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Simon Portegies Zwart

Publications and source records attributed to Simon Portegies Zwart.

At least 145 records · Page 8Linked to original sources
arXiv2013

Structure and evolution of high-mass stellar mergers

In young dense clusters repeated collisions between massive stars may lead to the formation of a very massive star (above 100 Msun). In the past the study of the long-term evolution of merger remnants has mostly focussed on collisions between low-mass stars (up to about 2 Msun) in the context of blue-straggler formation. The evolution of collision products of more massive stars has not been as thoroughly investigated. In this paper we study the long-term evolution of a number of stellar mergers formed by the head-on collision of a primary star with a mass of 5-40 Msun with a lower mass star at three points in its evolution in order to better understand their evolution. We use smooth particle hydrodynamics (SPH) calculations to model the collision between the stars. The outcome of this calculation is reduced to one dimension and imported into a stellar evolution code. We follow the subsequent evolution of the collision product through the main sequence at least until the onset of helium burning. We find that little hydrogen is mixed into the core of the collision products, in agreement with previous studies of collisions between low-mass stars. For collisions involving evolved stars we find that during the merger the surface nitrogen abundance can be strongly enhanced. The evolution of most of the collision products proceeds analogously to that of normal stars with the same mass, but with a larger radius and luminosity. However, the evolution of collision products that form with a hydrogen depleted core is markedly different from that of normal stars with the same mass. They undergo a long-lived period of hydrogen shell burning close to the main-sequence band in the Hertzsprung-Russell diagram and spend the initial part of core helium burning as compact blue supergiants.

astro-ph.SR↗
arXiv2013

The Cosmogrid Simulation: Statistical Properties of Small Dark Matter Halos

We present the results of the "Cosmogrid" cosmological N-body simulation suites based on the concordance LCDM model. The Cosmogrid simulation was performed in a 30Mpc box with 2048^3 particles. The mass of each particle is 1.28x10^5 Msun, which is sufficient to resolve ultra-faint dwarfs. We found that the halo mass function shows good agreement with the Sheth & Tormen fitting function down to ~10^7 Msun. We have analyzed the spherically averaged density profiles of the three most massive halos which are of galaxy group size and contain at least 170 million particles. The slopes of these density profiles become shallower than -1 at the inner most radius. We also find a clear correlation of halo concentration with mass. The mass dependence of the concentration parameter cannot be expressed by a single power law, however a simple model based on the Press-Schechter theory proposed by Navarro et al. gives reasonable agreement with this dependence. The spin parameter does not show a correlation with the halo mass. The probability distribution functions for both concentration and spin are well fitted by the log-normal distribution for halos with the masses larger than ~10^8 Msun. The subhalo abundance depends on the halo mass. Galaxy-sized halos have 50% more subhalos than ~10^{11} Msun halos have.

astro-ph.CO↗
arXiv2013

The Effect of Many Minor Mergers on the Size Growth of Compact Quiescent Galaxies

Massive galaxies with a half-mass radius <~ 1kpc are observed in the early universe (z~>2), but not in the local universe. In the local universe similar-mass (within a factor of two) galaxies tend to be a factor of 4 to 5 larger. Dry minor mergers are known to drive the evolution of the size of a galaxy without much increasing the mass, but it is unclear if the growth in size is sufficient to explain the observations. We test the hypothesis that galaxies grow through dry minor mergers by simulating merging galaxies with mass ratios of q=1:1 (equal mass) to q=1:160. In our N-body simulations the total mass of the parent galaxy doubles. We confirm that major mergers do not cause a sufficient growth in size. The observation can be explained with mergers with a mass ratio of q=1:5--1:10. Smaller mass ratios cause a more dramatic growth in size, up to a factor of ~17 for mergers with a mass ratio of 1:80. For relatively massive minor mergers q ~> 1:20 the mass of the incoming child galaxies tend to settle in the halo of the parent galaxy. This is caused by the tidal stripping of the child galaxies by the time they enter the central portion of the parent. When the accretion of minor galaxies becomes more continuous, when q <~ 1:40, the foreign mass tends to concentrate more in the central region of the parent galaxy. We speculate that this is caused by dynamic interactions between the child galaxies inside the merger remnant and the longer merging times when the difference in mass is larger. These interactions cause dynamical heating which results in accretion of mass inside the galaxy core and a reduction of the parent's circular velocity and density.

astro-ph.GA↗
arXiv2012

Planet-Mediated Precision-Reconstruction of the Evolution of the Cataclysmic Variable HU Aquarius

