1,721,024 research outputs found
High Precision, High Performance Simulations of Astrophysical Stellar Systems
Full text linkThe main target of this work is the discussion of the modern techniques (software and hardware) apt to solve numerically the -body problem in order to develop a numerical code with highest as possible speed and accuracy performance. In particular, we will introduce a new high precision, high performance, code (called \code) which solves the -body problem exploiting both a high order time integration algorithm (the Hermite's 6th order integrator) and the modern hardware represented by Graphics Processing Units (GPUs), which work as powerful computing accelerators. I will describe in details \code showing how GPUs can be efficiently exploited for gravitational -body simulations up to a large number of particles () with a degree of precision and speed impossible to reach until 5 years ago. Being quite new technologies, the GPUs have not been fully exploited so far; this is why, in this Thesis, I will discuss modern numerical techniques associated with the -body problem, starting from the set up of initial conditions up to the computation of the dynamical evolution of dense and populous stellar systems using GPUs and the two main languages (OpenCL and CUDA) apt to program them.
I will present also results of the application of \code to study the emerging state, and rapid mass segregation, of intermediate-, young, stellar systems after their violent relaxation process. These objects have been investigated simulating systems composed by stars of different masses, including a central star-mass black hole as well as a model of gas residual of the mother cloud, starting from \lq cold\rq to \lq warm\rq initial conditions. Moreover, thanks to the high adaptability of the developed software, our group is investigating the formation and the evolution of the innermost region of galaxies (Nuclear Star Clusters). This is, surely, a modern topic, which has not yet received an adequate self-consistent explanation neither from theoretical nor a numerical point of view
The emerging state of open clusters upon their violent relaxation
No full textThe state after virialization of a small-to-intermediate N-body system depends on
its initial conditions; in particular, systems that are, initially, dynamically “cool”
(virial ratios Q below 0:3) relax violently in few crossing times.
This leads to a metastable system (virial ratio ~ 1) which carries a clear signature
of mass segregation much before the gentle 2-body relaxation time scale. This
result is obtained by a set of high precision N-body simulations of isolated clusters
composed of stars of two different masses (in the ratio mh=ml = 2), and is
confirmed also in presence of a massive central object (simulating a black hole of
stellar size). We point out that this (quick) mass segregation occurs in two phases:
the first one shows up in clumps originated by sub-fragmentation before the deep
overall collapse; this segregation is erased during the deep collapse to re-emerge,
abruptly, during the second phase that occurs after the first bounce of the system.
This way to segregate masses, actual result of a violent relaxation, is an interesting feature also on the astronomical-observational side. In those stellar systems that start their dynamical evolution from cool conditions, this kind of mass segregation adds to the following, slow, secular segregation as induced by 2- and 3- body encounters
The Dense Stellar Systems Around Galactic Massive Black Holes
No full textThe central regions of galaxies show the presence of massive black holes and/or dense stellar systems. The question about their modes of formation is still under debate. A likely explanation of the formation of the central dense stellar systems in both spiral and elliptical galaxies is based on the orbital decay of massive globular clusters in the central region of galaxies due to kinetic energy dissipation by dynamical friction. Their merging leads to the formation of a nuclear star cluster, like that of the Milky Way, where a massive black hole (Sgr A*) is also present. Actually, high precision N-body simulations (Antonini, Capuzzo-Dolcetta et al. 2012, ApJ, 750, 111) show a good fit to the observational characteristics of the Milky Way nuclear cluster, giving further reliability to the cited `migratory' model for the formation of compact systems in the inner galaxy regions
High Precision Simulations of the Evolution of a Super Star Cluster Around a Massive Black Hole
No full textWe present preliminary results of the application of a new sophisticated code which allows high precision integration of orbits of stars belonging to a dense stellar system moving in the vicinity of a massive black hole. This mimics the situation observed in the center of many galaxies, where a nuclear star cluster contains a massive black hole which, in the past, was, likely, an active engine of violent emission of radiation. The main scope of our work is the investigation of the relaxation of the super star cluster on a sufficiently long time, together with the investigation of its feedback with the massive black hole
HiGPUs: Hermite's N-body integrator running on Graphic Processing Units
No full textHiGPUs is an implementation of the numerical integration of the classical, gravitational, N-body problem, based on a 6th order Hermite's integration scheme with block time steps, with a direct evaluation of the particle-particle forces. The main innovation of this code is its full parallelization, exploiting both OpenMP and MPI in the use of the multicore Central Processing Units as well as either Compute Unified Device Architecture (CUDA) or OpenCL for the hosted Graphic Processing Units. We tested both performance and accuracy of the code using up to 256 GPUs in the supercomputer IBM iDataPlex DX360M3 Linux Infiniband Cluster provided by the italian supercomputing consortium CINECA, for values of N ≤ 8 millions. We were able to follow the evolution of a system of 8 million bodies for few crossing times, task previously unreached by direct summation codes.
