and dense massive star cluster in which individual stars can be resolved. Often suggested as a globular cluster in formation, its study is of great interest and promises to provide insights into the early dynamical evolution of massive star clusters. This is crucial to understand more extreme and distant starburst clusters, which contribute to a significant fraction of all current star formation in the Local Universe, in particular in interacting galaxies. The majority of this thesis is based on multi-epoch spectroscopic observations in and around R136 obtained as part of the VLT-FLAMES Tarantula Survey (VFTS), an ambitious programme which targeted nearly 1 000 massive stars in the intricate 30 Doradus star-forming region. The motivations and observing strategy of this survey, designed to address key questions about the evolution of massive stars and clusters, are first introduced. The data reduction procedures applied to VFTS data are described, with an emphasis on the tasks accomplished in the context of this thesis. The VFTS data are first used to perform a detailed kinematic study of R136, determine its dynamical state, and evaluate the importance of gas expulsion in the early evolution of massive star clusters. Orbital motions of binary stars are found to dominate the line- of-sight velocity dispersion of the cluster, illustrating the risk of interpreting velocity dispersion measurements for unresolved extragalactic young massive clusters. However, once the detected binaries are rejected and the contribution of undetected binaries is accounted for through Monte Carlo simulations, the true velocity dispersion of the cluster is found to be low and consistent with it being in virial equilibrium. This suggests that gas expulsion has not had a dramatic effect on the early dynamical evolution of R136. Using the velocity measurements of R136 as a test case, a maximum likelihood method that fits the velocity dispersion of a cluster from a single epoch of radial velocity data i is then tested. The method must be applied with care given the high binary fraction of massive stars and the large uncertainties in their binary orbital parameter distributions, but for typical velocity dispersions of young massive clusters (& 4 kms−1), it is shown that the velocity dispersion can be measured with an accuracy of 40% or better. This offers an efficient way of constraining the dynamics of these systems. The radial velocity measurements of apparently single stars in R136 are also used to investigate the internal rotation of the cluster, a potentially important but largely unexplored characteristic of young clusters. Evidence is found, at the 95% confidence level, for rotation of the cluster as a whole. A simple maximum likelihood method is presented to fit rotation curves to the data, from which a typical rotational velocity of 3 kms−1 is found. When compared to the low velocity dispersion of R136, this ∼ suggests that star clusters may form with as much as 20% of their kinetic energy in ∼ rotation. Finally, a smaller-scale survey of massive stars in the Wing of the Small Magellanic Cloud is introduced. As an example of the particularly interesting massive binaries that can be revealed by the synergy between large optical spectroscopic surveys of young clusters and observations at other wavelengths, the discovery of a new Be/X-ray pulsar binary and associated supernova remnant is reported. With a long spin period of over 1 000 seconds and a young age of 104 years constrained by its association with ∼ the supernova remnant, the pulsar in this system is quickly emerging as a unique object that challenges our understanding of the spin evolution of accreting neutron stars. ii Lay Summary Stars often form in groups called clusters, which are sometimes considered as the building blocks of galaxies. Understanding in detail how these clusters form, evolve, and are destroyed can provide important insights on the properties and evolution of their host galaxies. The so-called globular star clusters, for example, typically contain hundreds of thousands of stars each and, given their very old ages of several billion years, are relics of the early assembly of galaxies. To understand the early phases of the evolution of such clusters, our best hope is to study much younger clusters that are similarly massive. The central part of this thesis consists of a study of the velocities of stars in an important young massive cluster, called R136, one of the few nearby enough for individual stars to still be distinguishable with large telescopes. The stellar velocities hold precious information about the history of such systems. However, to successfully recover that history, care must be taken to isolate unwanted effects that could blur the information contained in the velocities. One particularly harmful effect is the rapid motions of stars orbiting each other in close binary systems. An important aspect of this thesis is to explore methods to take binary stars into account. Once this is done for the case of the young massive cluster R136, it is found that it had a quieter infancy than originally thought despite the rapid expulsion of gas from its natal cocoon. Consequently, it will probably be a long-lived cluster. It is also shown that R136 is probably rotating, and the implications of that for the formation and evolution of star clusters are discussed. Finally, as an example of the interesting objects that can be unveiled when studying young clusters in detail, the discovery of a pulsar in a binary system is presented. This pulsar is the compact rotating remnant of a massive star that died at a young age in a supernova explosion. The age of the remnant could be constrained by studying the shell of gas swept by the explosion, which was also discovered as part of this thesis. The pulsar is intriguingly found to be rotating much slower than expected given its young age, and promises to reveal important clues about the physics of such stellar remnants. iii R´esum´een fran¸cais pour les non-sp´ecialistes Les ´etoiles sont souvent form´ees en groupes appel´es amas, qui peuvent ˆetre consid´er´es comme des composantes fondamentales des galaxies. Comprendre en d´etails comment ces amas naissent, ´evoluent et sont d´etruits peut fournir d’importants indices sur les propri´et´es et l’´evolution des galaxies dans lesquelles ils se trouvent. Les amas dits globulaires, par exemple, contiennent typiquement plusieurs centaines de milliers d’´etoiles chacun et, compte tenu de leur ˆage de plusieurs milliards d’ann´ees, constituent des reliques d’une ´epoque lointaine de la formation des galaxies. Pour comprendre les d´ebuts de l’´evolution de tels amas, notre meilleur espoir est d’´etudier des amas beaucoup plus jeunes mais tout aussi massifs. La partie central de cette th`ese est port´ee sur l’´etude des vitesses des ´etoiles dans un important amas jeune et massif situ´edans le Grand Nuage de Magellan, une galaxie voisine de notre Voie Lact´ee. Appel´eR136, cet amas est un des rares qui soit assez proche pour que des ´etoiles individuelles puissent ˆetre discern´ees avec l’aide de grands t´elescopes. Les vitesses des ´etoiles contiennent de pr´ecieuses informations sur l’histoire de tels syst`emes. Par contre, pour en profiter, il faut prendre bien soin d’isoler certains effets ind´esirables qui pourraient brouiller ces informations. Un effet particuli`erement n´efaste est le mouvement rapide d’´etoiles en orbite dans des syst`emes binaires. Un aspect important de cette th`ese consiste `aexplorer des m´ethodes pour prendre en compte les mouvements des ´etoiles dans ces syst`emes binaires. Une fois ces m´ethodes appliqu´ees `aR136, il est d´emontr´eque l’amas a eu une enfance plus calme qu’autrefois anticip´ee, et ce malgr´el’expulsion rapide du gaz de son cocon natal. En cons´equence, R136 devrait survivre pour plusieurs milliards d’ann´ees. Il est aussi d´emontr´eque R136 montre un taux de rotation ´elev´ee, et les implications de cette d´ecouverte pour la formation et l’´evolution des amas sont discut´ees. Finalement, en tant qu’exemple d’astres int´eressants qui peuvent ˆetre d´evoil´es par iv l’´etude d´etaill´ee de jeunes amas, la d´ecouverte d’un pulsar membre d’un syst`eme binaire est pr´esent´ee. Ce pulsar est le cadavre stellaire compact d’une ´etoile beaucoup plus massive que le Soleil morte `aun jeune ˆage suite `ason explosion en supernova. L’ˆage du pulsar a pu ˆetre estim´een ´etudiant la coquille de gaz balay´ee par l’explosion, ´egalement d´ecouverte dans le cadre de cette th`ese.