Cataclysmic variables (CVs) are binaries in which a compact white dwarf accretes material from a low-mass companion star. The discovery of two planets in orbit around the CV HU Aquarii opens unusual opportunities for understanding the formation and evolution of this system. In particular the orbital parameters of the planets constrains the past and enables us to reconstruct the evolution of the system through the common-envelope phase. During this dramatic event the entire hydrogen envelope of the primary star is ejected, passing the two planets on the way. The observed eccentricities and orbital separations of the planets in HU Aqr enable us to limit the common-envelope parameter $αλ= 0.45\pm 0.17$ or $γ= 1.77\pm0.02$ and measure the rate at which the common envelope is ejected, which turns out to be copious. The mass in the common envelope is ejected from the binary system at a rate of ${\dot m} = 1.9\pm 0.3\,\MSun/yr$. The reconstruction of the initial conditions for HU Aqr indicates that the primary star had a mass of $M_{\rm ZAMS} = 1.6\pm0.2$\,\MSun\, and a $m_{\rm ZAMS} = 0.47\pm 0.04$\,\MSun\, companion in a $a=25$--160\,\RSun\, (best value $a=97$\,\RSun) binary. The two planets were born with an orbital separation of $a_a=541\pm44$\,\RSun\, and $a_b=750\pm72$\,\Rsun\, respectively. After the common envelope, the primary star turns into a $0.52\pm0.01$\,\MSun\, helium white dwarf, which subsequently accreted $\sim 0.30$\,\MSun\, from its Roche-lobe filling companion star, grinding it down to its current observed mass of $0.18\,\MSun$.

astro-ph.EP↗
arXiv2012

Multi-physics simulations using a hierarchical interchangeable software interface

We introduce a general-purpose framework for interconnecting scientific simulation programs using a homogeneous, unified interface. Our framework is intrinsically parallel, and conveniently separates all component numerical modules in memory. This strict separation allows automatic unit conversion, distributed execution of modules on different cores within a cluster or grid, and orderly recovery from errors. The framework can be efficiently implemented and incurs an acceptable overhead. In practice, we measure the time spent in the framework to be less than 1% of the wall-clock time. Due to the unified structure of the interface, incorporating multiple modules addressing the same physics in different ways is relatively straightforward. Different modules may be advanced serially or in parallel. Despite initial concerns, we have encountered relatively few problems with this strict separation between modules, and the results of our simulations are consistent with earlier results using more traditional monolithic approaches. This framework provides a platform to combine existing simulation codes or develop new physical solver codes within a rich "ecosystem" of interchangeable modules.

astro-ph.IM↗
arXiv2012

Supernova Type Ia progenitors from merging double white dwarfs: Using a new population synthesis model

The study of Type Ia supernovae (SNIa) has lead to greatly improved insights into many fields in astrophysics, however a theoretical explanation of the origin of these events is still lacking. We investigate the potential contribution to the SNIa rate from the population of merging double carbon-oxygen white dwarfs. We aim to develope a model that fits the observed SNIa progenitors as well as the observed close double white dwarf population. We differentiate between two scenarios for the common envelope (CE) evolution; the alpha-formalism based on the energy equation and the gamma-formalism that is based on the angular momentum equation. In one model we apply the alpha-formalism always. In the second model the gamma-formalism is applied, unless the binary contains a compact object or the CE is triggered by a tidal instability for which the alpha-formalism is used. The binary population synthesis code SeBa was used to evolve binary systems from the zero-age main sequence to the formation of double white dwarfs and subsequent mergers. SeBa has been thoroughly updated since the last publication of the content of the code. The limited sample of observed double white dwarfs is better represented by the simulated population using the gamma-formalism than the alpha-formalism. For both CE formalisms, we find that although the morphology of the simulated delay time distribution matches that of the observations within the errors, the normalisation and time-integrated rate per stellar mass are a factor 7-12 lower than observed. Furthermore, the characteristics of the simulated populations of merging double carbon-oxygen white dwarfs are discussed and put in the context of alternative SNIa models for merging double white dwarfs.

astro-ph.HE↗
arXiv2012

A pilgrimage to gravity on GPUs

In this short review we present the developments over the last 5 decades that have led to the use of Graphics Processing Units (GPUs) for astrophysical simulations. Since the introduction of NVIDIA's Compute Unified Device Architecture (CUDA) in 2007 the GPU has become a valuable tool for N-body simulations and is so popular these days that almost all papers about high precision N-body simulations use methods that are accelerated by GPUs. With the GPU hardware becoming more advanced and being used for more advanced algorithms like gravitational tree-codes we see a bright future for GPU like hardware in computational astrophysics.

