arXiv subjects

Simon Portegies Zwart

Publications and source records attributed to Simon Portegies Zwart.

At least 127 records · Page 7Linked to original sources

arXiv2015

The fragility of planetary systems

We specify the range to which perturbations penetrate a planetesimal system. Such perturbations can originate from massive planets or from encounters with other stars. The latter can have an origin in the star cluster in which the planetary system was born, or from random encounters once the planetary system has escaped its parental cluster. The probability of a random encounter, either in a star cluster or in the Galactic field depends on the local stellar density, the velocity dispersion and the time spend in that environment. By adopting order of magnitude estimates we argue that the majority of planetary systems born in open clusters will have a {\em Parking zone}, in which planetesimals are affected by encounters in their parental star cluster but remain unperturbed after the star has left the cluster. Objects found in this range of semi-major axis and eccentricity preserve the memory of the encounter that last affected their orbits, and they can therefore be used to reconstruct this encounter. Planetary systems born in a denser environment, such as in a globular cluster are unlikely to have a Parking zone. We further argue that some planetary systems may have a {\em Frozen zone}, in which orbits are not affected either by the more inner massive planets or by external influences. Objects discovered in this zone will have preserved information about their formation in their orbital parameters.

astro-ph.EP↗

arXiv2015

A debris disk under the influence of a wide planetary mass companion: The system of HD106906

The 13 Myr old star HD106906 is orbited by a debris disk of at least 0.067 M_Moon with an inner and outer radius of 20 AU and 120 AU, respectively, and by a planet at a distance of 650 AU. We use this curious combination of a close low-mass disk and a wide planet to motivate our simulations of this system. We study the parameter space of the initial conditions to quantify the mass loss from the debris disk and its lifetime under the influence of the planet. We find that when the planet orbits closer to the star than about 50 AU and with low inclination relative to the disk (less than about 10 degrees), more disk material is perturbed outside than inside the region constrained by observations on timescales shorter than 1 Myr. Considering the age of the system, such a short lifetime of the disk is incompatible with the timescale for planet--planet scattering which is one of the scenarios suggested to explain the wide separation of the planet. For some configurations when the planet's orbit is inclined with respect to the disk, the latter will start to wobble. We argue that this wobbling is caused by a mechanism similar to the Kozai--Lidov oscillations. We also observe various resonant structures (such as rings and spiral arms) induced in the disk by the planet.

astro-ph.EP↗

arXiv2015

Intermediate-mass black holes in globular clusters: observations and simulations

The study of intermediate-mass black holes (IMBHs) is a young and promising field of research. Formed by runaway collisions of massive stars in young and dense stellar clusters, intermediate-mass black holes could still be present in the centers of globular clusters, today. Our group investigated the presence of intermediate-mass black holes for a sample of 10 Galactic globular clusters. We measured the inner kinematic profiles with integral-field spectroscopy and determined masses or upper limits of central black holes in each cluster. In combination with literature data we further studied the positions of our results on known black-hole scaling relations (such as M_bh - sigma) and found a similar but flatter correlation for IMBHs. Applying cluster evolution codes, the change in the slope could be explained with the stellar mass loss occurring in clusters in a tidal field over its life time. Furthermore, we present results from several numerical simulations on the topic of IMBHs and integral field units (IFUs). We ran N-body simulations of globular clusters containing IMBHs in a tidal field and studied their effects on mass-loss rates and remnant fractions and showed that an IMBH in the center prevents core collapse and ejects massive objects more rapidly. These simulations were further used to simulate IFU data cubes. For the specific case of NGC 6388 we simulated two different IFU techniques and found that velocity dispersion measurements from individual velocities are strongly biased towards lower values due to blends of neighbouring stars and background light. In addition, we use the Astrophysical Multipurpose Software Environment (AMUSE) to combine gravitational physics, stellar evolution and hydrodynamics to simulate the accretion of stellar winds onto a black hole.

