arXiv subjects

M. Rampp

Publications and source records attributed to M. Rampp.

At least 19 recordsLinked to original sources

arXiv2020

Intermediate Mass Black Hole Formation in compact Young Massive Star Clusters

Young dense massive star clusters are a promising environment for the formation of intermediate mass black holes (IMBHs) through collisions. We present a set of 80 simulations carried out with Nbody6++GPU of 10 initial conditions for compact $\sim 7 \times 10^4 M_{\odot}$ star clusters with half-mass radii $R_\mathrm{h} \lesssim 1 pc$, central densities $\rho_\mathrm{core} \gtrsim 10^5 M_\odot pc^{-3}$, and resolved stellar populations with 10\% primordial binaries. Very massive stars (VMSs) with masses up to $\sim 400 M_\odot$ grow rapidly by binary exchange and three-body scattering events with main sequences stars in hard binaries. Assuming that in VMS - stellar BH collisions all stellar material is accreted onto the BH, IMBHs with masses up to $M_\mathrm{BH} \sim 350 M_\odot$ can form on timescales of $\lesssim 15$ Myr. This process was qualitatively predicted from Monte Carlo MOCCA simulations. Despite the stochastic nature of the process - typically not more than 3/8 cluster realisations show IMBH formation - we find indications for higher formation efficiencies in more compact clusters. Assuming a lower accretion fraction of 0.5 for VMS - BH collisions, IMBHs can also form. The process might not work for accretion fractions as low as 0.1. After formation, the IMBHs can experience occasional mergers with stellar mass BHs in intermediate mass-ratio inspiral events (IMRIs) on a 100 Myr timescale. Realised with more than $10^5$ stars, 10 \% binaries, the assumed stellar evolution model with all relevant evolution processes included and 300 Myr simulation time, our large suite of simulations indicates that IMBHs of several hundred solar masses might form rapidly in massive star clusters right after their birth while they are still compact.

astro-ph.GA↗

arXiv2020

FaVAD: A software workflow for characterisation and visualizing of defects in crystalline structures

The analysis of defects and defect dynamics in crystalline materials is important for fundamental science and for a wide range of applied engineering. With increasing system size the analysis of molecular-dynamics simulation data becomes non-trivial. Here, we present a workflow for semi-automatic identification and classification of defects in crystalline structures, combining a new approach for defect description with several already existing open-source software packages. Our approach addresses the key challenges posed by the often relatively tiny volume fraction of the modified parts of the sample, thermal motion and the presence of potentially unforeseen atomic configurations (defect types) after irradiation. The local environment of any atom is converted into a rotation-invariant descriptive vector ('fingerprint'), which can be compared to known defect types and also yields a distance metric suited for classification. Vectors which cannot be associated to known structures indicate new types of defects. As proof-of-concept we apply our method on an iron sample to analyze the defects caused by a collision cascade induced by a 10 keV primary-knock-on-atom. The obtained results are in good agreement with reported literature values.

physics.comp-ph↗

arXiv2014

The 3D MHD code GOEMHD3 for large-Reynolds-number astrophysical plasmas

The numerical simulation of turbulence and flows in almost ideal, large-Reynolds-number astrophysical plasmas motivates the implementation of almost conservative MHD computer codes. They should efficiently calculate, use highly parallelized schemes scaling well with large numbers of CPU cores, allows to obtain a high grid resolution over large simulation domains and which can easily be adapted to new computer architectures as well as to new initial and boundary conditions, allow modular extensions. The new massively parallel simulation code GOEMHD3 enables efficient and fast simulations of almost ideal, large-Reynolds-number astrophysical plasma flows, well resolved and on huge grids covering large domains. Its abilities are validated by major tests of ideal and weakly dissipative plasma phenomena. The high resolution ($2048^3$ grid points) simulation of a large part of the solar corona above an observed active region proved the excellent parallel scalability of the code using more than 30.000 processor cores.

astro-ph.SR↗

arXiv2006

Two-Dimensional Hydrodynamic Core-Collapse Supernova Simulations with Spectral Neutrino Transport II. Models for Different Progenitor Stars