astro-ph.IM↗
arXiv2012

Bonsai: A GPU Tree-Code

We present a gravitational hierarchical N-body code that is designed to run efficiently on Graphics Processing Units (GPUs). All parts of the algorithm are executed on the GPU which eliminates the need for data transfer between the Central Processing Unit (CPU) and the GPU. Our tests indicate that the gravitational tree-code outperforms tuned CPU code for all parts of the algorithm and show an overall performance improvement of more than a factor 20, resulting in a processing rate of more than 2.8 million particles per second.

astro-ph.IM↗
arXiv2012

A sparse octree gravitational N-body code that runs entirely on the GPU processor

We present parallel algorithms for constructing and traversing sparse octrees on graphics processing units (GPUs). The algorithms are based on parallel-scan and sort methods. To test the performance and feasibility, we implemented them in CUDA in the form of a gravitational tree-code which completely runs on the GPU.(The code is publicly available at: http://castle.strw.leidenuniv.nl/software.html) The tree construction and traverse algorithms are portable to many-core devices which have support for CUDA or OpenCL programming languages. The gravitational tree-code outperforms tuned CPU code during the tree-construction and shows a performance improvement of more than a factor 20 overall, resulting in a processing rate of more than 2.8 million particles per second.

astro-ph.IM↗
arXiv2012

Mapping the Galactic Center with Gravitational Wave Measurements using Pulsar Timing

We examine the nHz gravitational wave (GW) foreground of stars and black holes (BHs) orbiting SgrA* in the Galactic Center. A cusp of stars and BHs generates a continuous GW spectrum below 40 nHz; individual BHs within 1 mpc to SgrA* stick out in the spectrum at higher GW frequencies. The GWs and gravitational near-field effects can be resolved by timing pulsars within a few pc of this region. Observations with the Square Kilometer Array (SKA) may be especially sensitive to intermediate mass black holes (IMBHs) in this region, if present. A 100ns-10mus timing accuracy is sufficient to detect BHs of mass 1000 Msun with pulsars at distance 0.1-1 pc in a 3 yr observation baseline. Unlike electromagnetic imaging techniques, the prospects for resolving individual objects through GW measurements improve closer to SgrA*, even if the number density of objects increases inwards steeply. Scattering by the interstellar medium will pose the biggest challenge for such observations.

astro-ph.GA↗
arXiv2012

High-Performance Distributed Multi-Model / Multi-Kernel Simulations: A Case-Study in Jungle Computing

High-performance scientific applications require more and more compute power. The concurrent use of multiple distributed compute resources is vital for making scientific progress. The resulting distributed system, a so-called Jungle Computing System, is both highly heterogeneous and hierarchical, potentially consisting of grids, clouds, stand-alone machines, clusters, desktop grids, mobile devices, and supercomputers, possibly with accelerators such as GPUs. One striking example of applications that can benefit greatly of Jungle Computing Systems are Multi-Model / Multi-Kernel simulations. In these simulations, multiple models, possibly implemented using different techniques and programming models, are coupled into a single simulation of a physical system. Examples include the domain of computational astrophysics and climate modeling. In this paper we investigate the use of Jungle Computing Systems for such Multi-Model / Multi-Kernel simulations. We make use of the software developed in the Ibis project, which addresses many of the problems faced when running applications on Jungle Computing Systems. We create a prototype Jungle-aware version of AMUSE, an astrophysical simulation framework. We show preliminary experiments with the resulting system, using clusters, grids, stand-alone machines, and GPUs.

cs.DC↗
arXiv2012

PSDF: Particle Stream Data Format for N-Body Simulations

We present a data format for the output of general N-body simulations, allowing the presence of individual time steps. By specifying a standard, different N-body integrators and different visualization and analysis programs can all share the simulation data, independent of the type of programs used to produce the data. Our Particle Stream Data Format, PSDF, is specified in YAML, based on the same approach as XML but with a simpler syntax. Together with a specification of PSDF, we provide background and motivation, as well as specific examples in a variety of computer languages. We also offer a web site from which these examples can be retrieved, in order to make it easy to augment existing codes in order to give them the option to produce PSDF output.

