arXiv subjects

R. Spurzem

Publications and source records attributed to R. Spurzem.

At least 37 records · Page 2Linked to original sources

arXiv2010

Dynamical evolution of rotating dense stellar systems with embedded black holes

Evolution of self-gravitating rotating dense stellar systems (e.g. globular clusters, galactic nuclei) with embedded black holes is investigated. The interaction between the black hole and stellar component in differential rotating flattened systems is followed. The interplay between velocity diffusion due to relaxation and black hole star accretion is investigated together with cluster rotation using 2D+1 Fokker-Planck numerical methods. The models can reproduce the Bahcall-Wolf solution $f \propto E^{1/4}$ ($n \propto r^{-7/4}$) inside the zone of influence of the black hole. Gravo-gyro and gravothermal instabilities conduce the system to a faster evolution leading to shorter collapse times with respect to the non-rotating systems. Angular momentum transport and star accretion support the development of central rotation in relaxation time scales. We explore system dissolution due to mass-loss in the presence of an external tidal field (e.g. globular clusters in galaxies).

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arXiv2010

On the isolated dwarf galaxies: from cuspy to flat dark matter density profiles and metallicity gradients

The chemodynamical evolution of spherical multi-component self-gravitating models for isolated dwarf galaxies is studied. We compare their evolution with and without feedback effects from star formation processes. We find that initially cuspy dark matter profiles flatten with time as a result of star formation, without any special tuning conditions. Thus the seemingly flattened profiles found in many dwarfs do not contradict the cuspy profiles predicted by cosmological models. We also calculate the chemical evolution of stars and gas, to permit comparisons with observational data.

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arXiv2008

Dynamics of Planetary Systems in Star Clusters

At least 10-15% of nearby sun-like stars have known Jupiter-mass planets. In contrast, very few planets are found in mature open and globular clusters such as the Hyades and 47 Tuc. We explore here the possibility that this dichotomy is due to the post-formation disruption of planetary systems associated with the stellar encounters in long-lived clusters. One supporting piece of evidence for this scenario is the discovery of freely floating low-mass objects in star forming regions. We use two independent numerical approaches, a hybrid Monte Carlo and a direct $N$-body method, to simulate the impact of the encounters. We show that the results of numerical simulations are in reasonable agreement with analytical determinations in the adiabatic and impulsive limits. They indicate that distant stellar encounters generally do not significantly modify the compact and nearly circular orbits. However, moderately close stellar encounters, which are likely to occur in dense clusters, can excite planets' orbital eccentricity and induce dynamical instability in systems which are closely packed with multiple planets. We compute effective cross sections for the dissolution of planetary systems and show that, for all initial eccentricities, dissolution occurs on time scales which are longer than the dispersion of small stellar associations, but shorter than the age of typical open and globular clusters. Although it is much more difficult to disrupt short-period planets, close encounters can excite modest eccentricity among them, such that subsequent tidal dissipation leads to orbital decay, tidal inflation, and even disruption of the close-in planets.

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arXiv2008

Post-Newtonian simulations of super-massive black hole binaries in galactic nuclei

We present some preliminary results from recent numerical simulations that model the evolution of super-massive black hole (SMBH) binaries in galactic nuclei. Including the post-Newtonian terms for the binary system and adopting appropriate models for the galaxies allows us, for the first time, to follow the evolution of SMBH binaries from kpc scales down to the coalescence phase. We use our results to make predictions of the detectability of such events with the gravitational wave detector LISA.

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arXiv2007

Satellite survival in cold dark matter cosmology

We study the survival of substructures (clumps) within larger self-gravitating dark matter halos. Building on scaling relations obtained from N-body calculations of violent relaxation, we argue that the tidal field of galaxies and halos can only destroy substructures if spherical symmetry is imposed at formation. We explore other mechanisms that may tailor the number of halo substructures during the course of virialization. Unless the larger halo is built up from a few large clumps, we find that clump-clump encounters are unlikely to homogenize the halo on a dynamical timescale. Phase mixing would proceed faster in the inner parts and allow for the secular evolution of a stellar disk.