1D and 2D supernova simulations for stars between 11 and 25 solar masses are presented, making use of the Prometheus/Vertex neutrino-hydrodynamics code, which employs a full spectral treatment of the neutrino transport. Multi-dimensional transport aspects are treated by the ``ray-by-ray plus'' approximation described in Paper I. Our set of models includes a 2D calculation for a 15 solar mass star whose iron core is assumed to rotate rigidly with an angular frequency of 0.5 rad/s before collapse. No important differences were found depending on whether random seed perturbations for triggering convection are included already during core collapse, or whether they are imposed on a 1D collapse model shortly after bounce. Convection below the neutrinosphere sets in about 40 ms p.b. at a density above 10**12 g/cm^3 in all 2D models, and encompasses a layer of growing mass as time goes on. It leads to a more extended proto-neutron star structure with accelerated lepton number and energy loss and significantly higher muon and tau neutrino luminosities, but reduced mean energies of the radiated neutrinos, at times later than ~100 ms p.b. In case of an 11.2 solar mass star we find that low (l = 1,2) convective modes cause a probably rather weak explosion by the convectively supported neutrino-heating mechanism after ~150 ms p.b. when the 2D simulation is performed with a full 180 degree grid, whereas the same simulation with 90 degree wedge fails to explode like all other models. This sensitivity demonstrates the proximity of our 2D models to the borderline between success and failure, and stresses the need of simulations in 3D, ultimately without the axis singularity of a polar grid. (abridged)

astro-ph↗

arXiv2005

Two-dimensional hydrodynamic core-collapse supernova simulations with spectral neutrino transport. I. Numerical method and results for a 15 M_sun star

Supernova models with a full spectral treatment of the neutrino transport are presented, employing the Prometheus/Vertex neutrino-hydrodynamics code with a ``ray-by-ray plus'' approximation for treating two- (or three-) dimensional problems. The method is described in detail and critically assessed with respect to its capabilities, limitations, and inaccuracies in the context of supernova simulations. In this first paper of a series, 1D and 2D core-collapse calculations for a (nonrotating) 15 M_sun star are discussed, uncertainties in the treatment of the equation of state -- numerical and physical -- are tested, Newtonian results are compared with simulations using a general relativistic potential, bremsstrahlung and interactions of neutrinos of different flavors are investigated, and the standard approximation in neutrino-nucleon interactions with zero energy transfer is replaced by rates that include corrections due to nucleon recoil, thermal motions, weak magnetism, and nucleon correlations. Models with the full implementation of the ``ray-by-ray plus'' spectral transport were found not to explode, neither in spherical symmetry nor in 2D with a 90 degree lateral wedge. The success of previous 2D simulations with grey, flux-limited neutrino diffusion can therefore not be confirmed. Omitting the radial velocity terms in the neutrino momentum equation leads to ``artificial'' explosions by increasing the neutrino energy density in the convective gain layer by about 20--30% and thus the integral neutrino energy deposition in this region by about a factor of two. (abbreviated)

astro-ph↗

arXiv2005

On ion-ion correlation effects during stellar core collapse

The role of ion-ion correlations in suppressing neutrino-nucleus elastic scattering during stellar core collapse is reinvestigated, using two different equations of state. We test the improved description by Itoh et al. against the treatment suggested by Horowitz and find that the stronger cross section reduction for small momentum transfer in the former case does not lead to noticeable changes of the core deleptonization and entropy increase during collapse, because the improvements are relevant below neutrino trapping conditions only for very low neutrino energies, corresponding to a very small phase space volume. Treating screening effects for ionic mixtures by the linear mixing rule, applied to the collection of representative heavy nucleus, alpha particles, and free nucleons, which is assumed to characterize the composition in nuclear statistical equilibrium, we cannot determine mentionable differences during stellar collapse, because alpha particles are not sufficiently abundant and their coherent scattering opacity is too small.

astro-ph↗

arXiv2004

Exploiting the neutronization burst of a galactic supernova

One of the robust features found in simulations of core-collapse supernovae (SNe) is the prompt neutronization burst, i.e. the first $\sim 25$ milliseconds after bounce when the SN emits with very high luminosity mainly $\nu_e$ neutrinos. We examine the dependence of this burst on variations in the input of current SN models and find that recent improvements of the electron capture rates as well as uncertainties in the nuclear equation of state or a variation of the progenitor mass have only little effect on the signature of the neutronization peak in a megaton water Cherenkov detector for different neutrino mixing schemes. We show that exploiting the time-structure of the neutronization peak allows one to identify the case of a normal mass hierarchy and large 13-mixing angle $\theta_{13}$, where the peak is absent. The robustness of the predicted total event number in the neutronization burst makes a measurement of the distance to the SN feasible with a precision of about 5%, even in the likely case that the SN is optically obscured.