astro-ph.IM↗
arXiv2011

Simulations of Dense Stellar Systems with the AMUSE Software Toolkit

We describe AMUSE, the Astrophysical Multipurpose Software Environment, a programming framework designed to manage multi-scale, multi-physics simulations in a hierarchical, extensible, and internally consistent way. Constructed as a collection of individual modules, AMUSE allows computational tools for different physical domains to be easily combined into a single task. It facilitates the coupling of modules written in different languages by providing inter-language tools and a standard programming interface that represents a balance between generality and computational efficiency. The framework currently incorporates the domains of stellar dynamics, stellar evolution, gas dynamics, and radiative transfer. We present some applications of the framework and outline plans for future development of the package.

astro-ph.IM↗
arXiv2011

The Origin of OB Runaway Stars

About 20% of all massive stars in the Milky Way have unusually high velocities, the origin of which has puzzled astronomers for half a century. We argue that these velocities originate from strong gravitational interactions between single stars and binaries in the centers of star clusters. The ejecting binary forms naturally during the collapse of a young ($\aplt 1$\,Myr) star cluster. This model replicates the key characteristics of OB runaways in our galaxy and it explains the $\apgt 100$\,\Msun\, runaway stars around young star clusters, e.g. R136 and Westerlund~2. The high proportion and the distributions in mass and velocity of runaways in the Milky Way is reproduced if the majority of massive stars are born in dense and relatively low-mass (5000-10000 \Msun) clusters.

astro-ph.GA↗
arXiv2011

Multi-physics simulations using a hierarchical interchangeable software interface

We introduce a general-purpose framework for interconnecting scientific simulation programs using a homogeneous, unified software interface. Our framework is intrinsically parallel, and conveniently separates all components in memory. It performs unit conversion between different modules automatically and defines common data structures to communicate across different codes. We use the framework to simulate embedded star clusters. For this purpose we couple solvers for gravitational dynamics, stellar evolution and hydrodynamics to self consistently resolve the dynamical evolution simultaneousy with the internal nuclear evolution of the stars and the hydrodynamic response of the gas. We find, in contrast to earlier studies, that the survival of a young star cluster depends only weakly on the efficiency of star formation. The main reason for this weak dependency is the asymmetric expulsion of the embedding gas from the cluster.

astro-ph.GA↗
arXiv2011

High performance cosmological simulations on a grid of supercomputers

We present results from our cosmological N-body simulation which consisted of 2048x2048x2048 particles and ran distributed across three supercomputers throughout Europe. The run, which was performed as the concluding phase of the Gravitational Billion Body Problem DEISA project, integrated a 30 Mpc box of dark matter using an optimized Tree/Particle Mesh N-body integrator. We ran the simulation up to the present day (z=0), and obtained an efficiency of about 0.93 over 2048 cores compared to a single supercomputer run. In addition, we share our experiences on using multiple supercomputers for high performance computing and provide several recommendations for future projects.

cs.DC↗
arXiv2011

The formation of the eccentric-orbit millisecond pulsar J1903+0327 and the origin of single millisecond pulsars

The millisecond pulsar J1903+0327 is accompanied by an ordinary G-dwarf star in an unusually wide ($P_{\rm orb} \simeq 95.2$\,days) and eccentric ($e \simeq 0.44$) orbit. The standard model for producing MSPs fails to explain the orbital characteristics of this extraordinary binary, and alternative binary models are unable to explain the observables. We present a triple-star model for producing MSPs in relatively wide eccentric binaries with a normal (main-sequence) stellar companion. We start from a stable triple system consisting of a Low-Mass X-ray Binary (LMXB) with an orbital period of at least 1 day, accompanied by a G-dwarf in a wide and possibly eccentric orbit. Variations in the initial conditions naturally provide a satisfactory explanation for the unexplained triple component in the eclipsing soft X-ray transient 4U~2129+47 or the cataclysmic variable EC 19314-5915. The best explanation for J1903, however, results from the expansion of the orbit of the LMXB, driven by the mass transfer from the evolving donor star to its neutron star companion, which causes the triple eventually to becomes dynamically unstable. Using numerical computations we show that, depending on the precise system configuration at the moment the triple becomes dynamically unstable, the ejection of each of the three components is possible. If the donor star of the LMXB is ejected, a system resembling J1903, will result. If the neutron star is ejected, a single MSP results. This model therefore also provides a straightforward mechanism for forming single MSP in the Galactic disk. We conclude that the Galaxy contains some 30--300 binaries with characteristics similar to J1903, and about an order of magnitude fewer single millisecond pulsars produced with the proposed triple scenario.

astro-ph.SR↗