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arXiv2006

2D Fokker-Planck models of rotating clusters

Globular clusters rotate significantly, and with the increasing amount of detailed morphologicaland kinematical data obtained in recent years on galactic globular clusters many interesting features show up. We show how our theoretical evolutionary models of rotating clusters can be used to obtain fits, which at least properly model the overall rotation and its implied kinematics in full 2D detail (dispersions, rotation velocities). Our simplified equal mass axisymmetric rotatingmodel provides detailed two-dimensional kinematical and morphological data for star clusters. The degree of rotation is not dominant in energy, but also non-negligible for the phase space distribution function, shape and kinematics of clusters. Therefore the models are well applicable for galactic globular clusters. Since previously published papers on that matter by us made it difficult to do detailed comparisons with observations we provide a much more comprehensive and easy-to-use set of data here, which uses as entries dynamical age and flattening of observed cluster andthen offers a limited range of applicable models in full detail. The method, data structure and some exemplary comparison with observations are presented. Future work will improve modelling anddata base to take a central black hole, a mass spectrum and stellar evolution into account.

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arXiv2006

Encounter-Triggered Disc Mass Loss in the ONC

The relevance of encounters on the destruction of protoplanetary discs in the Orion Nebula Cluster (ONC) is investigated by combining two different types of numerical simulation. First, star-cluster simulations are performed to model the stellar dynamics of the ONC, the results of which are used to investigate the frequency of encounters, the mass ratio and separation of the stars involved, and the eccentricity of the encounter orbits. The results show that interactions that could influence the star-surrounding disc are more frequent than previously assumed in the core of the ONC, the so-called Trapezium cluster. Second, a parameter study of star-disc encounters is performed to determine the upper limits of the mass loss of the discs in encounters. For simulation times of $\sim$ 1-2 Myr (the likely age of the ONC) the results show that gravitational interaction might account for a significant disc mass loss in dense clusters. Disc destruction is dominated by encounters with high-mass stars, especially in the Trapezium cluster, where the fraction of discs destroyed due to stellar encounters can reach 10-15%. These estimates are in accord with observations of (Lada et al. 2000) who determined a stellar disc fraction of 80-85%. Thus, it is shown that in the ONC - a typical star-forming region - stellar encounters do have a significant effect on the mass of protoplanetary discs and thus affect the formation of planetary systems.

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arXiv2005

Anisotropic gaseous models of tidally limited star clusters -- comparison with other methods

We present new models of the evolution and dissolution of star clusters evolving under the combined influence of internal relaxation and external tidal fields, using the anisotropic gaseous model based on the Fokker-Planck approximation, and a new escaper loss cone model. This model borrows ideas from loss cones of stellar distributions near massive black holes, and describes physical processes related to escaping stars by a simple model based on two timescales and a diffusion process. We compare our results with those of direct $N$-body models and of direct numerical solutions of the orbit-averaged Fokker-Planck equation. For this comparative study we limit ourselves to idealized single point mass star clusters, in order to present a detailed study of the physical processes determining the rate of mass loss, core collapse and other features of the system's evolution. With the positive results of our study the path is now open in the future to use the computationally efficient gaseous models for future studies with more realism (mass spectrum, stellar evolution).

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arXiv2005

Weighing the cusp at the Galactic Centre

This paper has been withdrawn temporarily by the authors. As stars close to the galactic centre have short orbital periods it has been possible to trace large fractions of their orbits in the recent years. Previously the data of the orbit of the star S2 have been fitted with Keplerian orbits corresponding to a massive black hole (MBH) with a mass of M$_{BH}$=3-4$\times10^6$M_sun implying an insignificant cusp mass.However, it has also been shown that the central black hole resides in a ~1'' diameter stellar cluster of a priori unknown mass. In a spherical potential which is neither Keplerian nor harmonic, orbits will precess resulting in inclined rosetta shaped trajectories on the sky. In this case, the assumption of non-Keplerian orbits is a more physical approach. It is also the only approach through which cusp mass information can be obtained via stellar dynamics of the cusp members. This paper presents the first exemplary modelling efforts in this direction. Using positional and radial data of star S2, we find that there could exist an unobserved extended mass component of several 10$^5$M$_{\odot}$ forming a so-called 'cusp' centred on the black hole position. Considering only the fraction of the cusp mass M$_{S2_{apo}}$ within the apocenter of the S2 orbit we find as an upper limit that M$_{S2_{apo}}$/(M$_{BH}$ + M$_{S2_{apo}}$) $\le$ 0.05. A large extended cusp mass, if present, is unlikely to be composed of sub-solar mass constituents, but could be explained rather well by a cluster of high M/L stellar remnants, which we find to form a stable configuration.