astro-ph.GA↗

arXiv2015

The fate of supernova remnants near quiescent supermassive black holes

There is mounting observational evidence that most galactic nuclei host both supermassive black holes (SMBHs) and young populations of stars. With an abundance of massive stars, core-collapse supernovae are expected in SMBH spheres of influence. We develop a novel numerical method, based on the Kompaneets approximation, to trace supernova remnant (SNR) evolution in these hostile environments, where radial gas gradients and SMBH tides are present. We trace the adiabatic evolution of the SNR shock until 50% of the remnant is either in the radiative phase or is slowed down below the SMBH Keplerian velocity and is sheared apart. In this way, we obtain shapes and lifetimes of SNRs as a function of the explosion distance from the SMBH, the gas density profile and the SMBH mass. As an application, we focus here exclusively on quiescent SMBHs, because their light may not hamper detections of SNRs and because we can take advantage of the unsurpassed detailed observations of our Galactic Centre. Assuming that properties such as gas and stellar content scale appropriately with the SMBH mass, we study SNR evolution around other quiescent SMBHs. We find that, for SMBH masses over ~10^7 Msun, tidal disruption of SNRs can occur at less than 10^4 yr, leading to a shortened X-ray emitting adiabatic phase, and to no radiative phase. On the other hand, only modest disruption is expected in our Galactic Centre for SNRs in their X-ray stage. This is in accordance with estimates of the lifetime of the Sgr A East SNR, which leads us to expect one supernova per 10^4 yr in the sphere of influence of Sgr A*.

astro-ph.HE↗

arXiv2014

24.77 Pflops on a Gravitational Tree-Code to Simulate the Milky Way Galaxy with 18600 GPUs

We have simulated, for the first time, the long term evolution of the Milky Way Galaxy using 51 billion particles on the Swiss Piz Daint supercomputer with our $N$-body gravitational tree-code Bonsai. Herein, we describe the scientific motivation and numerical algorithms. The Milky Way model was simulated for 6 billion years, during which the bar structure and spiral arms were fully formed. This improves upon previous simulations by using 1000 times more particles, and provides a wealth of new data that can be directly compared with observations. We also report the scalability on both the Swiss Piz Daint and the US ORNL Titan. On Piz Daint the parallel efficiency of Bonsai was above 95%. The highest performance was achieved with a 242 billion particle Milky Way model using 18600 GPUs on Titan, thereby reaching a sustained GPU and application performance of 33.49 Pflops and 24.77 Pflops respectively.

astro-ph.GA↗

arXiv2014

On the Reliability of N-body Simulations

The general consensus in the N-body community is that statistical results of an ensemble of collisional N-body simulations are accurate, even though individual simulations are not. A way to test this hypothesis is to make a direct comparison of an ensemble of solutions obtained by conventional methods with an ensemble of true solutions. In order to make this possible, we wrote an N-body code called Brutus, that uses arbitrary-precision arithmetic. In combination with the Bulirsch--Stoer method, Brutus is able to obtain converged solutions, which are true up to a specified number of digits. We perform simulations of democratic 3-body systems, where after a sequence of resonances and ejections, a final configuration is reached consisting of a permanent binary and an escaping star. We do this with conventional double-precision methods, and with Brutus; both have the same set of initial conditions and initial realisations. The ensemble of solutions from the conventional simulations is compared directly to that of the converged simulations, both as an ensemble and on an individual basis to determine the distribution of the errors. We find that on average at least half of the conventional simulations diverge from the converged solution, such that the two solutions are microscopically incomparable. For the solutions which have not diverged significantly, we observe that if the integrator has a bias in energy and angular momentum, this propagates to a bias in the statistical properties of the binaries. In the case when the conventional solution has diverged onto an entirely different trajectory in phase-space, we find that the errors are centred around zero and symmetric; the error due to divergence is unbiased, as long as the time-step parameter, eta <= 2^(-5) and when simulations which violate energy conservation by more than 10% are excluded.

astro-ph.IM↗

arXiv2014

The cosmic web in CosmoGrid void regions

We study the formation and evolution of the cosmic web, using the high-resolution CosmoGrid \$Λ\$CDM simulation. In particular, we investigate the evolution of the large-scale structure around void halo groups, and compare this to observations of the VGS-31 galaxy group, which consists of three interacting galaxies inside a large void. The structure around such haloes shows a great deal of tenuous structure, with most of such systems being embedded in intra-void filaments and walls. We use the Nexus+ algorithm to detect walls and filaments in CosmoGrid, and find them to be present and detectable at every scale. The void regions embed tenuous walls, which in turn embed tenuous filaments. We hypothesize that the void galaxy group of VGS-31 formed in such an environment.

