SCYON Abstract

Received on December 12 2011

Mass segregation and fractal substructure in young massive clusters: (I) the McLuster code and method calibration

Authors	Andreas H.W. Küpper (¹)
Affiliation	(¹) Argelander Institut für Astronomie (AIfA), Auf dem Hügel 71, 53121 Bonn, Germany
Thesis work conducted at	University of Bonn, University of Edinburgh, ESO Vitacura; Ph.D. Thesis directed by: Pavel Kroupa (1st supervisor), Holger Baumgardt (2nd supervisor), Douglas C. Heggie & Steffen Mieske; Ph.D. Degree awarded: 21st October 2011
Contact	akuepper@astro.uni-bonn.de
URL	http://www.astro.uni-bonn.de/~akuepper/index.html
Links	The McLuster code is available from the following URL: http://www.astro.uni-bonn.de/~akuepper/mcluster/mcluster.html

Abstract

The majority of stars is born in star clusters of different sizes and masses. But star clusters are dynamically unstable objects; they dissolve due to a continuos exchange of energy between the constituent stars. In this thesis, the dissolution process of massive star clusters is studied by means of high-performance N-body computations. Such investigations have only recently become feasible due to the availability of sophisticated accelerator hardware and adapted codes like Nbody6.
First, the formation of tidal tails by star clusters which orbit about a galaxy is studied. It is found that stars, when they escape, move on epicyclic orbits away from the cluster. This motion produces a standing wave in the tidal tails when performed by a continuous stream of escapers, and can be detected as overdense and underdense regions in such stellar streams. This mechanism is investigated for a broad range of cluster types and orbits. Moreover, it is tested whether other mechanisms, e.g. tidal shocks, can produce overdensities in tidal tails as well. It is shown that epicyclic motion is the main reason for substructure in the tidal tails of all investigated star clusters. Hence, the substructures existent in observed tidal tails of Milky-Way globular clusters (e.g. Palomar 5) are likely to be epicyclic overdensities.
Furthermore, it is demonstrated that the dissolution process has a strong influence on the appearance of star clusters. Prior to escape, energetically unbound stars orbit for some time within the cluster and cause the cluster's velocity dispersion to deviate from its Newtonian expectation in the outer parts of the cluster. Moreover, for star clusters on eccentric orbits, the distance between stars in the tails and the cluster changes periodically due to differential acceleration. That is, between apogalacticon and perigalacticon this distance increases, whereas it decreases on the way back to apogalacticon. This orbital compression causes a periodically changing stellar density in the outer part of such a cluster. While star clusters usually show a slope of R^-5 at large cluster radii within their projected radial surface density profile, this slope increases up to R^-1 for clusters on eccentric orbits close to apogalacticon. Therefore, the stellar density in the outer parts of a cluster can be used to gain information on its orbital phase. In a case study of the globular cluster Palomar 13 this behaviour is illustrated.
Finally, the publicly available code McLuster is presented, which can be used to set-up star cluster models for N-body computations or for direct study. It originated from the work done throughout this thesis and is now officially part of the Nbody6 package by Dr. Sverre Aarseth from Cambridge, UK. The capabilities of this code are demonstrated by testing methods from the literature for detecting and quantifying mass segregation and substructure in star clusters. For this purpose, several models of the young massive cluster R136 in the Large Magellanic Cloud are generated and analysed.