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arXiv2004

Physical Processes in Star-Gas Systems

First we present a recently developed 3D chemodynamical code for galaxy evolution from the K**2 collaboration. It follows the evolution of all components of a galaxy such as dark matter, stars, molecular clouds and diffuse interstellar matter (ISM). Dark matter and stars are treated as collisionless N-body systems. The ISM is numerically described by a smoothed particle hydrodynamics (SPH) approach for the diffuse (hot) gas and a sticky particle scheme for the (cool) molecular clouds. Physical processs such as star formation, stellar death or condensation and evaporation processes of clouds interacting with the ISM are described locally. An example application of the model to a star forming dwarf galaxy will be shown for comparison with other codes. Secondly we will discuss new kinds of exotic chemodynamical processes, as they occur in dense gas-star systems in galactic nuclei, such as non-standard ``drag''-force interactions, destructive and gas producing stellar collisions. Their implementation in 1D dynamical models of galactic nuclei is presented. Future prospects to generalize these to 3D are work in progress and will be discussed.

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arXiv2003

A stochastic Monte Carlo approach to model real star cluster evolution, III. Direct integrations of three- and four-body interactions

Spherically symmetric equal mass star clusters containing a large amount of primordial binaries are studied using a hybrid method, consisting of a gas dynamical model for single stars and a Monte Carlo treatment for relaxation of binaries and the setup of close resonant and fly-by encounters of single stars with binaries and binaries with each other (three- and four-body encounters). What differs from our previous work is that each encounter is being integrated using a highly accurate direct few-body integrator which uses regularized variables. Hence we can study the systematic evolution of individual binary orbital parameters (eccentricity, semi-major axis) and differential and total cross sections for hardening, dissolution or merging of binaries (minimum distance) from a sampling of several ten thousands of scattering events as they occur in real cluster evolution including mass segregation of binaries, gravothermal collapse and reexpansion, binary burning phase and ultimately gravothermal oscillations. For the first time we are able to present empirical cross sections for eccentricity variation of binaries in close three- and four-body encounters. It is found that a large fraction of three-body and four-body encounters results in merging. Previous cross sections obtained by Spitzer and Gao for strong encounters can be reproduced, while for weak encounters non-standard processes like formation of hierarchical triples occur.

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arXiv2003

A Multi-Phase Chemo-Dynamical SPH Code for Galaxy Evolution

In this paper we present some test results of our newly developed Multi-Phase Chemo-Dynamical Smoothed Particle Hydrodynamics (MP- CD-SPH) code for galaxy evolution. At first, we present a test of the ``pure'' hydro SPH part of the code. Then we describe and test the multi-phase description of the gaseous components of the interstellar matter. In this second part we also compare our condensation and evaporation description with the results of a previous 2d multi-phase hydrodynamic mesh code.

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arXiv2002

Chemodynamical Modeling of Dwarf Galaxy Evolution

We present our recently developed 3-dimensional chemodynamical code for galaxy evolution. It follows the evolution of all components of a galaxy such as dark matter, stars, molecular clouds and diffuse interstellar matter (ISM). Dark matter and stars are treated as collisionless $N$-body systems. The ISM is numerically described by a smoothed particle hydrodynamics (SPH) approach for the diffuse (hot) gas and a sticky particle scheme for the (cool) molecular clouds. Additionally, the galactic components are coupled by several phase transitions like star formation, stellar death or condensation and evaporation processes within the ISM. As an example here we present the dynamical, chemical and photometric evolution of a star forming dwarf galaxy with a total baryonic mass of $2 \times 10^9 {\rm M}_\odot$.