astro-ph.CO↗

arXiv2014

Computational Gravitational Dynamics with Modern Numerical Accelerators

We review the recent optimizations of gravitational $N$-body kernels for running them on graphics processing units (GPUs), on single hosts and massive parallel platforms. For each of the two main $N$-body techniques, direct summation and tree-codes, we discuss the optimization strategy, which is different for each algorithm. Because both the accuracy as well as the performance characteristics differ, hybridizing the two algorithms is essential when simulating a large $N$-body system with high-density structures containing few particles, and with low-density structures containing many particles. We demonstrate how this can be realized by splitting the underlying Hamiltonian, and we subsequently demonstrate the efficiency and accuracy of the hybrid code by simulating a group of 11 merging galaxies with massive black holes in the nuclei.

astro-ph.IM↗

arXiv2014

On the Effects of Subvirial Initial Conditions and the Birth Temperature of R136

We investigate the effect of different initial virial temperatures, Q, on the dynamics of star clusters. We find that the virial temperature has a strong effect on many aspects of the resulting system, including among others: the fraction of bodies escaping from the system, the depth of the collapse of the system, and the strength of the mass segregation. These differences deem the practice of using "cold" initial conditions no longer a simple choice of convenience. The choice of initial virial temperature must be carefully considered as its impact on the remainder of the simulation can be profound. We discuss the pitfalls and aim to describe the general behavior of the collapse and the resultant system as a function of the virial temperature so that a well reasoned choice of initial virial temperature can be made. We make a correction to the previous theoretical estimate for the minimum radius, $R_{min}$, of the cluster at the deepest moment of collapse to include a Q dependency, $R_{min}\approx Q + N^{(-1/3)}$, where $N$ is the number of particles. We use our numerical results to infer more about the initial conditions of the young cluster R136. Based on our analysis, we find that R136 was likely formed with a rather cool, but not cold, initial virial temperature ($Q\approx 0.13$). Using the same analysis method, we examined 15 other young clusters and found the most common initial virial temperature to be between 0.18 and 0.25.

astro-ph.GA↗

arXiv2014

Binary white dwarfs in the halo of the Milky Way

Aims: We study single and binary white dwarfs in the inner halo of the Milky Way in order to learn more about the conditions under which the population of halo stars was born, such as the initial mass function (IMF), the star formation history, or the binary fraction. Methods: We simulate the evolution of low-metallicity halo stars at distances up to ~ 3 kpc using the binary population synthesis code SeBa. We use two different white dwarf cooling models to predict the present-day luminosities of halo white dwarfs. We determine the white dwarf luminosity functions (WDLFs) for eight different halo models and compare these with the observed halo WDLF of white dwarfs in the SuperCOSMOS Sky Survey. Furthermore, we predict the properties of binary white dwarfs in the halo and determine the number of halo white dwarfs that is expected to be observed with the Gaia satellite. Results: By comparing the WDLFs, we find that a standard IMF matches the observations more accurately than a top-heavy one, but the difference with a bottom-heavy IMF is small. A burst of star formation 13 Gyr ago fits slightly better than a star formation burst 10 Gyr ago and also slightly better than continuous star formation $10-13$ Gyrs ago. Gaia will be the first instrument to constrain the bright end of the field halo WDLF, where contributions from binary WDs are considerable. Many of these will have He cores, of which a handful have atypical surface gravities ($\log g < 6$) and reach luminosities $\log(L/L_\odot) > 0$ in our standard model for WD cooling. These so called pre-WDs, if observed, can help us to constrain white dwarf cooling models and might teach us something about the fraction of halo stars that reside in binaries.

astro-ph.GA↗

arXiv2014

On the minimal accuracy required for simulating self-gravitating systems by means of direct N-body methods

The conservation of energy, linear momentum and angular momentum are important drivers for our physical understanding of the evolution of the Universe. These quantities are also conserved in Newton's laws of motion under gravity \citep{Newton:1687}. Numerical integration of the associated equations of motion is extremely challenging, in particular due to the steady growth of numerical errors (by round-off and discrete time-stepping, \cite{1981PAZh....7..752B,1993ApJ...415..715G,1993ApJ...402L..85H,1994LNP...430..131M}) and the exponential divergence \citep{1964ApJ...140..250M,2009MNRAS.392.1051U} between two nearby solution. As a result, numerical solutions to the general N-body problem are intrinsically questionable \citep{2003gmbp.book.....H,1994JAM....61..226L}. Using brute force integrations to arbitrary numerical precision we demonstrate empirically that ensembles of different realizations of resonant 3-body interactions produce statistically indistinguishable results. Although individual solutions using common integration methods are notoriously unreliable, we conjecture that an ensemble of approximate 3-body solutions accurately represents an ensemble of true solutions, so long as the energy during integration is conserved to better than 1/10. We therefore provide an independent confirmation that previous work on self-gravitating systems can actually be trusted, irrespective of the intrinsic chaotic nature of the N-body problem.