HiGPUs is also available as part of the AMUSE project
Rapid mass segregation in small stellar clusters
No full textIn this paper we focus our attention on small-to-intermediate N-body systems that are, initially, distributed uniformly in space and dynamically ‘cool’ (virial ratios Q= 2 T/ | Ω| below ∼ 0.3). In this work, we study the mass segregation that emerges after the initial violent dynamical evolution. At this scope, we ran a set of high precision N-body simulations of isolated clusters by means of HiGPUs, our direct summation N-body code. After the collapse, the system shows a clear mass segregation. This (quick) mass segregation occurs in two phases: the first shows up in clumps originated by sub-fragmentation before the deep overall collapse; this segregation is partly erased during the deep collapse to re-emerge, abruptly, during the second phase, that follows the first bounce of the system. In this second stage, the proper clock to measure the rate of segregation is the dynamical time after virialization, which (for cold and cool systems) may be significantly different from the crossing time evaluated from initial conditions. This result is obtained for isolated clusters composed of stars of two different masses (in the ratio mh/ ml= 2), at varying their number ratio, and is confirmed also in presence of a massive central object (simulating a black hole of stellar size). Actually, in stellar systems starting their dynamical evolution from cool conditions, the fast mass segregation adds to the following, slow, secular segregation which is collisionally induced. The violent mass segregation is an effect persistent over the whole range of N (128 ≤ N≤ 1 , 024) investigated, and is an interesting feature on the astronomical-observational side, too. The semi-steady state reached after virialization corresponds to a mass segregated distribution function rather than that of equipartition of kinetic energy per unit mass as it should result from violent relaxation
Very massive stars, pair-instability supernovae and intermediate-mass black holes with the SEVN code
Full text linkUnderstanding the link between massive (≳30M⊙) stellar black holes (BHs) and their progenitor stars is a crucial step to interpret observations of gravitational-wave events. In this paper, we discuss the final fate of very massive stars (VMSs), with zero-age main sequence (ZAMS) mass > 150 M⊙, accounting for pulsational pair-instability supernovae (PPISNe) and for pair-instability supernovae (PISNe).We describe an updated version of our population synthesis code SEVN, in which we added stellar evolution tracks for VMSs with ZAMS mass up to 350M⊙ and we included analytical prescriptions for PPISNe and PISNe. We use the new version of SEVN to study the BH mass spectrum at different metallicity Z, ranging from Z = 2.0 × 10-4 to 2.0 × 10-2. The main effect of PPISNe and PISNe is to favour the formation of BHs in the mass range of the first gravitational-wave event (GW150914), while they prevent the formation of remnants with mass 60-120M⊙. In particular, we find that PPISNe significantly enhance mass-loss of metal-poor (Z ≤ 2.0 × 10-3) stars with ZAMS mass 60 = MZAMS/M⊙ ≤ 125. In contrast, PISNe become effective only for moderately metal-poor (Z < 8.0 × 10-3) VMSs. VMSs with mZAMS ≳ 220 M⊙ and Z < 10-3 do not undergo PISNe and form intermediate-mass BHs (with mass ≳200M⊙) via direct collapse
The mass spectrum of compact remnants from the parsec stellar evolution tracks
Full text linkThe mass spectrum of stellar mass black holes (BHs) is highly uncertain. Dynamical mass measurements are available only for few (similar to 10) BHs in X-ray binaries, while theoretical models strongly depend on the hydrodynamics of supernova (SN) explosions and on the evolution of massive stars. In this paper, we present and discuss the mass spectrum of compact remnants that we obtained with sevn, a new public population-synthesis code, which couples the parsec stellar evolution tracks with up-to-date recipes for SN explosion (depending on the carbon-oxygen mass of the progenitor, on the compactness of the stellar core at pre-SN stage and on a recent two-parameter criterion based on the dimensionless entropy per nucleon at pre-SN stage). sevn can be used both as a stand-alone code and in combination with direct-summation N-body codes (starlab, higpus). The parsec stellar evolution tracks currently implemented in sevn predict significantly larger values of the carbon-oxygen core mass with respect to previous models. For most of the SN recipes we adopt, this implies substantially larger BH masses at low metallicity (a parts per thousand currency sign2 x 10(-3)), than other population synthesis codes. The maximum BH mass found with sevn is similar to 25, 60 and 130 M-aS (TM) at metallicity Z = 2 x 10(-2), 2 x 10(-3) and 2 x 10(-4), respectively. Mass loss by stellar winds plays a major role in determining the mass of BHs for very massive stars (a parts per thousand yen90 M-aS (TM)), while the remnant mass spectrum depends mostly on the adopted SN recipe for lower progenitor masses. We discuss the implications of our results for the transition between neutron star and BH mass, and for the expected number of massive BHs (with mass > 25 M-aS (TM)) as a function of metallicity
The interaction between supermassive black holes and globular clusters
Full text linkAlmost all galaxies along the Hubble sequence host a compact massive object (CMO) in their center. The CMO can be either a supermassive black hole (SMBH) or a very dense stellar cluster, also known as nuclear star cluster (NSC). Generally, heavier galaxies (mass ≳ 1011M⊙) host a central SMBH while lighter show a central NSC. Intermediate mass hosts, instead, contain both a NSC and a SMBH. One possible formation mechanisms of a NSC relies on the dry-merger (migratory) scenario, in which globular clusters (GCs) decay toward the center of the host galaxy and merge. In this framework, the absence of NSCs in high-mass galaxies can be imputed to destruction of the infalling GCs by the intense tidal field of the central SMBH. In this work, we report preliminary results of N-body simulations performed using our high-resolution, direct, code HiGPUs, to investigate the effects of a central SMBH on a single GC orbiting around it. By varying either the mass of the SMBH and the mass of the host galaxy, we derived an upper limit to the mass of the central SMBH, and thus to the mass of the host, above which the formation of a NSC is suppressed
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