astro-ph↗

arXiv2004

Neutrino-Driven Supernovae: an Accretion Instability in a Nuclear Physics Controlled Environment

New simulations demonstrate that low-mode, nonradial hydrodynamic instabilities of the accretion shock help starting hot-bubble convection in supernovae and thus support explosions by the neutrino-heating mechanism. The prevailing conditions depend on the high-density equation of state which governs stellar core collapse, core bounce, and neutron star formation. Tests of this sensitivity to nuclear physics variations are shown for spherically symmetric models. Implications of current explosion models for r-process nucleosynthesis are addressed.

astro-ph↗

arXiv2004

Core-Collapse Supernovae: Modeling between Pragmatism and Perfectionism

We briefly summarize recent efforts in Garching for modeling stellar core collapse and post-bounce evolution in one and two dimensions. The transport of neutrinos of all flavors is treated by iteratively solving the coupled system of frequency-dependent moment equations together with a model Boltzmann equation which provides the closure. A variety of progenitor stars, different nuclear equations of state, stellar rotation, and global asymmetries due to large-mode hydrodynamic instabilities have been investigated to ascertain the road to finally successful, convectively supported neutrino-driven explosions.

astro-ph↗

arXiv2004

Core-Collapse Supernovae at the Threshold

Recent progress in modeling core-collapse supernovae is summarized and set in perspective. Two-dimensional simulations with state-of-the-art treatment of neutrino transport still fail to produce powerful explosions, but evidence is presented that they are very close to success.

astro-ph↗

arXiv2003

Supernova Simulations with Boltzmann Neutrino Transport: A Comparison of Methods

Accurate neutrino transport has been built into spherically symmetric simulations of stellar core collapse and postbounce evolution. The results of such simulations agree that spherically symmetric models with standard microphysical input fail to explode by the delayed, neutrino-driven mechanism. Independent groups implemented fundamentally different numerical methods to tackle the Boltzmann neutrino transport equation. Here we present a direct and detailed comparison of such neutrino radiation-hydrodynamical simulations for two codes, Agile-Boltztran of the Oak Ridge-Basel group and Vertex of the Garching group. The former solves the Boltzmann equation directly by an implicit, general relativistic discrete angle method on the adaptive grid of a conservative implicit hydrodynamics code with second-order TVD advection. In contrast, the latter couples a variable Eddington factor technique with an explicit, moving-grid, conservative high-order Riemann solver with important relativistic effects treated by an effective gravitational potential. The presented study is meant to test both neutrino radiation-hydrodynamics implementations and to provide a data basis for comparisons and verifications of supernova codes to be developed in the future. Results are discussed for simulations of the core collapse and post-bounce evolution of a 13 solar mass star with Newtonian gravity and a 15 solar mass star with relativistic gravity.

astro-ph↗

arXiv2003

Supernova neutrinos: Flavor-dependent fluxes and spectra

Transporting nu_mu and nu_tau in a supernova (SN) core involves several processes that have been neglected in traditional simulations. Based on a Monte Carlo study we find that the flavor-dependent spectral differences are much smaller than is often stated in the literature. A full-scale SN simulation using a Boltzmann solver and including all relevant neutrino reactions confirms these results. The flavor-dependent flux differences are largest during the initial accretion phase.

astro-ph↗

arXiv2003

Improved Models of Stellar Core Collapse and Still no Explosions: What is Missing?