astro-ph.IM↗

arXiv2014

Detecting gravitational waves from the galactic center with Pulsar Timing

Black holes orbiting the Super Massive Black Hole (SMBH) Sgr A* in the Milky-way galaxy center (GC) generate gravitational waves. The spectrum, due to stars and black holes, is continuous below 40 nHz while individual BHs within about 200 AU of the central SMBH stick out in the spectrum at higher frequencies. The GWs can be detected by timing radio pulsars within a few parsecs of this region. Future observations with the Square Kilometer Array of such pulsars with sufficient timing accuracy may be sensitive to signals from intermediate mass BHs (IMBH) in a 3 year observation baseline. The recent detection of radio pulsations from the magnetar SGR J1745-29 very near the GC opens up the possibilities of detecting millisecond pulsars (which can be used as probes of the GWs) through lines of sight with only moderate pulse and angular broadening due to scattering.

astro-ph.HE↗

arXiv2013

MPWide: a light-weight library for efficient message passing over wide area networks

We present MPWide, a light weight communication library which allows efficient message passing over a distributed network. MPWide has been designed to connect application running on distributed (super)computing resources, and to maximize the communication performance on wide area networks for those without administrative privileges. It can be used to provide message-passing between application, move files, and make very fast connections in client-server environments. MPWide has already been applied to enable distributed cosmological simulations across up to four supercomputers on two continents, and to couple two different bloodflow simulations to form a multiscale simulation.

cs.DC↗

arXiv2013

Are Super-Luminous supernovae and Long GRBs produced exclusively in young dense star clusters?

Super Luminous supernovae (SLSN) occur almost exclusively in small galaxies (SMC/LMC-like or smaller), and the few SLSN observed in larger star-forming galaxies always occur close to the nuclei of their hosts. Another type of peculiar and highly energetic supernovae are the broad-line type Ic SNe (SN Ic-BL) that are associated with long-duration gamma-ray bursts (LGRBs). Also these have a strong preference for occurring in small (SMC/LMC-like or smaller) star-forming galaxies, and in these galaxies LGRBs always occur in the brightest spots. Studies of nearby star-forming galaxies that are similar to the hosts of LGRBs show that these brightest spots are giant HII regions produced by massive dense young star clusters with many hundreds of O- and Wolf-Rayet-type stars. Such dense young clusters are also found in abundance within a few hundred parsecs from the nucleus of larger galaxies like our own. We argue that the SLSN and the SN Ic-BL/LGRBs are exclusive products of two types of dynamical interactions in dense young star clusters. In our model the high angular momentum of the collapsing stellar cores required for the "engines" of a SN Ic-BL results from the post-main sequence mergers of dynamically produced cluster binaries with almost equal-mass components. The merger produces a critically rotating single helium star with sufficent angular momentum to produce a LGRB; the observed "metal aversio" of LGRBs is a natural consequence of the model. We argue that, on the other hand, SLSN could be the products of runaway multiple collisions in dense clusters, and we present (and quantize) plausible scenarios of how the different types of SLSNs can be produced.

astro-ph.HE↗

arXiv2013

Simulating Star Clusters with the AMUSE Software Framework: I. Dependence of Cluster Lifetimes on Model Assumptions and Cluster Dissolution Modes

We perform a series of simulations of evolving star clusters using AMUSE (the Astrophysical Multipurpose Software Environment), a new community-based multi-physics simulation package, and compare our results to existing work. These simulations model a star cluster beginning with a King model distribution and a selection of power-law initial mass functions, and contain a tidal cut-off. They are evolved using collisional stellar dynamics and include mass loss due to stellar evolution. After determining that the differences between AMUSE results and prior publications are understood, we explored the variation in cluster lifetimes due to the random realization noise introduced by transforming a King model to specific initial conditions. This random realization noise can affect the lifetime of a simulated star cluster by up to 30%. Two modes of star cluster dissolution were identified: a mass evolution curve that contains a run-away cluster dissolution with a sudden loss of mass, and a dissolution mode that does not contain this feature. We refer to these dissolution modes as "dynamical" and "relaxation" dominated respectively. For Salpeter-like initial mass functions, we determined the boundary between these two modes in terms of the dynamical and relaxation time scales.