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arXiv2002

Collisional Stellar Dynamics, Gas Dynamics and Special Purpose Computing

Challenging stellar dynamical problems, such as the study of gravothermal oscillations in star clusters, have in the past initiated the very successful building of GRAPE special purpose computers. It is discussed, that present day tasks such as the formation and evolution of galactic nuclei with one or more massive black holes and the coupled stellar and gas dynamical processes in the formation of nuclei and star clusters, demand a new kind of hybrid architecture, using both GRAPE and a reconfigurable logics board called RACE. For such a system we have developed first implementations and floating point performance studies in the case of the SPH algorithm (smoothed particle hydrodynamics), which will be of great advantage for SPH modelling and also for direct $N$-body simulations using the more efficient Ahmad-Cohen neighbour schemes.

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arXiv2001

Chemodynamical Modelling of Galaxy Formation and Evolution

We present our recently developed 3-dimensional chemodynamical code for galaxy evolution. This code follows the evolution of different galactic components like stars, dark matter and different components of the interstellar medium (ISM), i.e. a diffuse gaseous phase and the molecular clouds. Stars and dark matter are treated as collisionless N-body systems. The ISM is numerically described by a smoothed particle hydrodynamics (SPH) approach for the diffuse gas and a sticky particle scheme for the molecular clouds. Additionally, the galactic components are coupled by several phase transitions like star formation, stellar death or condensation and evaporation processes within the ISM. As an example we show the dynamical and chemical evolution of a star forming dwarf galaxy with a total baryonic mass of $2 \cdot 10^9 M_\odot$. After a moderate collapse phase the stars and the molecular clouds follow an exponential radial distribution, whereas the diffuse gas shows a central depression as a result of stellar feedback. The metallicities of the galactic components behave quite differently with respect to their temporal evolution as well as their radial distribution. Especially, the ISM is at no stage well mixed.

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arXiv2001

Gas in the central region of AGNs: ISM and supermassive gaseous objects

This is a starting point of future work on a more detailed study of early evolutionary phases of galactic nuclei. We put into port the study of the evolution of star accretion onto a supermassive gaseous object in the central region of an active galactic nucleus, which was previously addressed semi-analytically. For this purpose, we use a gaseous model of relaxing dense stellar systems, whose equations are solved numerically. The model is shortly described and some first model calculations for the specific case of black hole mass growth in time are given. Similar concepts will be applied in future work for a system in which there is a dense interstellar medium as a possible progenitor of a black hole in the galactic centre.

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arXiv2001

Merging Timescales and Merger Rates of Star Clusters in Dense Star Cluster Complexes

Interacting galaxies like the famous Antennae (NGC 4038/4039) or Stephan's Quintet (HCG 92) show considerable star forming activity in their tidal arms. High resolution images (e.g. from HST-observations) indicate that these regions consist of up to hundreds of massive stellar clusters or tidal dwarf galaxies (TDG). In this paper we want to investigate the future fate of these clusters of massive star clusters (in this work called super-clusters). We simulate compact super-clusters in the tidal field of a host-galaxy and investigate the influence of orbital and internal parameters on the rate and timescale of the merging process. We show that it is possible that such configurations merge and build a dwarf galaxy, which could be an important mechanism of how long-lived dwarf satellite galaxies form. A detailed study of the merger object will appear in a follow-up paper.

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arXiv2000

Superbox - An Efficient Code for Collisionless Galactic Dynamics

We present Superbox, a particle-mesh code with high resolution sub-grids and an NGP (nearest grid point) force-calculation scheme based on the second derivatives of the potential. Superbox implements a fast low-storage FFT-algorithm, giving the possibility to work with millions of particles on desk-top computers. Test calculations show energy and angular momentum conservation to one part in 10^5 per crossing-time. The effects of grid and numerical relaxation remain negligible, even when these calculations cover a Hubble-time of evolution. As the sub-grids follow the trajectories of individual galaxies, the code allows a highly resolved treatment of interactions in clusters of galaxies, such as high-velocity encounters between elliptical galaxies and the tidal disruption of dwarf galaxies. Excellent agreement is obtained in a comparison with a direct-summation N-body code running on special-purpose Grape3 hardware. The orbital decay of satellite galaxies due to dynamical friction obtained with Superbox agrees with Chandrasekhar's treatment when the Coulomb logarithm is approximately 1.5.

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