Two-dimensional hydrodynamic simulations of stellar core-collapse with and without rotation are presented which for the first time were performed by solving the Boltzmann equation for the neutrino transport including a state-of-the-art description of neutrino interactions. Although convection develops below the neutrinosphere and in the neutrino-heated region behind the supernova shock, the models do not explode. This suggests missing physics, possibly with respect to the nuclear equation of state and weak interactions in the subnuclear regime. However, it might also indicate a fundamental problem of the neutrino-driven explosion mechanism.

astro-ph↗

arXiv2003

Electron capture rates on nuclei and implications for stellar core collapse

Supernova simulations to date have assumed that during core collapse electron captures occur dominantly on free protons, while captures on heavy nuclei are Pauli-blocked and are ignored. We have calculated rates for electron capture on nuclei with mass numbers A=65-112 for the temperatures and densities appropriate for core collapse. We find that these rates are large enough so that, in contrast to previous assumptions, electron capture on nuclei dominates over capture on free protons. This leads to significant changes in core collapse simulations.

astro-ph↗

arXiv2002

Explosion Mechanisms of Massive Stars

One of the central problems in supernova theory is the question how massive stars explode. Understanding the physical processes that drive the explosion is crucial for linking the stellar progenitors to the final remnants and for predicting observable properties like explosion energies, neutron star and black hole masses, nucleosynthetic yields, explosion anisotropies, and pulsar kicks. Here we review different suggestions for the explosion mechanism and discuss the constraints that can or cannot be deduced from observations. The most refined and detailed hydrodynamical models are available for neutrino-driven supernova explosions, although the viability of this mechanism has yet to be demonstrated convincingly. Since spherical models do not explode, the hope rests on the helpful effects of convection inside the nascent neutron star and in the neutrino-heated region behind the stalled shock. We present the first two-dimensional simulations of these processes which have been performed with a Boltzmann solver for the neutrino transport and a state-of-the-art description of neutrino-matter interactions. Our most complete models fail, but convection brings them encouragingly close to a success. An explosion could be obtained by just a minor modification of the neutrino transport, in which case the exploding model fulfills important requirements from observations (abridged).

astro-ph↗

arXiv2002

Core Collapse and Then? The Route to Massive Star Explosions

The rapidly growing base of observational data for supernova explosions of massive stars demands theoretical explanations. Central of these is a self-consistent model for the physical mechanism that provides the energy to start and drive the disruption of the star. We give arguments why the delayed neutrino-heating mechanism should still be regarded as the standard paradigm to explain most explosions of massive stars and show how large-scale and even global asymmetries can result as a natural consequence of convective overturn in the neutrino-heating region behind the supernova shock. Since the explosion is a threshold phenomenon and depends sensitively on the efficiency of the energy transfer by neutrinos, even relatively minor differences in numerical simulations can matter on the secular timescale of the delayed mechanism. To enhance this point, we present some results of recent one- and two-dimensional computations, which we have performed with a Boltzmann solver for the neutrino transport and a state-of-the-art description of neutrino-matter interactions. Although our most complete models fail to explode, the simulations demonstrate that one is encouragingly close to the critical threshold because a modest variation of the neutrino transport in combination with postshock convection leads to a weak neutrino-driven explosion with properties that fulfill important requirements from observations.

astro-ph↗

arXiv2002

The Mechanism of Core-Collapse Supernovae and the Ejection of Heavy Elements

We present here the first results of two-dimensional hydrodynamical simulations of the neutrino-heating phase in the collapsed core of a 15 solar mass star, where the neutrino transport is treated with a variable Eddington factor method for solving the Boltzmann transport equation, and the neutrino interactions include nucleon-nucleon bremsstrahlung, nucleon recoils and correlations, and weak-magnetism effects as well as direct interactions between neutrinos of different flavors. With the given input physics (neutrino reactions and nuclear equation of state), our best simulations do not develop strong convection in the neutrino-heating layer behind the shock and do not yield explosions. With about 30% higher neutrino-energy deposition behind the shock, however, an explosion occurs on a timescale of 150 ms after core bounce. It leaves behind a neutron star with an initial baryonic mass of 1.4 solar masses and ejects the N = 50 isotopes of Sr, Y and Zr in amounts consistent with Galactic abundances.

astro-ph↗

arXiv2002

Core-collapse supernova simulations: Variations of the input physics

Spherically symmetric simulations of stellar core collapse and post-bounce evolution are used to test the sensitivity of the supernova dynamics to different variations of the input physics. We consider a state-of-the-art description of the neutrino-nucleon interactions, possible lepton-number changing neutrino reactions in the neutron star, and the potential impact of hydrodynamic mixing behind the supernova shock.

astro-ph↗