astro-ph.GA↗

arXiv2013

Evolution of star clusters in a cosmological tidal field

We present a method to couple N-body star cluster simulations to a cosmological tidal field, using the Astrophysical Multipurpose Software Environment. We apply this method to star clusters embedded in the CosmoGrid dark matter-only LambdaCDM simulation. Our star clusters are born at z = 10 (corresponding to an age of the Universe of about 500 Myr) by selecting a dark matter particle and initializing a star cluster with 32,000 stars on its location. We then follow the dynamical evolution of the star cluster within the cosmological environment. We compare the evolution of star clusters in two Milky-Way size haloes with a different accretion history. The mass loss of the star clusters is continuous irrespective of the tidal history of the host halo, but major merger events tend to increase the rate of mass loss. From the selected two dark matter haloes, the halo that experienced the larger number of mergers tends to drive a smaller mass loss rate from the embedded star clusters, even though the final masses of both haloes are similar. We identify two families of star clusters: native clusters, which become part of the main halo before its final major merger event, and the immigrant clusters, which are accreted upon or after this event; native clusters tend to evaporate more quickly than immigrant clusters. Accounting for the evolution of the dark matter halo causes immigrant star clusters to retain more mass than when the z=0 tidal field is taken as a static potential. The reason for this is the weaker tidal field experienced by immigrant star clusters before merging with the larger dark matter halo.

astro-ph.CO↗

arXiv2013

The evolution of triples with a Roche-lobe filling outer star

The evolution of triples has not attracted much attention in the literature, although their evolution can be dramatically different from binaries and single stars. Triples are quite common, and we find that for about 1% of the triples in the Tokovinin catalogue of multiple stellar systems in the solar neighbourhood, the tertiary star will overflow its Roche lobe at some time in its evolution, before any of the inner stars leave the main sequence. For two of these systems, Xi Tauri and HD97131 we simulate in detail this phase of mass transfer, during which stellar evolution, gravitational dynamics and hydrodynamics all play an important role. We have used the Astrophysical Multi-purpose Software Environment (AMUSE) to solve these physical processes in a self-consistent way. The resulting evolution, mass transfer and the effects on the inner as well as on the outer orbit are profound, although it is not trivial to predict the eventual consequence of the phase of mass transfer and the appearance of the resulting system.

astro-ph.SR↗

arXiv2013

Assembly of filamentary void galaxy configurations

We study the formation and evolution of filamentary configurations of dark matter haloes in voids. Our investigation uses the high-resolution LambdaCDM simulation CosmoGrid to look for void systems resembling the VGS_31 elongated system of three interacting galaxies that was recently discovered by the Void Galaxy Survey (VGS) inside a large void in the SDSS galaxy redshift survey. HI data revealed these galaxies to be embedded in a common elongated envelope, possibly embedded in intravoid filament. In the CosmoGrid simulation we look for systems similar to VGS_31 in mass, size and environment. We find a total of eight such systems. For these systems, we study the distribution of neighbour haloes, the assembly and evolution of the main haloes and the dynamical evolution of the haloes, as well as the evolution of the large-scale structure in which the systems are embedded. The spatial distribution of the haloes follows that of the dark matter environment. We find that VGS_31-like systems have a large variation in formation time, having formed between 10 Gyr ago and the present epoch. However, the environments in which the systems are embedded evolved resemble each other substantially. Each of the VGS_31-like systems is embedded in an intra-void wall, that no later than z = 0.5 became the only prominent feature in its environment. While part of the void walls retain a rather featureless character, we find that around half of them are marked by a pronounced and rapidly evolving substructure. Five haloes find themselves in a tenuous filament of a few Mpc/h long inside the intra-void wall. Finally, we compare the results to observed data from VGS_31. Our study implies that the VGS_31 galaxies formed in the same (proto)filament, and did not meet just recently. The diversity amongst the simulated halo systems indicates that VGS_31 may not be typical for groups of galaxies in voids.

astro-ph.CO↗